Surging Idle when Hot

[ecniv]

First post, and I am new to the world of BMWs, so please forgive the newbishness.

Car is an '87 325e with somewhere around 188k miles. It runs great, with the exception of a surging idle once the car gets up to temp. All vacuum hoses have been replaced, (couldn't find any leaks before they were replaced, but vacuum hose is cheap). I cleaned the ICV with carb cleaner, no luck there.

I did a search, and saw lots of things to check, but I am ignorant as to what all the acronyms mean, and the location of the parts. DME? AFM? Which coolant temp sensor should I check? If anyone can provide me some insight, I'd really appreciate it.

Thanks,
Vince

[LTDScott]

Just to add (I am Vince's friend), when the plug to the ICV is pulled, the idle speed does speed up, but still surges. You can actually feel the surge somewhat while driving too. The economy gauge on the dash board swings like a pendulum when the engine is doing this.

When spraying carb cleaner near the vicinity of the thermostat housing, the idle actually calms down. I thought this to be odd since normally spraying carb cleaner near a vacuum leak will cause the engine speed to increase, not decrease.

Any ideas?

[Goge]

Is it like clockwork, I mean does it zoom up and down like 700-1500 on a regular cycle?

I had a bad idle control computer (not sure if the '87 has one separate from the main computer or not). The problem was a solder joint on the circuit board had cracked, and when it got warm the connection would take a turn for the worse. The idle would rack up and down constantly when you took your foot off the gas. It would cycle like every two seconds or so.

The connection in question was incoming power to the circuit board, so I guess when it got hot the idle computer was just not even alive.

-Todd

[LTDScott]

Yes, the surging idle is cyclical as you described once the engine has warmed up.

This weekend is half off at the junkyard and we're picking up spares of just about everything mechanical. I'll make sure the idle control unit is on the list.

[Goge]

Do you know where the idle computer is? If not, it's above the glovebox, to the left of the big computer. Held on by one big bolt. Not too hard to get to. Evidently the 'best' one to have (so I've read) has a dark green cover on it. Mine was dark green, and I have an '86 eta.

I don't think cars with Motronic 1.1 have these, the idle control is built into the main computer with 1.1.

Did eta's ever come with 1.1? I dunno.

-TH

[scabzzzz]

Super Etas came with the upgraded motronic. Did you check the intake elbow for leaks? It cracks underneath.

Sounds like to me theres a vacuum leak. I dont know for sure if the 87 E cars came with that idle control module, but that doesnt sound like its the problem.

[LTDScott]

The intake boot is in good shape. I have inspected all the lines and boots and all appear to be good. Have replaced most of the smaller vacuum lines. Will replace the injector O-ring seals for cheap insurance.

I work for a parts supplier and according to our catalog, the car does have a separate idle control unit:



The dashboard of our car is completely gone (gutted race car), so finding the idle control unit next to the main computer shouldn't be hard :)

[ecniv]

Here's a pic of the car with the entire dash removed. I didn't see anything like the pic that you posted, Scott. I definitely looked. That is what the Bentley manual pointed me toward after checking the ICV.

[Goge]

Did you look under here? Should be to the left of the main computer...

[ecniv]

Yep. I will look again on Friday when I get home. I'm going to feel like an idiot if it's staring me right in the face when I get there :)

Is it a part that is normally attached to the dashboard?

[LTDScott]

*shrug* dunno. Others seem to say it's right there.

[kishg]

did you test the o2 sensor?

[LTDScott]

did you test the o2 sensor?

Nope, but I really don't see that causing a rhythmic surging idle.

[ecniv]

Unplugging the O2 sensor has no effect on how the car runs.

[Goge]

Hey, maybe the idle computer is gone... that would explain some things. Is there a multi-pin plug hanging loose down there, like a smaller version of the big computer's? I think it has about 10 pins on the connector. Actually I guess the connector would have the female ends. But you get my drift.

-TH