You hope in your car, turn the key, and the engine shudders to life. But instead of a smooth idle, the car shakes noticeably for several seconds before stabilizing. This car shaking when starting engine can be alarming. What causes it and should you be concerned?
A vibrating engine on startup has several possible culprits. In most cases, it’s completely normal and not a major issue. But in some situations, shaking could indicate problems needing repair.
Read on to learn what makes your car shake when starting, what’s normal vs. abnormal, and how to diagnose and fix the most common causes.
Common Causes of a Car Shaking When Starting Engine
An engine may shake, vibrate, or run rough when first started for various reasons. Here are some of the most common explanations:
Cold Engine Idle
Almost all cars run rougher and vibrate more when first started cold. When cold, the pistons and cylinder walls contract, and clearances are tighter. Combine this with thicker, sluggish oil flow and high RPMs needed for cold startup, and significant vibration and instability result until the engine reaches operating temperature. This exaggerated idle shaking is most noticeable on cooler mornings after the car has sat overnight.
Low Oil Pressure
Oil pressure takes a second to build on startup before being distributed throughout the engine. Low oil levels can accentuate the Car Shaking When Starting Engine, allowing more vibration until pressure ramps up. On initial startup, oil pressure takes a moment to build in the system and make its way through all the passageways to lubricate vital components. If the oil level is low, it takes longer to generate sufficient pressure. This brief lack of lubrication allows more metal-on-metal vibration until oil coats all the contact surfaces and pressure rises.
Weak Spark
Weak ignition coil(s), bad plugs or wires, or faulty ignition timing can cause engine misfires on startup. Weak or faulty ignition system components like the coil, plugs, and wires can misfire one or more cylinders when cold. When the engine is already contracting and prone to vibration when cold, these misfires compound the shaking effect. As things expand and warm up, the ignition improves, smoothing things out.
Failing Motor Mounts
Bad motor mounts allow more engine movement and vibration to transfer through the chassis when starting. The shaking gradually smooths out as RPMs stabilize. Over time, the rubber in motor mounts degrades and hardens. This prevents them from adequately absorbing engine vibration, especially during cold startups. Bad mounts transfer more shaking motion directly to the chassis and body, resulting in a rougher, more noticeable startup vibration.
Battery and Alternator Issues
If the battery is weak or the alternator is failing, the system can’t deliver consistent electrical power when starting. This creates uneven firing and shaking that may persist for a few seconds after the engine starts. Weak battery charge and lower startup voltage can cause ignition and fuel system components variability. Combined with a failing alternator unable to properly respond and charge the system when the engine first turns over, this can contribute to uneven firing, misfires, and a shaky startup.
Fuel Delivery Problems
Issues like a clogged filter, failing fuel pump, or malfunctioning injectors can cause the engine to stumble from weak fuel delivery when first started. Problems like a clogged fuel filter, failing pump, or dirty injectors cause inconsistencies in the air/fuel mixture when starting up cold. This leads to cylinders misfiring and exaggerates vibrations before the fuel delivery smooths out and the engine warms up.
So in most cases, the startup vibrations are just temporary effects of a cold engine with normal variability in ignition, fuel delivery, oil pressure, and electrics until systems stabilize. But constant shaking or violent vibrations likely indicate a problem.
When is Startup Shaking Not Normal?
While most cars exhibit some engine vibration and instability when first started, take note if the shaking seems excessive or persists. Here are some indicators of abnormal vibration or Car Shaking When Starting Engine:
- Entire car rocks back and forth violently
- Startup vibration lasts more than 10-15 seconds
- RPMs fluctuate and the engine struggles to stay running
- Check engine light illuminated
- Burning smells coming from the engine bay
- Loud rattling or knocking sounds from the engine
- Oil light flickers on/off
- RPMs surge up and down randomly
These symptoms typically point to issues like:
- Multiple cylinder misfires from faulty ignition components
- Misfiring from low fuel pressure or injector problems
- Damaged motor mounts unable to isolate engine movement
- Bent or broken fan blade striking shroud
- Failing main bearings allowing crankshaft to play
- Thrown rod from low oil or bearing failure
Any shaking that seems excessive, lasts longer than 15 seconds or is accompanied by other problems likely indicates that something needs repair or investigation. Don’t ignore persistent unusual vibrations.
What Causes Shaking Only When Starting Cold?
An engine will often seem fine starting up once warmed, but shakes and vibrates noticeably on initial cold startup. Some common reasons this happens:
Cold Engine Idle
Engines inherently run rougher when cold. The pistons and cylinders contract overnight which reduces clearances. Combine this with high cold idle speeds and you get much more vibration.
Oil Viscosity
Thicker, Cold oil flows slower and offers less cushion between metal parts. This allows more vibration until the oil thins and distributes as the engine warms up.
Weak Spark
On a cold start, the ignition system has to fire the plugs without the benefit of heat and expansion. Misfires are more likely until they reach operating temperature.
Cold Fuel Condensation
Moisture in the fuel can collect and pool overnight leading to erratic cold running for the first few minutes.
Cold Battery
A weak cold battery cranks the engine slower and delivers lower voltage on startup. This can cause uneven firing and shaking until the charging system responds.
So while cold startup shaking is annoying, it’s often just a characteristic of cold engine operation and not indicative of a serious problem.
What Makes a Car Shaking When Starting Engine Warm?
On the other hand, consistent shaking and vibration even after the engine has warmed up typically signals a mechanical or maintenance issue. Some possibilities include:
- Unstable idle from dirty fuel injectors or intake
- Misfiring cylinders due to bad ignition coil(s) or spark plugs
- Clogged fuel filter causing insufficient fuel delivery
- Vacuum leak drawing in extra air and disrupting airflow
- Faulty idle air control valve unable to regulate idle smoothly
- Engine too lean or rich from a failed oxygen sensor
- Bad motor/transmission mounts failing to dampen engine movement
- Bent or imbalanced driveshaft allowing vibration into the cabin
If the vibration persists when warm, don’t ignore it. Have the engine inspected and repaired to avoid further damage.
How to Diagnose the Cause of Engine Shaking
Pinpointing what makes your Car Shaking When Starting Engine requires a step-by-step diagnostic process:
1. Visual Inspection
Look under the hood for any obvious issues like damaged mounts, loose parts interfering with the fan, frayed belts, etc.
2. Feel Engine Movement
Have an assistant start the car while you feel for excessive engine rocking that could indicate mount failure.
3. Check Dashboard Lights
Note check engine or other warning lights that could point to ignition, fuel, or emissions-related problems.
4. Measure Startup Time
Use a stopwatch to time how long the vibration lasts after starting. Over 15 seconds likely indicates an issue.
5. Monitor RPM Behavior
Notice if RPMs surge randomly, take too long to settle, or struggle to stabilize during startup.
6. Test Drive
Drive the vehicle and monitor if shaking lessens or worsens during acceleration. Listen for any abnormal sounds.
7. Scan Computer Codes
Use an OBD-II scanner tool to check for any diagnostic trouble codes stored in the computer that could help identify faulty components.
8. Inspect Ignition System
Remove and test spark plugs, wires, and ignition coil(s) for faults that could cause misfiring and vibration.
9. Check Fuel Pressure
A weak fuel pump can cause startup surging and vibration. Confirm fuel pressure is within spec.
10. Look for Vacuum Leaks
Listen and inspect all hoses, gaskets, PCV valves, etc. that could potentially leak in air and disrupt engine airflow.
Following this methodical diagnostic routine can help narrow down the root cause of vibration and direct you to the necessary repairs.
How to Fix a Shaking Engine on Startup
Once you’ve diagnosed the specific cause, here are some typical repairs to stop the car from Shaking When Starting Engine:
- Replace faulty ignition parts like the coil, plugs, or spark plug wires
- Clean or replace dirty fuel injectors disrupting fuel distribution
- Fix vacuum leaks allowing unmetered air into the intake
- Replace damaged or worn-out motor mounts
- Repair engine issues causing misfires in multiple cylinders
- Clean the MAF sensor and throttle body causing incorrect airflow
- Install a new battery if the weak voltage is contributing to rough starts
- Drain old gas and add fresh if moisture contamination is suspected
- Swap out a failing fuel pump or clogged filter impairing fuel delivery
- Adjust engine timing if incorrect timing is determined to be the cause
- Clean carbon buildup in cylinders allowing uneven compression
Routine maintenance like new plugs, wires, inspecting hoses, etc. can smooth out a shaky cold startup. However significant ongoing shaking requires professional diagnosis to pinpoint and remedy the underlying problem.
Frequently Asked Questions
Why does my car shake when I start the car?
Common reasons for shaking on startup include normal cold engine vibration, weak battery/alternator, engine misfires from ignition or fuel system faults, failing motor mounts, or vacuum leaks.
Why does my car shake when I start it in the morning?
The most likely culprit is normal cold idle shaking that’s exaggerated on cooler morning starts before the engine fully warms up and smooths out.
Why does my car shake when I start it cold?
Again, all engines vibrate more when cold. The tighter clearances, cold oil, uneven fuel distribution, and high RPMs contribute to more vibration that lessens as the operating temperature is reached.
Can a bad starter cause your car to shake?
It’s unlikely the starter itself would cause shaking. However, a weak battery being strained by a failing starter can produce lower voltage at startup, contributing to unstable engine operation.
How do I fix my car shaking when idle?
For idle shaking, check ignition components, engine mounts, vacuum leaks, clean fuel injectors, adjust idle speed, inspect airflow sensors, and clear any DTC trouble codes with a diagnostic scanner.
Is it OK to drive a car while shaking?
You should avoid driving a car with persistent shaking or extreme vibration that lasts more than 10-15 seconds after starting. Prolonged shaking can worsen existing damage.
Can spark plugs cause the car to shake?
Yes, faulty or worn-out spark plugs can misfire in one or more cylinders and cause noticeable shaking, especially at idle on startup. Replacing plugs typically smooth things out.
Why does my car shake and jerk at idle?
Similar to startup shaking, this is often caused by ignition problems, motor mounts, fuel delivery issues, intake leaks, dirty throttle bodies, failing sensors, or computer trouble codes affecting idle speed.
So in summary, monitor your car’s behavior closely during startups. Some initial vibration is normal, but consistent, violent shaking or stalling likely indicates that repairs are needed. Consult a certified mechanic for any persistent engine vibrations.
Conclusion
It’s very common for car engines to exhibit some instability and vibration for the first 5-10 seconds after startup, especially when cold. This shaking is generally harmless and subsides as the engine warms up and systems stabilize. However excessive, prolonged shaking or vibration after startup can signal problems needing diagnosis and repair.
Monitor your engine closely during startups and look for visual issues, abnormal noises, dashboard warning lights, trouble codes, misfires, performance changes, and other clues pointing to maintenance needs or faulty components. Addressing any underlying issues promptly will maximize engine health and minimize the annoying vibration during startups. For more information related to Car Shaking When Starting Engine check this article Car Shakes When Starting Then Runs Fine: Reasons and Solutions
