Engine misfiring or rough idle: causes and diagnosis
By Altay Hasanli, auto electrician
Short answer
Shaking and loss of power usually mean one cylinder isn't firing properly: ignition (plug, coil), an injector, a vacuum leak or a sensor. A scan tool shows which cylinder is misfiring, and an oscilloscope tells an electrical cause from a mechanical one without taking the engine apart.
The most common causes
- 1
Ignition system
Worn plugs, a cracked coil or a bad lead weaken the spark. The shaking is usually worse on a cold engine and under load.
- 2
Injectors
A dirty or leaking injector doesn't deliver fuel to the cylinder properly. Shaking at idle, higher consumption and hard starting are typical.
Fix: Injector cleaning
- 3
Vacuum leaks and sensors
An intake leak, a dirty throttle body or a faulty MAF or MAP sensor upsets the mixture, and the idle hunts up and down.
Fix: Computer diagnostics
- 4
Mechanical causes
Low compression, valve problems or worn engine mounts also cause shaking. Compression can be compared with an oscilloscope without stripping the engine.
What you can check yourself
- Note when it shakes: on a cold engine, at idle or when accelerating.
- Check whether the Check Engine light is on or flashing.
- If you've recently changed fuel stations, mention it.
- If the shaking gets worse with the air conditioning on, tell us that too: it helps the diagnosis.
When to stop driving
- If Check Engine is flashing, go easy on the engine: unburnt fuel destroys the catalytic converter.
- If you smell petrol, there may be a fuel leak: don't use the car.
How we find the cause
- 1We read per-cylinder misfire counters with a scan tool.
- 2We check ignition and injector signals with an oscilloscope.
- 3We analyse fuel trims and look for vacuum leaks.
- 4We run a relative compression test without stripping the engine.
This guide is general information: one symptom can have several causes, and only a diagnostic shows the exact one.
Services and prices
Computer diagnostics
from 20 AZNCar diagnostics: fault codes and sensors
Oscilloscope diagnostics
from 40 AZNAnalysing sensor, ignition and injector signals
Injector cleaning
from 40 AZNCleaning and bench testing of MPI and GDI injectors
Frequently asked questions
I replaced the spark plugs but it still shakes. Why?
The cause may be a coil, a lead, an injector or a vacuum leak. Finding it with diagnostics is cheaper than swapping parts on a guess.
Will cleaning the injectors help?
If they're dirty, yes. The test bench shows each injector's spray pattern and flow, so you can see the result before and after cleaning.
Why use an oscilloscope?
A scan tool only shows the fault code; an oscilloscope shows the signal itself. That's how intermittent faults with no code get found.
Other problems
- Car won't start
Starter won't crank, clicks, or the engine won't catch
- Battery keeps dying
The battery goes flat overnight or after a few days parked
- Check Engine light is on
The engine warning light is on or flashing
- Alternator not charging
Battery light on while driving, lights dimming
- Electrical faults
Lights, windows or dashboard not working, fuses blowing
- Fuel consumption is up
The tank empties faster than usual with the same driving
- Engine stalls by itself
The engine cuts out while driving, at a stop or right after starting