Bad Idling Problem

My brother’s car had a bad idling problem. In neutral, the RPM would hunt around 600-800. It’ll rev to 800 then drop down to 600 and the cycle repeats ever second. In drive it’s even worse, going from 600-1200 and sometimes stalling. Revving the engine made it worse. The RPM would jump and after letting go of the pedal, it takes a few seconds before stabilizing at 600-800RPM. All the time the engine felt like it was going to stall.

The car hasn’t been started for a month because he was at Perth for that duration. The mechanic suspected that it was due to fouled plugs but couldn’t look at it till next week because his appointment was full. Since I had plenty of time to kill I decided to troubleshoot

Bad idling can be caused by the following:

  1. Bad spark plugs
  2. Bad idle control valve
  3. ECU malfunction
  4. Damaged spark plug cables
  5. Damaged ignition coils
  6. Damaged ECU wiring harness in the engine
  7. O2 sensor damage
  8. MAF sensor damage
  9. Vacuum leaks

I didn’t have a fault code reader so I had to go through each and every item on the list to isolate the problem.

MAF Sensor

The first thing I checked was the MAF (Mass Air Flow) sensor. A dirty or faulty MAF sensor could cause a slow response to the ECU, resulting in overcompensation by the ECU and the engine to idle badly.


This is the Bosch Hot Film MAF sensor. If you look into the tube you can see a thin film. This film is heated up to a certain constant temperature. As air flows through it, the film is cooled, therefore more power is required to keep the film at the predetermined temperature. The ECU then uses this information to calculate how much air is flowing through to the engine. There are two other types of MAF sensors, namely the hot wire type and the spring-loaded vane type.

I reasoned that if the film has been covered in dust, it would be effectively insulated from the flowing air and thus give out erroneous readings. Cleaned it with some sensor-safe carb cleaner. Installed it back. Same results. So it wasn’t the problem with the MAF sensor.

Spark Plugs

Fouled or mis-gapped spark plugs can also cause bad idling problems because the mis-gap will affect the ignition timing. I took out the spark plugs and also checked the condition of the wiring and coil packs but they were in good condition. The plugs were in exceptional condition, the ceramic portion showing perfect combustion (slightly brown deposits on the white ceramic). I didn’t take any photos of the plugs but the following diagram will help you in determining your plug condition:


So I had to rule out the ignition system. Wiring were all good. Engine harness was still in good condition.

Vacuum Leaks

In most cars, vacuum from the intake manifold is tapped for the usage of other systems. One common vacuum user is the master brake cylinder. The vacuum is used to assist the driver in applying brakes. That’s why when your engine is off, it’s very hard to push down on the brake pedal.

Since idling is somewhat affected by the airflow into the manifold, any vacuum leaks will introduce extra air into the engine, thus messing up the fuel scheduling by the ECU. The ECU thinks that there is that much air going in, but in fact there is more because of the vacuum leak.


The problem was caused by a disconnected vacuum line to ABS module (shown by the red arrow). I’m not sure why I didn’t notice that before. Could have saved me one afternoon.

The engine idled perfectly after reconnecting that vacuum line. No hunting at all, RPM stabilizes instantly once the gas pedal is released after revving the engine.

Sometimes the simplest of solution solves the problem. So the next time before you go check your plugs or idle control valve, check that all the vacuum lines are connected properly. Might just save you a lot of time troubleshooting.


And I love working on this car because it comes with it’s own set of tools!