DefinitelyMiami

Hey Guys (and Gals),
I bought a pristine '90 C4 a couple of weeks ago with 80K on it. I've been driving it daily without issue. Friday night, I parked the car as normal. Saturday morning, it wouldn't start. I checked the fuel pump, and it was working. I had compression as well. I did some digging, and decided to replace the ICM and the rotor. After that, the car started- but wouldn't idle! I feathered the accelerator for a bit, and when the car heated up to around 190 degrees, I could take my foot off the pedal and the car would maintain an even idle on its own. If I shut it off, I could start it with no problem, and it would idle on its own.

This morning I went back out to drive the car to work (it was cold out), and again, it wouldn't maintain an idle on its own. I warmed the car up, and drove it to a repair shop. They haven't been able to find out what's causing this? Do you guys have any thoughts? This is my first Vette, and I'm looking at it like a learning experience- or, as my one car buddy said it, "Daily driving a C4 is considered an extreme sport by most people."

ANY help would be appreciated. My thought is maybe it's the throttle position sensor...?

Thanks in advance.

I'm starting to wonder if it's not the Coolant Temp Sensor? I noticed my second fan does not turn on, even with the A/C on. Could that be sending faulty signals to the ECM and causing issues with ignition, idle, and that second fan? I bought the FSM but it hasn't arrived yet.

Is this the culprit? It leads back to what I think is either the MAF Burnoff Relay or the MAF Power Relay, and I bet it was connected to that terminal.

Whaleman

That looks like an aftermarket connection. Not factory. Dan

DefinitelyMiami

Thanks!

DefinitelyMiami

Update:

Now the car starts and idles with no problem

JasBass

Congrats on the purchase and welcome to C4 life! Also, consider the MAT in the future (manifold air temp sensor). It is in the back of the plenum, pretty close to your FPR. I found it worthwhile to get an OBD I connector and software that can scan these cars. Pulling codes is pretty easy, but it is always nice to verify what the sensors are sending

DefinitelyMiami

Aaaaaaaannnnnddd this morning it started doing the same thing

bosshog8

There is no MAF on a 90. You have a MAP sensor. I would suspect the IAC (idle air controller). Typically a cold engine needs different idle air than a hot engine. The IAC might be stuck or sticking in the hot position or gummed up with carbon. You can remove it and clean it (which is free). You will not get a code because if it's working electrically but stuck mechanically the computer can't see that.

DefinitelyMiami

JasBass,

Ran diag and the car isn't throwing any codes (C12, normal operation). The MAT would throw a code, no?

DefinitelyMiami

Makes sense; I ran the diag today and the car didn't throw any codes.

aklim

Depends. If it is outside the plausible range, it would. If not, very hard to say. As an example, if the sensor reports it is 80 degrees when it is 50, it probably won't throw a code. OTOH, if the temp reported is 500, sure.

To do this, we need to scan the data and see what it is being told the conditions are.

If you want to clean the TB and hope that is the issue, get the gaskets. Take the TB off and probably do the coolant bypass so your next time will be neater. Remove the top plate, IAC and the IAC housing. Clean all passages with brake cleaner. GENTLE spray of brake cleaner on the IAC pintle and wipe it clean GENTLY. You will need a new set of gaskets when you reinstall. If that doesn't work, it won't hurt.

After this, you need a scanner to see what data the sensors are reporting. For coolant and air sensors, you need an infrared thermometer. So if the ECM is told that the air temp is 88, hit it with the thermometer and see if it is around that temp. Same with coolant temp. After that, you can reset the minimum idle. If you cannot, perhaps there is an air leak. Hard to say.

DefinitelyMiami

Thank you. If you don't mind answering one last question that's been bothering me? Why do you think this problem disappears after the car is driven? Once I drive it, it's good for another week or two, and then it crops back up again. Is there anything about that that would lead you to believe it's the TB or the IAC? Would those issues disappear once the car ran for a while? I don't mean to bother you; this is my first Vette, and I'm still sharpening up my skill set. I work at a Porsche dealership and the easy answer would have been just leasing a Boxster but that's not nearly as interesting or educational haha.

aklim

Problem is you are assuming conditions are static. The air quality is NOT. The ECM is constantly adjusting so you know there is an issue when it cannot adjust because it goes out the window where it can compensate. Within that window, you won't notice anything without scanning the data strea.. Not sure if that makes sense to you.

Your emissions stuff is in the intake so it might be that the oil fumes coat the pintle and there is build up? It could also be that there is an air leak somewhere. Kinda why I suggested looking at the data. If the IAC counts are 0 and you close the throttle body "butterfly" to as far closed as possible and it is still 0, you know that it gets more air than it needs so there is a leak somewhere. IF that sets right, look elsewhere.

Is your timing correct? Fuel pressure and WOT fuel pressure?

JasBass

I agree with Ak, could be IAC. Just FYI to answer your previous question, the ECM is programmed to recognize intake air temps ranging from -40 to 210F, -40 to 100C. You wont throw a code unless you get readings outside of a recognizable parameter. That's why it is good to be able to look at actual input data from a scan setup. You could see the steps IAC is programmed to complete.

Don't apologize for wanting to learn about your Vette. I cut my teeth under these guys, that's why the forum exists! You wanted a project and not that shiny Porsche, it's worth a little investment in learning and being ok not knowing

DefinitelyMiami

Thanks, JasBass.

UPDATE:

Lacking the immediate facilities and equipment (my dad and I usually wrench together but my parents sold their house and he's in the midst of building his dream garage with two lifts (!) in it), I armed myself with a box of gourmet pastries from a local bakery and headed to a little repair shop on the far north side of Chicago. These guys have fixed a couple of other cars for me (mostly electrical work, which is NOT my forte) and I thought they might be able to help. I brought the pastries because, as I explained to the mechanics, I didn't want to just drop the car off and have them fix it. I wanted to learn. They were a little shocked, honestly, but after a couple pastries, some coffee, and some car talk, we got to work. Timing checked out, as did pretty much everything else. We cleaned the TB and the IAC valve. All said and done, it appears (as someone mentioned) there may be associated with the EGR valve. As we are currently in the midst of a bit of a cold spring here in Chicago, we disconnected it, and I'm going to drive it for a week and see if that fixes the issue. I'll keep you guys updated.

aklim

By cleaning, did you take the IAC out, IAC housing and top plate and clean the passages and reassemble with fresh gaskets?

Did you reset the minimum idle with scan counts?