M42 hunting idle!

Post Tue Jun 10, 2014 7:14 pm

My m42 has decided this week to start being a pain in the arse and has now started hunting for idle.

It will sit at 2k rpm and then drop to 1.5k rpm or just under and then back up to high idle. Not good! Very intermittent and happens when the car is cold and warm, earlier when I first left work it was happening then after a 10 minute run down a dual carriage way it was idling fine at 1k rpm.

Also using some coolant/water.

So far I've cleaned the ICV and throttle body, done the carb cleaner air leak check.. Both had no results.

Any ideas? Thermostat? Temp sensor? Worse?

Post Tue Jun 10, 2014 9:09 pm

1K is quite a high idle speed for an M42. Are both butterflies fully shut when your foot is off the accelerator?

Idle speed is controlled by the ECU and isnt adjustable.

Post Tue Jun 10, 2014 9:36 pm

I'll have to check that when I catch it running well.

I'd be happy with 1k rpm compared to the 2k idle and up and down revs haha!

Post Wed Jun 11, 2014 3:43 pm

Happened again to and from work today, it's making it quite difficult to drive in traffic, but weirdly after the stint down the dual carriageway on the way home it stopped doing it again.

Any suggestions what to try next?

Post Wed Jun 11, 2014 4:01 pm

I've found a slight leak in the radiator, could this cause the idle issue?

Post Wed Jun 11, 2014 4:48 pm

Doubtful. Try disconnecting the battery or ECU for an hour or so to reset it, then let it idle for at least 10 minutes to enable base idle settings to be learnt, switch off, then take it for a drive throughout the rev range.

Post Wed Jun 11, 2014 9:14 pm

If the butterflies are not fully shut the ECU doesn't know the car is at idle so you cannot begin to cure the problem.

Post Thu Jun 12, 2014 2:31 pm

Thanks for the replies.

Speedtouch ill try that later when i get home from work and ill look at the butterflies.

Post Thu Jun 12, 2014 5:03 pm

I can confirm that the butterflies are fully closed, I tried speed touches ecu reset by disconnecting the battery with no joy.

I think it's the ICV, I tested it with a multimeter and it's measuring 8ohms, aren't they supposed to be 40?

Post Thu Jun 12, 2014 8:02 pm

It's only a 2 pin ICV, I'll have to check in the morning in the O2 sensor but I believe I do so could be that.

How do you check that the throttle position sensors set right/working?

I will re-check for air leaks but I've tried twice already and did wiggle the pipes both times.

Thanks for the help.

Post Thu Jun 12, 2014 9:30 pm

Sounds like an air leak could be the culprit.

Is the bung over the adjuster screw still in the AFM or has someone removed it?

Post Fri Jun 13, 2014 1:49 pm

My mate found a spare ICV in his garage, swapped that over and seems to have sorted it ! I'll take it for a drive later to double check

Post Fri Jun 13, 2014 11:45 pm

Definitely was the Icv, it's running like a dream now.

Thanks for the help!

Post Sat Jun 14, 2014 12:42 pm

Good to hear that, Dave.