Strange Idle Problem

[flumph]

So I've been working on my Eta. I discovered that the idle control module (the little green box next to the ECU/DME) was completely missing from my car, so I went to the scrap yard and pulled one, along with a throttle body and TPS that I haven't installed yet. The previous owner had substituted in a plumbing ball valve in series with the ICV.

So the ICV works and is getting power. About 6 volts at about 100 Hz, my multimeter says. Reads 10 ohms as well, which is in spec. Also cleaned it with carb cleaner. Throttle body plate is also spotless.

The idle was pretty bad before, but it was somewhat stable. Now it's doing this. Surging about once per second between 800 and 1500 rpm. I pulled the vac hose to the FPR and plugged it, and nothing outwardly obvious happened, so I doubt that's the problem unless I'm missing something.

If I disconnect the ICV, the idle rises dramatically (~3000).

Could it be the MAF track being dirty?

I don't know where to look next.

[MrBurgundy]

A surging idle is typically a vacuum leak.

Use that carb cleaner and check.

[flumph]

I just sprayed it anywhere I could think of. Brake booster vac hose, on and around the throttle body, PCV tube, ICV, and fuel rail. If there was a change, I couldn't see it. Also tried plugging the ICV in the other direction because the socket is broken off, but it didn't change anything (which I feel like it should have).

On cold start, it idles at a stable RPM, but when you crack the throttle even a little, that oscillating starts. Could the TPS be causing mischief? Maybe signaling an open throttle, going to a richer fuel map, increasing idle, and then the O2 sensor sees that it's running rich, which makes the ECU cut fuel...

The previous owner also removed the catalytic converter, and there's an audible exhaust leak. Could that be doing something to the O2 sensor?

I'm sorry if these are dumb questions, and feel free to tell me to RTFM.

Edit: I did go and bathe my engine bay in carb cleaner at your suggestion. Hadn't done that before.

[e30davie]

The e30 tps just has a throttle switch and a wide open throttle switch, nothing in between.

So it might be a case that when you crack the throttle and let it return the car isnt getting a "im idleing now" signal from the tps, and thus the idle control circuit and idle control valve isnt kicking in.

Best bet is to look up how to check your TPS and determine if the idle switch in the TPS is working as it should. You might need to adjust your throttle stop or the tps might be had it.

[flumph]

You were totally right!

I just swapped in the throttle body with the fresh sensor that I found at the junk yard, and it idles very nicely at 850 RPM. It wanders a little, but it's not nearly as bad as it was. Also, the brake booster vacuum hose literally just sheared off when I moved it out of the way to get to one of the throttle body nuts (which were also quite loose). Everything still passes the carb cleaner bath check, but I'm positive that I have a vacuum leak. Sometime soon, I'll be replacing that booster hose, because it's 33 years old and is almost certainly leaking.

I will leave that video up for anyone who finds this in the future. That's what it sounds like when your TPS goes bad on your eta.

Thanks a bunch guys. Super happy with how this is going.

[jlevie]

There is a good chance of intake leaks. The only reliable way to find them is with a smoke test, properly executed. That means plugging the exhaust. replacing the AFM wit a plug having a nipple for smoke injection, and pressurizing the intake system with smoke for about 10 minutes.