M42 Idle Fluctuations - WTF

[hamburglars]

Hi there, so I have had idle problems with my car since I got it, and recently went through the "mess under the intake" job and put everything back together nicely.



You can see my ICV plumbing above.

• I have made absolutely certain there are no leaks here at all, hose clamping every junction I thought might possibly need it.
  
• Sprayed the whole area with carby cleaner to see if the RPMs climbed and nothing.
  
• I've made sure there are no leaks anywhere on the intake boot or any other vacuum hoses that I can see.
  
• I recently removed the ICV and cleaned the hell out of it with carby/TB cleaner and a toothbrush.

But the idle still hunts all over the place, usually between ~600 and ~900. It feels like it's almost dying at points and the whole car shakes. Sometimes I will start the car and it will be fine (no hunting) and other times I will go for a drive and it will sort itself out, other times it will continue to hunt like crazy even when the car is nice and warm. The car also seems to run more smoothly and seems to have more power when the idle is not hunting, though this may be some kind of placebo effect.

If I disconnect the plug to the ICV the idle stabilises at ~1000rpm (running rich as hell I assume) but as soon as I plug it back in it goes back to hunting. For this reason I am sure that my idle problems are something to do with the ICV, and not due to a leak/malfunction elsewhere.

Do I need a new ICV or is there something massive I've missed? I've searched through these forums and over on M42club but haven't found anything that has helped yet.

Thanks heaps in advance, any help at all is appreciated :D

[DER E30]

Did you make sure the little hose that goes from the front of the fuel rail to the underside of the throttle body is hooked up? It makes a decent vacuum leak when it isn't...

Other than that I can't help :( But it does sound like the ICV

[hamburglars]

Yep, checked there haha. Went for a drive this afternoon and it seems worse than ever. So frustrating. :(

[jrobie79]

theres a connector on the passenger side of the harness against the firewall...make sure its DISconnected

[hamburglars]

theres a connector on the passenger side of the harness against the firewall...make sure its DISconnected

Thanks for the response, could you elaborate? Which connector do you mean and why does it need to be disconnected? I've never heard this before

edit: also when you say passenger... My car is RHD, do you mean exhaust side?

[roguetoaster]

He is talking about that little single pin connector under the long plastic cover at the top of the engine bay/firewall on the exhaust side. It is designed to be connected only if no O2 sensor is present.

Still, I am not sure that is your issue.

[G FORCE]

Any codes stored?

[hamburglars]

He is talking about that little single pin connector under the long plastic cover at the top of the engine bay/firewall on the exhaust side. It is designed to be connected only if no O2 sensor is present.

Still, I am not sure that is your issue.

I will have to check this out, I'm not sure I've ever seen it haha.

Any codes stored?

I don't have a check-engine light so I can't do a stomp test, is there any other way of garnering this info?

I drove the car for most of the day today (WTAC if anyone else went), the idle was all over the place when I started the car in the morning but seemed to sort itself out after half an hour of driving. I left the car for about 5 hours then when I started it again the idle was perfect and the car ran great. It does kind of seem to be heat related.. Would a bad intake manifold gasket cause this kind of behaviour? Still super puzzled.

[Boris3]

Jrobie and Roguetoaster are talking about this but again doesn't seen to fit in your case




You mentioned idle problems before you did the mess under the intake. Did the problems change at all when you did the mess delete or are still the same? Did you check the temp sensor?

[hamburglars]

You mentioned idle problems before you did the mess under the intake. Did the problems change at all when you did the mess delete or are still the same? Did you check the temp sensor?

The problem is exactly the same as it was before the job, very frustrating.

I will test resistance across the temp sensor asap, I hadn't thought of this. Would a fault there cause the symptoms I'm experiencing?

Thanks for the help

[Balleristic31]

Did you get a new intake boot? If so, did you remember to plug the hole on the bottom side of the boot close to the throttle body? Most new boots don't come with a plug in there and it's a massive vac leak and easy to miss.

[hamburglars]

Did you get a new intake boot? If so, did you remember to plug the hole on the bottom side of the boot close to the throttle body? Most new boots don't come with a plug in there and it's a massive vac leak and easy to miss.

Original boot, the plug's there and everything else seems to be sealed tight.

As a bit of an update, the car now seems to run fine once it's warm every time now. Sometimes I still get a serious fluctuation when first starting and it's hard to stop the car from stalling, other times it idles perfectly from a cold start. I have no idea why this is the case but I am sick of trying to solve this issue and it's nowhere near as bad as it was, so I'm just going to leave it.

[G FORCE]

I still haven't found my idle hunting after hot start. I few hard revs usually takes care of it but it is annoying. Cold starts are just fine. I will update if I ever figure it out as well.

[uturn]

Mine used to idle funny when cold until I changed out the AFM with one from a compatible e36.