Need help with M20 idle speed

[LuckyHenriksen]

So I have been holding off on making this thread hoping I could find the answer in the vast database that is r3v, but I am going to break down and make a thread about it.

Car specs: 1992 325i convertible. Auto trans.

Symptoms: Idles around 1500 in Park or Neutral, drops down to around 700 in any of the gears. Also idles rough in gear, sometimes sounding like it will stall. I can drive the car around, but it has horrible throttle response. The car runs this way hot or cold, doesn't matter. And doesn't changed even after I ran it hard up and down the road for several minutes

Things I have tried so far: New plugs, seafoamed motor and changed oil/filter, new fuel filter, several gallons of fresh fuel and injector cleaner. I also cleaned the ICV and throttle body, changed the throttle sensor plug out, and have tried switching AFM's

What else should I be changing or checking? I am lost now, hopefully someone can set me straight. Also, it is a bit challenging using the Bentley as it doesn't cover anything later than a '89 or '90

Thanks

[SoEm0]

Have you tried adjusting the TPS? I have very similar issues sometimes and tomorrow I will be hooking up the meter to the TPS and checking if its right. The procedure is covered in the Bentley.

[LuckyHenriksen]

TPS is the throttle position sensor right? The little flat plug under the throttle body? I changed that out with one out of a good running car, didn't have any affect on the idle. Is it possible that the rocker shaft plugs could be letting enough vacuum by to cause it? I can't find any other vac leaks anywhere

[jlevie]

The first thing I'd do would be to have a smoke test run to check for intake leaks.

[Brandon12V]

check vacuum leaks, resistance test on ICV, check charcoal canister hose for cracks, check valve eccentric lash, cap & rotor, check fuel pressure, are you chipped?

[LuckyHenriksen]

Not chipped. Tried swapping out fuel regulators last night, also tested the engine temp sensor. Nothing changed. I can pull the vac hose off the FPR causing a huge vac leak and the idle speed doesn't change. And if I pull the oil cap off the VC it smooth's the idle way down

[PNWDan]

I think the fact that the idle speed drops to normal when its in gear points to electrical. I would think that if it were a straight vacuum leak it would still idle high and try and move while in gear. The TPS or somewhere on the idle circuit would be my first suspect. Since you swapped it out with another maybe it is an issue along that circuit. I cant recall how the automatic one works but I think the ICV and TPS are tied to the transmission so it can compensate the idle for when you put in into gear. My E34 540i use to do wierd things like that too except when i turned the headlights on the idle would even out.

[jlevie]

The cause could be electrical, but the rough idle and poor throttle response suggest other things are going on. My first action would be a smoke test and repair of the leaks it finds. After that I'd install a new O2 sensor if the one on the car has close to or more than 100k, adjust the valves, test the fule system, and test the TPS to see if the idle switch is operating.

[Brandon12V]

If the idle starts to smooth out when you take off the oil cap then I would also check your oil pressure. and as said, check the TPS. I have a feeling there's probably some blow-by in your throttle body. pull your plugs back out and check them. the motor is probably running rich to compensate. are you getting knock under load or at said idle?

[LuckyHenriksen]

The plugs are very wet, and I am getting blow by into the TB. Someone suggested that my rocker shaft plugs could be bad, causing the vacuum leak? If anyone knows how to find/replace these plugs that would be great. I can't find anything about their location by searching

[jlevie]

The rocker arm plugs are available from your dealer or most any of the on-line parts sources. They should be replaced at each valve adjust (along with the gasket), which should be done every 10k.

[rodpaine]

The plugs are very wet, and I am getting blow by into the TB. Someone suggested that my rocker shaft plugs could be bad, causing the vacuum leak? If anyone knows how to find/replace these plugs that would be great. I can't find anything about their location by searching

-Rod

[Brandon12V]

there are two at the back of the head as well(4 total). just order a valve gasket kit from bav auto. their valve cover is much nicer than the victor reinz. it's made of rubber fills any imperfections with better results. it used to come with rocker shaft plugs. if it doesn't, they will reccomend them when you buy it. make sure you check your valve lash while you're in there.

[LuckyHenriksen]

Cool, can the plugs be replaced from the top of the cylinder head without taking anything else apart?

And thank you everyone for your help so far

[Brandon12V]

yes. aside from the vavle cover and manifold bracket, and oil hose, nothing has to be removed. I doubt this is your only issue, but it's worth doing since it's a cheap maintenance job.