Random Revving

Hello, 

I have a Suzuki Jimny 1998, recently it is revving up to 1500 rpm and it some times drops the revs almost turning off but then it stabilizes immediately. I noticed it happening when it starts to warm up, because in the morning it does not do it but after driving for a bit it starts to rev, even whilst driving I go to change gears and when I press the clutch it stays revved until I put in gear and it is normal. Whilst in traffic it revs up, stabilizes and sometimes drops the revs and stabilizes again. I took it to my mechanic and he cleaned the throttle body and idle sensor and it still did it. 

Is there some one who maybe experienced the same issue or know what I need to do next.

Thank you in advance,

Cain Spiteri

Whilst I do not own a Jimny of that generation I do own another vehicle of that same year. Vehicles of that period use an 'idle control valve' to manage idle speed during warm up and under various load conditions rather than just a throttle body managed by the ECU as we have now. It is a physically separate valve from the throttle body

A common problem is that this valve often just needs a clean or its seals have developed an air leak. What you describe sounds just like a dirty (possibly faulty) idle control valve. The mechanic might have cleaned the throttle body but did he clean the idle control valve? If it was me I would remove it, clean and replace the seals.

Thank you. Yes he in fact took it all out and cleaned it we were actually surprised that it started to rev again. I will be taking it again to see maybe it needs to be replaced (Idle control Valve) but we'll see. After 28 years it is expected that I have to start replacing some come parts.

There's a little impeller type thing in the edge of the throttle body in a chamber that bypasses the butterfly plate in the air stream that drives a hall effect sensor. They get sticky and don't run freely which confuses the ecu.

I don’t know if this engine has a MAF sensor, but a dirty sensor or one on the way out can cause ‘hunting’ at tick over.

My first guess would be throttle body (as Lambert suggested) but if that fails to fix the problem then it might be a MAF sensor, if it has one.

Does you car have AC? When AC is engaged it lifts the revs at tick over to overcome the increased load. If the AC is getting triggered, but no AC in use that might do it.