Help! Bad idle, check engine light, 1990 325i!

[Tad]

Hi i've got a 1990 325i m20 motor, all stock.

I've got a really cruddy idle.
it keeps dropping really low then bouncing back up, over and over.

and as i'm driving around the check engine light will come on for a while then go back off.

Any clue what's going on?
I had a previous problem with the car dying out and not making power at all in higher rpms for a while. But I replaced some coolant sensors, and changed the oil and that seems to have fixed that.

it's only this current problem that persists..

[e30this]

ICV?!?! Take it out and clean it up really well, then put it back in.

[Tad]

I think I already did that actually!

[jlevie]

That could be caused by intake leaks.

[Adrian_Visser]

prolly intake leaks, check what code you're getting, if its an O2 sensor code its likely an intake leak.

[ob_318is]

If there was an 02 sensor code, say 1222...what are some possible solutions?

[Adrian_Visser]

see jlevie's post above.

[ob_318is]

yea.... just a little disbelief, don't think that's the case with my car, but will check it out..

[Adrian_Visser]

Why not? a vacuum leak from one of the numerous hose connections into the intake manifold boot or oil cap or dipstick o-ring or breather hose or injector o-rings yada yada would cause a leaner than normal a/f ratio rough idle or lower than normal idle, or on an early car with the spearate idle control unit the idle may rise and start hunting all of this can be caused by a vacuum leak. The DME senses the leaner mixture through the O2 sensor and falsely "thinks" the O2 sensor is out of spec and throws the 1222 lambda code.

finding the vac leaks isn't always very easy either you'll need to spray something flammable (ether or starting fluid works) onto all the connections and listen very closely for a change in idle.

[e30car]

Although rare the two times that this has happened to me it turned out to be the faulty ECU (I have owned the car for 14years and 180k miles)

[twright]

My lousy idle and check engine light were caused by a bad o rings at the oil return tube between the intake manifold and the block.

I could not detect a leak there before disassembling it by spraying it with Gumout during idle: the idle didn't change. The bottom o ring was definitely bad.

The only reason I suspected it was because it seemed to be seeping oil and I couldn't find a leak anywhere else.