M50 Idle issues

[Cronopoulos]

Engine - 1994 M50B25 w/ vanos completely stock.

Problem -

Car starts and idles perfectly ~900rpms when cold. When the car warms up, the idle gradually increases until around 1500 rpms, by that time the engine is at operating temperature. The idle doesn't increase nor decrease from this point.

The idle does not bounce and the car runs perfectly even with the idle "sticking" high and no check engine light.

If I unplug the ICV, the car runs like poo. If i unplug the MAF sensor, the car runs like poo.

What gives? 02 sensor? Temp sensor? FPR?

[Cynical 1]

i'm having a similar issue. in for discussion.

[Cronopoulos]

i'm having a similar issue. in for discussion.

I am leaning towards a vacuum leak(s). The only air that is bypassed around the throttle body is through the IAC valve, i cleaned it and it rotates fine, also buzzes when plugged in during key on.

I am going to check the boots for cracks and go from there. Also when the car was put together, i had unplugged the MAF while it was running, the idle bounced but steadied out and seemed to idle lower, is that a sign that the MAF has gone bad?

I did go around the runners/boots/iac/tb areas with carb clean and noticed no change in RPM... The hunt continues.

** edit, with the IAC plugged in, the valve shouldn't be able to freely move? I assume this is a electromagnet which locks the valve in a certain position based on voltage from TPS?? If this is the case, ill find another ICV... would make sense seeing if that isn't working, its like one giant vacuum leak.

[dashboardmonkey]

Yes, start checking vac leaks and fix them if any.

[Cronopoulos]

Yes, start checking vac leaks and fix them if any.

Both the inlet and outlet boots for the IAC valve check out ok, the vacuum source for the FPR has been replaced already. The Air temp sensor has also been replaced.

I am narrowing it down to a failure in the IAC valve and/or the TPS. I would assume the IAC valve gets a reference signal from the TPS or something along those lines letting it know when its above/below 20% throttle.

Could also be the MAF, lol.

[butters]

I had this issue. ICV was stuck open, but getting a new one only reduced rpm and idle still rose rather than dropping as the engine warmed up. Replaced the engine coolant temperature sensor (sends to the ECU, not the cluster) and that solved it for me.

[Cronopoulos]

I had this issue. ICV was stuck open, but getting a new one only reduced rpm and idle still rose rather than dropping as the engine warmed up. Replaced the engine coolant temperature sensor (sends to the ECU, not the cluster) and that solved it for me.

I will give it a shot, appreciate the help! Thanks!

[Cynical 1]

my issue is solved by pinching off the hose from the iac motor to the intake boot - pinch it shut and idle drops to "normal". haven't had a chance to diagnose. bad iac?

otherwise, my idle is 1000 rpm at start up, and creeps to 13-1400 rpm when warm.

[Cronopoulos]

my issue is solved by pinching off the hose from the iac motor to the intake boot - pinch it shut and idle drops to "normal". haven't had a chance to diagnose. bad iac?

otherwise, my idle is 1000 rpm at start up, and creeps to 13-1400 rpm when warm.

If that is the case then yes i'd say the iac is toast. Does your idle still creep with it pinched?

[Cronopoulos]

So I replaced the IAC valve with a known working one along its boots. Also replaced both temp sensors and air intake temp sensor. The car starts up and idles PERFECTLY without the MAF plugged in. It doesnt skip a beat until you plug the MAF in and the idle jumps to 1200 and hangs, unplug the maf, it hunts for idle and then runs perfectly again.

02 sensor and or MAF..... fuuuuuu