Have you ever started your car on a cold morning only to feel it shake and vibrate more intensely than usual? It’s frustrating when your car runs perfectly fine after warming up, but struggles on those first cold starts.
A cold engine needs more spark and fuel to start up, which can cause rougher idling and shaking. Fortunately, this annoying shake is often an easy fix.
In this guide, we’ll walk through the common causes and solutions for cars that shake when cold but run fine when warmed up.
PS: You can also read my guide on car idling rough but smooths out while driving.
What You Will Learn:
Some Key Highlights For You
- Engine mounts contracting when cold can cause temporary vibrations and misalignment. Replace worn mounts to resolve.
- Thick, dirty engine oil or low oil level makes it harder for oil pump to circulate oil when it is cold. Use correct viscosity oil and change regularly.
- Failing hydraulic lifters allow more valve train slop when cold until oil pressure builds up. Listen for ticking noises.
- Faulty coolant temp sensor sends wrong data to computer, confusing fuel trim and timing settings when cold.
- Moisture on ignition parts makes spark weaker when cold until it evaporates once warmed up.
- Fuel injectors clogged with carbon and deposits struggle to spray fuel properly on a cold engine startup.
- When transmission fluid is too thick, it can’t flow properly on a cold engine startup. Due to this reason, there is delaying gear engagement which will cause car shaking.
- Clogged air filter restricts needed airflow when air density higher while cold.
- Clogged fuel filter combined with cold thickened fuel reduces fuel supply to the engine when cold.
Is It Normal For A Car To Shake On Cold Startup?
It’s completely normal for your car engine to shake a little bit for a very few seconds when starting up cold.
When you first start your cold engine, the car’s computer (ECU) operates in open loop mode. This means it ignores sensor inputs and relies on pre-programmed fuel maps to run the engine. These fuel maps are set rich to help the engine warm up quickly.
In a cold engine, the fuel doesn’t vaporize and mix with air properly. To help the engine warm up quickly, the car’s computer (ECU) injects extra fuel when cold.
The extra fuel also helps heat up the catalytic converter faster. The catalytic converter needs to reach operating temperature to start reducing emissions. When cold, it doesn’t work well.
In open loop operation, the ECU doesn’t monitor oxygen sensors to adjust and optimize fuel trim. Its only goal is a fast warm-up. This over-rich fuel mixture can cause the engine to shake and vibrate slightly on initial start-up for a very few seconds.
Once the engine reaches normal operating temperature, the ECU switches to closed loop mode. Now it starts using live sensor data from oxygen sensors to precisely tune the fuel mixture for optimal efficiency and emissions.
Causes Of Car Shaking Up On Cold Start But Running Fine When Warmed Up
1. Bad Motor and Transmission Mounts
Engine and transmission mounts are responsible for firmly securing these heavy components in place and reducing vibration transferred into the chassis.
The engine is secured by mounts at the top near the front, on the sides, and at the bottom rear. These isolate engine movement and prevent vibration transfer into the chassis:
- Top mounts: Attached at front of engine near radiator support. Restrict vertical and lateral movement. Often most prone to failure.
- Side mounts: Mount engine to subframe or body along the length. Control fore/aft movement.
- Rear mount: Secures rear of engine, often with hydraulic or rubber cushion to allow engine torque reaction. Reduces engine roll.
When mounts become worn out or damaged, it allows excessive movement of the powertrain when idling and accelerating. This can cause noticeable vibration, steering wheel shake, and cabin noise especially when the engine is cold.
Once the engine warms up and the rubber of the mounts becomes more pliable, symptoms may improve or disappear entirely. Moreover, some mounts have gel filling whose dampening ability enhances as the engine warms up.
What are the signs?
Since worn mounts allow more engine movement, testing involves observing engine movement under load:
- Visual inspection: Check for cracked/torn rubber, leaks, looseness. Requires some disassembly.
- Rev test: Have someone rev the engine moderately while watching for excessive rocking/shaking (make sure that parking brakes are applied). Do the same in gear with the parking brake applied to put a load on the engine.
- Pry test: Use pry bar to stress mounts and check for cracks or excessive motion.
How to fix?
Replacing mounts is the only solution. Labor costs vary based on the vehicle and the number of mounts replaced. Expect 1-3 hours of shop time. Use OEM or high-quality aftermarket mounts for longevity.
Perform testing afterwards to verify resolution of vibration issues. Replacing mounts resolves the majority of cold start vibration problems caused by mount deterioration.
2. Thick or Dirty Engine Oil
When your car shakes or vibrates when you first start it up on a cold morning, but then runs smoothly after it warms up, the issue is likely related to the engine oil.
The thickness, level, and condition of the oil can all impact how your engine runs, especially when cold.
How engine oil is pumped?
To understand how oil impacts cold starts, you first need to understand how the oil pump works. The oil pump is driven by the engine’s crankshaft and works to circulate oil throughout the engine.
An oil pump draws oil from the oil pan and pushes it through the filter and oil galleries to lubricate bearings, camshafts, valvetrain components, and other internals.
At idle and low rpm, the oil pump spins slower and pumps less oil than at higher rpm. This is important when it comes to cold starts.
When you first start your cold engine, it idles at a low rpm for a minute or two while the engine warms up. During this time, the oil pump is delivering less oil flow than it does once the engine rpm increases.
How Thick Oil Can Cause Startup Shaking?
If the oil in your engine is too thick or viscous, it will be harder for the oil pump to push it through the engine when cold.
This can lead to temporary lack of lubrication in some areas, which increases friction and drag in the valvetrain and bearings. The resulting extra resistance when the engine is turning over is perceived as shaking or roughness.
Most modern engines are designed to use lighter viscosity oils like 0W-20 or 5W-20. These numbers refer to the ‘weight’ or thickness of the oil.
The lower the number, the easier it flows. Using heavier oils like 10W-30 or 15W-40 in an engine designed for 5W-20 will increase the resistance and friction when the engine is cold. You can read my guide on SAE 30 vs 10W30 to learn more.
Too thin of an oil can also cause problems, but most startup shaking issues are due to thick oil. Make sure you are using the manufacturer’s recommended oil viscosity for your vehicle.
Low oil levels can also be an issue
If your engine is low on oil, the oil pump may not be able to deliver sufficient oil volume or oil pressure at startup.
Bearings rely on that steady oil pressure to keep them separated. Low oil pressure allows extra motion and friction between bearing surfaces, translating into shaking and vibration.
Check your oil level regularly and top off when needed. If your oil level is good, the next thing to check is oil pressure. Low oil pressure may indicate a worn-out oil pump, clogged pickup, or excessive bearing clearances.
Do check your engine oil condition
Over time, oil breaks down and forms deposits, sludge, and varnish. This gunk collects in the engine and sticks to surfaces that need oil flow.
At cold startup, these deposits restrict oil flow even further in combination with the slower oil pump speed. This again increases friction and startup shaking. As the engine warms up, sludge and carbon deposits can burn due to heat, making your drive smoother.
If your oil has reached the recommended oil change interval, this is often the cause of cold start issues. Changing the oil and filter removes most of these deposits. Using a high-quality synthetic oil and shorter change intervals can reduce sludge buildup.
3. Failing Hydraulic Lifters
Hydraulic lifters play a critical role in ensuring proper valve train operation in many engines. When they start to fail, it can cause symptoms like shaking on cold start that go away once the engine warms up. This issue is quite common as lifters wear out over time.
What Hydraulic Lifters Do?
Hydraulic lifters, also called lash adjusters, are small cylindrical components located between the camshaft lobe and valve stem in the valve train assembly. Their job is to maintain zero lash (clearance) in the valve train as the engine heats up and parts expand.
On engines with a camshaft, valve clearances must be adjusted periodically as parts wear and tolerances change over time.
Hydraulic lifters eliminate this maintenance by automatically adjusting clearances. They use engine oil pressure to expand and take up any slack in the valve train.
Inside each lifter is a plunger suspended in oil. As the cam lobe presses down on the lifter, oil pressure pushes the plunger up to maintain contact with the lobe. This compensates for thermal expansion and wear as the engine operates.
To learn more, you can watch the following Youtube Video on working of hydraulic lifters:
How hydraulic lifters can fail?
While very durable, hydraulic lifters can still fail in a few common ways:
- Loss of oil pressure – Without sufficient oil pressure, the lifter cannot maintain its rigidity and pump up. This allows valves to make noise and potentially not open fully.
- Debris ingestion – Dirt inside the lifter can jam the plunger or inlet valve, preventing proper operation.
- Excessive wear – Over time, the lifter bore and plunger will wear and allow oil to leak down. This can cause ticking noises from hydraulic lifters.
- Plunger failure – The internal plunger itself can crack or come apart, destroying the lifter.
How bad failing lifters cause car shaking on cold startup?
When cold, engines have greater clearance in the valve train until the oil pressure builds up. Bad lifters that cannot pump up and compensate will exacerbate this.
On start up, the loose clearances combined with low oil viscosity allow the valvetrain to rattle around more violently. This shakes the engine and chassis.
Once oil pressure builds and the engine reaches operating temperature, clearances tighten up. The lifters also expand with heat and pump up to compensate. This smooths out the vibration and shaking felt on a cold start.
However, the underlying wear is still present and will progressively get worse over time. Even if the shaking disappears when warmed up, bad lifters should be addressed.
How to test?
Using stethoscope or long screwdriver, listen for any ticking noises from the lifter. Worn lifters need to be replaced. They require valve train disassembly for replacement.
To inspect lifters visually, pull the lifters and inspect for cupping or scoring on the roller tip. Another way to check them is rolling the roller tip on your forearm and feeling for an uneven surface.
For a mechanical test, you can push the hydraulic lifter. If it is soft, can be easily pressed with your thumb, and oil also starts leaking through the hole, it means lifter is bad.
4. Malfunctioning Engine Coolant Temperature Sensor
The engine coolant temperature (ECT) sensor, also known as the coolant temperature sensor, is an important component in a vehicle’s engine management system.
The ECT sensor contains a temperature-sensitive resistor that changes resistance value based on the coolant temperature.
At cold temperatures, the resistance is high. As the coolant warms up, the resistance decreases. This change in resistance signal is sent to the ECU.
The ECU uses this data to adjust the air-fuel mixture, ignition timing, and other parameters to optimize engine operation.
ECT sensor is usually located in the coolant passage between the engine and radiator, near the thermostat housing.
Note: Vehicles usually have primary and secondary ECT sensors. You have to check primary one which is located closer to engine head.
As the engine heats up or cools down during operation, the ECT sensor tracks these changes in coolant temperature.
A faulty ECT sensor can cause several driveability and performance issues, including cold start shaking that goes away once the engine warms up.
How it can cause problems?
The ECU enriches the mixture at cold start and leans it out as the engine warms up. This improves cold start performance, fuel economy, and emissions.
When the ECT sensor fails or sends incorrect data to the ECU, it can cause:
- Difficulty in cold starting: Without accurate coolant temperature data, the ECU cannot optimize ignition timing and fuel delivery for quick, reliable cold starts. This leads to extended cranking, misfires, stalling, and rough idling when cold.
- Improper ignition timing: Incorrect coolant temperature input will confuse the ECU, leading to premature ignition timing that causes sluggish acceleration and reduced power when cold.
- Hesitation and stumbling: The improper air-fuel mixture from a bad ECT sensor can cause a lean or rich condition, resulting in sporadic hesitation and stumbling issues when accelerating from a stop. This is most noticeable when the engine is cold.
- Check engine light: A faulty ECT sensor may trigger the check engine light and various engine-related error codes due to the ECU detecting abnormal readings.
How to test?
If cold start issues suspect a bad ECT sensor, it can be tested with a digital multimeter. Here is a simple process:
- Locate the ECT sensor and disconnect the electrical connector.
- Set the multimeter to check resistance and connect it to the sensor’s signal and ground terminals.
- Note the ambient temperature of the sensor. Check the resistance reading on the meter and compare to the factory spec chart.
- Warm up the sensor by blowing hot air on it. The resistance should drop as it gets warmer.
- Compare the readings at different temperatures to the spec chart. If they do not align, the ECT is faulty.
5. Poor Fuel Quality
Bad fuel quality often consists of impurities, which can disrupt the combustion process. When the engine is cold, it requires a rich air-fuel mixture for efficient ignition.
However, impurities in the fuel hinder the accurate mixture formation, causing incomplete combustion. This incomplete combustion leads to uneven engine firing, resulting in shaking and rough-running during cold starts.
What defines fuel quality?
When we talk about fuel quality, we primarily consider the octane rating and the presence of impurities.
Octane rating indicates the fuel’s resistance to knocking or pinging, preventing engine damage.
Impurities, such as water, debris, or contaminants, can hinder fuel combustion, resulting in vibration and shaking during ignition. Buying the cheapest gas at shady stations often means you’re getting low-quality fuel.
Always use the octane rating or higher recommended in your owner’s manual. Higher performance engines require premium 91+ octane fuel. Turbocharged engines need high-octane fuel with 95+ octane number.
How to prevent that?
If you have left the fuel in your fuel tank for longer, its quality will be affected due to contamination of water vapors and debris in the fuel tank. In that case, you should treat fuel tank with fuel stabilizers to keep it fresh.
Moreover, always re-fill your gas tank from the famous gas station. Seek out top tier brand stations in high traffic areas. Avoid any station that looks dirty, neglected or offers significantly cheaper gas.
6. Corroded Ignition Coils/Spark Plug Wires
When your car is first started on a cold morning, the ignition system components are still cool including the spark plugs, ignition coils and spark plug wires.
Cold temperatures can cause moisture and corrosion buildup on these parts, making it harder to produce a strong spark.
Without a good spark, the fuel mixture may not ignite properly in the cylinders. This can cause the engine to shake on cold start until it warms up and the moisture evaporates.
The ignition coils have to work harder to fire the spark plugs when cold. If the coils or spark plug wires are damaged, they may struggle to provide a consistent spark when cold causing a cylinder to misfire.
Bad ignition coils or spark plug wires will often set a P030X trouble code for a cylinder misfire and cause the engine to shake on cold start. As the engine warms up and the ignition components operate better, the misfire may disappear.
Depending on make and model of your vehicle, it ignition coils have two types:
- COP (Coil over plugs)
- Coil packs (these have spark plug wires)
How to test?
A successful approach for COP ignition systems involves exchanging the ignition coil of the problem cylinder with an ignition coil from another cylinder. Subsequently, monitor whether there is any alteration in the trouble code. If a change occurs, it suggests a defective ignition coil.
For ignition coil packs/spark plug wires, you can watch following video:
7. Fouled Spark Plugs
Just like ignition coil, moisture and corrosion can also be on spark plugs. When the engine is cold, the moisture makes it harder for the plugs to produce a strong spark to ignite the fuel mixture.
Without consistent sparking, the air-fuel mixture in one or more cylinders may fail to ignite properly. This can cause the engine to misfire and shake on cold start.
How Spark Plugs Become Contaminated?
Spark plugs can become contaminated from normal engine operation. Some common ways include:
- Carbon and fuel deposits build up on the electrodes and insulator over time. This fouling prevents the plugs from sparking correctly.
- Oil leaking into the cylinders can also foul the plugs causing a weak or non-existent spark. Excessive oil, worn piston rings, leaking seals, and short trips can all contribute to oil fouling on the plugs.
How to diagnose?
Visual inspection of the spark plugs can indicate bad plugs. Look for wet oily deposits, excessive carbon build up, electrode wear, or light grey ash deposits.
You can test spark plugs by using a multimeter and setting it to resistance measurement. You should get a reading between 4,000 to 8,000 ohms on the center electrode.
In addition, you should look for a short between the ground and center electrode of the spark plug. No short means a good spark plug.
You can watch following youtube video to learn more:
How to fix?
You can try cleaning the spark plugs using the wire brush and spark plug cleaner to carefully remove any carbon deposits or contaminants from the electrode. Ensure not to damage the electrode or insulator in the process.
First, use compressed air to clean off any dirt and debris. Then, keep the spark plug soaked in the brake cleaner till all the gunk on the spark plug is washed away.
8. Clogged Fuel Injectors Due to Carbon Buildup
Fuel injectors get pressurized fuel from the fuel pump and have a coil that acts as an electromagnet.
When the engine control module sends voltage to the injector coil, the magnetic field pulls up a valve and allows fuel to spray through the nozzle. The injector rapidly pulses on and off to control the amount of fuel.
Proper fuel injection provides an optimal air-fuel ratio for clean, efficient combustion across all engine operating conditions.
If the injectors get clogged or damaged, they can’t properly atomize and deliver fuel, causing driveability issues.
Why Do Fuel Injectors Get Clogged?
There are a few common reasons fuel injectors get restricted and fail to spray enough fuel:
- Carbon buildup: Combustion byproducts and fuel residues can bake onto injector tips
- Contaminated fuel: Particles and sediments can partially block nozzle holes
- Worn components: Affects atomization and spray pattern
Problems are most pronounced when the engine is cold because fuel doesn’t atomize as well compared to when hot. Clogged injectors accentuate fuel delivery issues when they’re already struggling to spray.
How to test?
First, you should perform visual inspection of fuel injectors as follows:
- Remove injectors and inspect nozzle ends and inlet screens for carbon deposits or corrosion. Clean if necessary.
- Check for cracks or damage around injector bodies and insulators.
- Ensure electrical connectors are tight and pins are not corroded.
You can check for damage by looking at the two terminals of the injector. These are the ends of the magnetic coil that carry electricity.
You should also check the o-ring of the fuel injectors. It is a small ring that prevents fuel from leaking. If it’s damaged, you’ll need to replace it. You can push the o-ring with your hand until it fits securely.
How to test?
To test if a fuel injector is bad, you can use a multimeter. Connect the probes to the terminals of the injector and set the multimeter to the right Ohms value (usually around 30 Ohms).
If the resistance shows “infinite,” it means the coil is open. If it shows “OL” or very low resistance, it means the coil has short-circuited. The right resistance means the coil is okay.
Another fuel injector tests include following:
- Fuel spray direction
- Fuel injector clicking sound
With the fuel pump running, pressurize the fuel rail and check each injector nozzle tip for a solid conical spray pattern without any interruptions.
Another test is to listen for a clicking noise. Use a long screwdriver near the injector and turn the key to prime the fuel pump. Then connect a 9V or 12V battery to each injector and listen for a ticking noise. That’s the sound of the injector opening and closing.
If you want a visual guide on how to test fuel injectors, check out a video starting from 2:40.
9. Vacuum Leaks
Unlike many other engine problems like ignition or fuel delivery issues that cause obvious symptoms all the time, vacuum leaks often only cause drivability problems under certain conditions.
A common pattern with vacuum leaks is that the engine will shake and run rough on a cold start, but then smooth out and run normally once it warms up.
The reason behind this behavior lies in understanding how the engine uses vacuum and how leaks can disrupt normal operation.
How vacuum leaks are present?
To understand vacuum leaks, you first need to understand what engine vacuum is and how it works. Engine vacuum is the negative air pressure created in the intake manifold when the pistons move downwards on the intake stroke.
As the pistons move down, they pull air in through the intake valves, essentially sucking air into the cylinders. This creates an area of low pressure in the intake manifold. As the throttle opens, more air passes through, resulting in vacuum drop.
Where Do Vacuum Leaks Occur?
There are a variety of places where vacuum leaks can occur. Some of the most common include:
- Vacuum hoses: The rubber hoses can become brittle and cracked over time, allowing vacuum leaks. The PCV hose and brake booster hose are common sources.
- Intake manifold gaskets: The gaskets where the intake manifold attaches to the cylinder heads are exposed to engine heat and can warp or develop leaks over time.
- EGR valves: The EGR valve allows some exhaust gases to recirculate into the intake. Leaks can occur past its gasket or internal component failure.
- Throttle body – The throttle body to intake manifold gasket surface can warp and leak vacuum.
- PCV valve and breather system: The PCV valve and hoses manage blowby gases. Leaks here expose the intake to oil vapor and unmetered air.
Why Do Vacuum Leaks Cause a Cold Start Shake?
So why do vacuum leaks seem to cause issues on cold start but not when warmed up? The answer lies in how the engine computer manages fuel trims.
At cold start, the engine computer uses pre-programmed baseline fuel maps to provide the proper air/fuel mixture for starting and warm-up.
Once the engine warms up to normal operating temperature, the computer switches to using feedback from the oxygen sensors to trim fuel delivery and maintain the ideal 14.7:1 air/fuel ratio.
A vacuum leak introduces unmetered, extra air into the intake that the computer does not account for when using its baseline fuel maps on a cold start. This makes the engine run lean and shake until it warms up.
Once warmed up, the oxygen sensor feedback allows the computer to adjust for the extra air by adding more fuel to compensate. This provides the proper air/fuel mix and the engine runs smoothly again.
How to detect?
While the engine is running, listen closely for any hissing sounds. A vacuum leak often makes a distinct high-pitched hissing noise. If you hear it, try to locate the source, as it can help you find the problem area.
Professionals sometimes use smoke machines to find vacuum leaks. These machines fill the intake system with smoke, making it easier to see where the smoke escapes from. While this method might not be readily available for most people, it’s highly effective in finding even the trickiest leaks.
You can also use carb cleaner to detect vacuum leaks.
10. Dirty and Thick Transmission Fluid
When the transmission fluid is dirty or too thick, it can lead to hard shifting and vibration on cold startup.
How Transmission Fluid Works?
Automatic transmissions require a special fluid called automatic transmission fluid (ATF) to function properly. ATF serves several crucial roles:
- Provides hydraulic pressure: ATF flows through the valve body to engage clutches and bands to shift gears. The transmission pump pressurizes the fluid to actuate these components.
- Lubricates moving parts: The fluid coats gears, shafts, bearings, seals and other internal parts to prevent excessive wear and overheating.
- Transfers power: The torque converter uses ATF to transfer rotational force from the engine to the transmission.
- Cools the transmission: As fluid circulates, it picks up heat from friction and moving components and dissipates it through the radiator.
How it causes problem?
When transmission fluid is thick and dirty, it cannot flow quickly through tiny passageways in the valve body or torque converter. The results are delayed engagement of gears and hard, jarring shifts.
Engine deposits, corrosion and different chemical reactions in transmission fluid while flowing through the transmission can affect its viscosity. That effect is more pronounced at lower temperatures.
You can also read my guide on transmission shifting hard when cold to learn more.
How do I spot?
- Dark/black color instead of red
- Burned or rancid smell
- Positive particle count on magnets inside transmission pan
- Sludge formation
In those cases, I would highly recommend you to perform a full transmission flush.
11. Dirty Engine Air Filter
As the engine draws in air, it passes through the air filter first to remove any contaminants. A clean, unclogged filter allows maximum airflow to the engine for optimal combustion and performance. Over time, the filter collects more and more gunk and gradually restricts airflow.
On cold startup, the air density is greater and the air alone has more difficulty passing through a severely clogged filter. The restricted cold air flow reduces the proper air-fuel ratio.
Once the engine reaches operating temperature, two things happen – the incoming air thins out as density decreases, and any moisture trapped in the filter has a chance to evaporate.
This combination allows improved airflow to smooth out engine operation. Of course, performance is still down due to the overall reduced airflow from dirt buildup.
How to inspect?
Here are some tips for checking your engine air filter:
- Remove the housing and take out the filter for visual inspection
- Look for very dark or black sections, which indicate dirt buildup
- Check for holes, cracks or tears in the filter material
Compare it to a new filter if unsure. The used one should not appear overly dirty or restrictive.
How to fix?
If the air filter is only slightly dirty, it can be cleaned using compressed air. Gently blow compressed air from the clean side of the filter, forcing the dirt and debris out through the dirty side.
If the air filter is heavily soiled or shows signs of damage, it is recommended to replace it. Regularly replacing the engine air filter ensures optimal airflow to the engine and helps maintain its performance.
The frequency of replacement may vary depending on the type of filter used. For OEM (original equipment manufacturer) filters, it is generally advised to replace them every 15,000 miles. However, if you are using a cheaper aftermarket filter, more frequent replacement, such as every 5,000 miles, may be necessary.
12. Clogged Fuel Filter
On cold start, the engine requires extra fuel to idle smoothly and accelerate. This is because the cold engine and components require more fuel to get moving and warming up.
The fuel injectors have to work harder, the ignition system needs higher energy sparks, and piston rings don’t seal as well until expanded from heat.
All of this leads to an increased fuel demand until the engine reaches normal operating temperature.
The viscosity or thickness of fuel increases at colder temperatures. Thicker and more viscous fuel doesn’t flow as easily through the fuel lines and filter.
This is especially true once the filter has accumulated some debris and is already partially clogged.
At the same time, cold temperatures cause condensation in the air which can collect in the gas tank and contaminate the fuel with water.
The combined effects of the thicker fuel and condensation make it even harder for fuel to pass through a clogged filter when cold.
Without enough fuel present, the combustion events are weak, uneven, and can misfire. This leads to rough idling and shaking as the engine speed surges up and down from the weak combustion events.
As the engine warms up and expands, the fuel viscosity decreases, providing better flow. The engine also needs less fuel overall when warm, so the restricted flow becomes adequate. This allows smooth running once at operating temperature despite the clogged fuel filter.
How to fix?
Replacing a contaminated fuel filter with a new one is the solution for cold start shaking in this case. The fresh filter will allow proper fuel flow even in cold weather.
For preventative maintenance, most manufacturers recommend replacing the filter every 60,000 to 80,000 miles or according to the maintenance schedule.
13. Clogged EGR Valve
The EGR valve opens to allow exhaust gases to enter the intake manifold and mix with the incoming air/fuel mixture.
It is controlled either electronically or by vacuum and normally only opens under light engine loads and speeds.
When open, the recirculated exhaust helps cool the combustion process, which in turn lowers cylinder temperatures and the formation of NOx.
On many modern engines, the EGR valve is closed by the engine computer during cold starts. This helps the engine warm up faster by allowing normal combustion temperatures.
Once warm, the EGR opens to do its job of reducing NOx emissions. When EGR opens, exhaust gases take place of fresh air with high oxygen content. This reduces combustion temperature, resulting in lesser NOx emissions.
However, with the EGR stuck open during a cold start, the combustion temperature is cooled significantly by the exhaust gases. This prevents engine warm up and causes your car to shake on cold startup.
How to test?
If your EGR valve is malfunctioning, the OBDII scan tool will display P0400, P0401 or P0402 error codes.
Now, remove the EGR valve and perform a visual inspection to check for any cracks or holes in the housing. A cracked or damaged EGR valve will need to be replaced.
To check if the EGR valve is working correctly, we will now use a hand-operated vacuum pump.
If the EGR valve can hold a vacuum, it indicates that the valve is functioning properly. However, if the vacuum pressure drops off after a while, it suggests a faulty EGR valve that needs to be replaced.
14. Bad Battery
Battery is being charged by the alternator when engine is running. If you are starting your car after a very long time and temperature is cold. The chances are that battery has been drained out, and is unable to provide sufficient power.
In that case, on cold startup, you should check your battery voltage. The voltage across battery terminals should be between 13.5 and 14.2 while the engine is running at idle and the lights are off.
In summary, there are many potential causes for a car shaking when cold but running smoothly once warmed up. The most common culprits include worn engine mounts, cold thick oil, failing hydraulic lifters, bad sensors, ignition problems, restricted fuel or air flow, vacuum leaks, and transmission fluid issues.
While symptoms may disappear when hot, the underlying causes still need to be addressed to prevent progressive damage over time. Proper maintenance and replacing worn parts is key, along with using good quality fluids.
Diagnosing the specific problem through inspections, tests and scans will reveal the needed repairs for smooth cold starts.