Intermittant overrevving - suggestions?

[Richard Deakin]

Hi
4l 1999 Chim.

Intermittantly when I have reversed out the garage and then drive off (having idled on the drive at at 1,000prm to warm up for a few mins), the idle revs increase to 2,200 odd and only drop when in motion to about 1,200 where she drives even with no pressure on the loud pedal. Dab the brakes and back upto 2,200. After a few mins she goes back to normal, but only after I've annoyed the neighbours.

I've checked the throttle linkage next to the plenum for possible sticking, but it is well greased and moves quite happily.

Anything obvious I'm missing (apart from the Chims genetic disposition to going faster?).

Stepper motor has been cleaned - also spare stepper swapped in - no different.

Thanks
Richard

>>> Edited by Richard Deakin on Tuesday 10th February 19:37

[v8 racing]

you need to adjust the base idle

[Richard Deakin]

v8 racing said:

you need to adjust the base idle

Err, OK...but it's intermittant - say 1 in 3 starts. Normal idle is within specified parameters.

[poorpeet]

Duff throttle position sensor (Potentiometer?) could be causing the problem. Not too sure how to test it though. I'm sure som one on here will know

[GreenV8S]

The idle seems to go mad when the ECU is struggling with a faulty sensor. A faulty throttle pot really seems to mess things up. A faulty road speed sensor can upset this too. Or it could be an intermittant fault with the stepper motor (dirty, sticking, or a wiring fault) or an air leak anywhere around the inlet manifold (stepper motor itself would be a candidate if it has been disturbed recently).

[v8 racing]

Sorry maybe i did not explain properly i was shooting of out, if the base idle is not quite low enough or close to its limit it will cause the engine to rev much higher generaly the symptoms will be if your driving up to a stop the engine will stay revving at approx 2/2500 rpm after 15 secs it will start dropping at 500 rpm intervals if it is doing this?? for the time it takes to set the base idle it is worth trying??

[Richard Deakin]

Thanks v8racing

After a bit more research on base idle I can see where you're coming from. I will today be mostly checking the throttle butterfly for sticking, reseating the stepper wth PTFE in case that causing an air leak, and if that doesn't work, generally scratching my head!

I'll let you know how I get on.

Cheers
Rich