My engine idles rough and the RPMs jump around
A rough idle means one or more cylinders are not firing smoothly, or the engine is getting the wrong mix of air and fuel. Worn spark plugs, a bad coil, a cracked vacuum hose, and a dirty throttle body are the usual suspects. Left alone, it can get worse and damage the catalytic converter.
Urgency
This week is fine
What to do right now
What to do right now
- 1Watch the check engine light. If it is flashing, the engine is misfiring badly. Slow down and get it in today.
- 2With the engine running, listen for a hiss under the hood. That can be a vacuum leak.
- 3Think back to when your plugs were last changed. If you do not know, they are probably due.
What it usually is
What it usually is
Tune-up and spark plugs
New plugs and, if needed, coils are the most common fix for a rough idle on a car with some miles on it.
See the serviceCheck engine light
Misfire codes tell us which cylinder is weak, so we replace the right part and not all of them.
See the serviceCar diagnostics
If plugs and coils test fine, we smoke test for vacuum leaks and check the throttle body and fuel injectors.
See the serviceQuestions people ask
What is the difference between a rough idle and a car that shakes at idle?
A rough idle means the engine itself runs uneven, so the RPM needle bounces. A car can also shake at a steady idle because of a worn engine mount. We check for both.
Can bad gas cause a rough idle?
Yes, but it is less common than people think. If it started right after a fill up, tell us where and when. Most of the time it is plugs, coils, or a leak.
How often should spark plugs be changed?
It depends on the car. Many newer cars use long life plugs, while older ones need them sooner. Check your owner's manual or ask us to look it up. Heavy traffic and rideshare miles wear them faster.
Will a rough idle hurt my engine?
A mild one mostly hurts gas mileage. A strong misfire sends raw fuel into the exhaust and can ruin the catalytic converter, which is a much bigger repair.