Why does my car shake when idling?
Updated 31 Oct 2019
At idle, your car should feel smooth, with little more than the sound of your stereo reaching your ears.
But problems can happen, like that annoying vibration you’re feeling in the steering wheel.
Since you’re sitting still right now, it can’t be the suspension or imbalanced tyres.
It has to be something with the powertrain.
If your car is shaking at idle, there are some common concerns that could be sneaking up on you.
Check out these six reasons that might need to be addressed, and how you should deal with them.
Fouled Spark Plug
Unless you have a diesel-fueled car, it has spark plugs that ignite the air-fuel mixture that sprays into the combustion chamber.
Spark plugs need to be replaced every 100,000 to 160,000km or so, depending on your vehicle, so it could just be time for maintenance.
It could also be fouled by oil or engine coolant, a sign that your car has bigger problems – perhaps a leaking cylinder head gasket or worn piston rings.
If it’s just replacing spark plugs, a DIYer with common mechanic’s tools can usually do the job.
But if the Check Engine light is on or there’s more to the problem than just changing spark plugs, it could be a large repair and an expensive fix.
Seek the help of an expert mechanic.
A knock sensor identifies vibration in your engine and alters the timing to improve how your car feels, at idle especially.
If the knock sensor isn’t correctly sensing vibrations, you’re likely to hear a loud ‘tap-tap-tap’ from your engine at idle, called engine knock. Now the name makes sense…
For some cars, to change the knock sensor is easy as.
It’s usually a single wire and requires only a ratchet and socket to change.
But for some cars, the knock sensor is tucked away under other parts and takes three hours or more to replace. The labour charges can rack up fast.
Variable Valve Timing
Like the knock sensor, a variable valve timing switch could shake your car at idle like a carnival ride.
It serves very much the same purpose – to adjust timing for smooth engine operation.
But if the VVT switch isn’t functioning, it can affect your car’s performance too.
It’s a tough job to do well and must be precise or you’ll do more damage than good.
It’s not a very costly repair but you should leave it to the pros.
Worn Motor Mounts
You don’t see it as often on modern cars as you did a decade or two ago.
Rubber insulators on motor mounts would deteriorate, letting the engine move around more than it should in the engine bay.
Left unrepaired, the twisting action can damage other parts like the exhaust, so it’s best fixed soon.
Parts are rather inexpensive for most models, and they aren’t overly time-intensive to change.
However, you’ll need equipment to support the engine, so maybe a mechanic is better suited to the job.
Dirty Fuel Injection System
Your engine won’t run well if it’s starved of fuel . . . seems like common sense, doesn’t it?
Impurities in fuel build up on fuel injectors, causing them to spray unevenly and needing more fuel to make the engine run smoothly.
Using a fuel injector cleaner occasionally when you fill your tank is helpful, but the best cleaning is with specialised chemicals, and done by a mechanic once per year or so.
Stretched Timing Belt
A rough idle could also be attributed to another serious condition – a stretched or skipped timing belt.
It keeps the camshaft and crankshaft synced together, but if it’s not timed correctly, the engine won’t run well.
And if the timing belt breaks, your car is dead in the water.
It’s a very detail-oriented job, and most home handymen aren’t equipped to deal with it.
To replace the timing belt is several hundred dollars and is definitely a job for your favourite mechanic.
If you’re sick of your shaking car, AutoGuru is here to make it easy to book a high-quality local mechanic for an inspection.
Jason is a Canadian automotive content writer with a background in the auto service industry, but he’s been hooked on cars and mechanics since childhood.
One of his first cars was an ’80 Mazda RX-7 that’s sorely missed to this day. A ’68 Ford Torino GT, a ’66 Ford Country Squire Woodie station wagon, and a ’96 Suzuki GSX-R 750 have spent time in his fleet of cars, bikes, and trucks over the past two decades.
Jason’s pride and joy is under construction – a turbocharged ’88 Mazda RX-7 convertible. Also on his resume is CASCAR official certification.