Just a week old new owner of a 90 coupe and loving every moment of it! Unfortunately, it came with a few kinks. One of them is that although it easily starts up, after 1.5 minutes the rpms drop from 10 to below 5 on the gauge and, if I don't rev the engine several times for several seconds or gun it if I'm in gear, it will die. Other than that the engine runs smooth. It was at the mechanics last week (before I narrowed down the problem) and they didn't detect anything obvious. It has passed CA smog test. Appreciate any ideas!

Im thinking that its your idle air control, they can be cleaned. But I prefer just to buy a new one from autozone for $60 or whatever they go for now.

If you prefer to save that $60.00 or so, Take it out and clean it. It's a simple fix. Two screws and it's out. although you need to put a rag under it so as not to drop the screws. They are NOT magnetic.

Thanks for the responses. I took the IAC out and it looked pretty well carbonized (if that's the term). Tried my best at cleaning it (including unscrewing the mini piston and WD40ing the whole thing). Unfortunately, the cleaning didn't do the job. But then neither did the brand new IAC. It continues to idle rough after about a minute of warming up (not absolutely always, but 9/10 times). Other than that, the engine runs really smooth.

I noticed two of the plugs into the coil were looking rusty. Scraped away as much as I could in hopes this may solve the problem. Couldn't get most, though. Could faulty connections cause it to idle rough without running rough?

Thanks for any other ideas! (BTW the tip about the screws was really, really helpful.)

When was the last complete tune up? (plugs, wires, O2 sensor, fuel filter, ect.)

Have you replaced the plugs and wires? Which coil pack do you have? Square (Magnvox) or round? (Delco) If you have the square, you should replace them with the round ones. (Delco) If you do this replace the unit under them at the same time. Junk yard is the best and cheapest source.

Thanks for the replies. The last complete tune-up was last week, but I'm wondering how complete it was now. I know they changed the air filter, but leaving wires with apparently rust-like corrosion on the connectors (I misspoke when I said plugs--sorry!) makes me doubt they changed much else. I'll bring it back.

The plugs on the squarish coil look good to me (drab gray). I'm sure it's the original and may be due for a replacement, if rough idling is a sign.

Thanks again. This is a great forum!

By the way, got new BF Goodrich touring tires on the car and it rides quieter than most cars I've been in--with the windows down!

Do the seafoam thru the plenum vaccum hose trick.

What is the "Do the seafoam thru the plenum vaccum hose trick" and specifically how do you do that? should and can this be done for maintanence type action?

Disconnect the hose that is just to the left of the Mass Airflow Sensor. Install a hose long enough to go into your Seafoam can. Start car,insert tubing into your Seafoam can, keeping a kink in the hose. Allow the vaccum to draw out the Seafoam. Directly above that hose is the throttle linkage. If you choose to, you can increase the RPM's of the motor.
Do not do this in an enclosed area as all kinds of carbon smoke will exit thru your exhaust system. This the easiest and best way to clean the plenum short of removing it.

Dave,

I think I may do the Seafoam thing just as preventative maintenance. I imagine the "kink in the hose" is to control the amount of Seafoam entering the engine (the Seafaom must bog down the engine, and try to cut it off....regulate with the "kink" and the acc cable???). How much Seafoam or how much of the can is necessary to clean appropirately please? Any other procedure tips please?

I believe the can is 12 ounces. Use it all.

Just an update on the original problem of the thread: Took the car back to the shop and had them replace the plug wires, but alas to no effect. Cleaned out the MAP (?) which regulates airflow and the IAC intake--no effect. Tested the fuel pressure and noticed decrease in pressure when revving the throttle during the rough idle. The mechanic is thinking faulty fuel pump. Fuel filter is brand new. So, I'll probably fork over some more Ben Franklins in the next couple days. Haven't tried the plenum cleaning, but intend to.

Did you clean the passageway behind the IAC where idle air flows when you cleaned the IAC? It still sounds like a IAC problem to me. If It runs good with the current fuel pump I should idle as well. Don't spend your money unless you know for sure the pump is bad.

If you had an 88 or 89 model you could open and close the IAC with the touch screen in diagnostics mode to trouble shoot it. I'm not sure about the 90 model.

It would really help us if you would edit your profile and put the model of your car in your signature line. That way everyone knows what model of car you are talking about at a glance.

Yes, the passageway (intake) got a good cleaning. There was the usual residue. The IAC is new.

The mechanic said that a faulty fuel pump may take a while to build up sufficient pressure (thus the rough idling initially), and once pressurized will run smoothly. I do have the slightest of lags at around 70mph and asked him if that might be related to the pump. He said when there's high fuel demand the pump may not be able to sustain pressure perfectly. Sounds reasonable to me, except that I think the lagging should occur at every acceleration from stop (greatest fuel flow demand, right?). It does lag initially, but after I've gunned it a couple times it smooths out. What do you think?

Check you fuel pressure before you buy a new pump.

Clarify a few things. Does the car idle rough all the time or just the first time it is started after sitting for a while? Does it idle good after the first start?

If it idles good after the first time it starts you could possibly have and injector that is leaking down while the engine is off (there is still some pressure in the system) and flooding one or more cylinders. Look for some black smoke when the car is first started and idling rough.

Just another idea.

Another update. I read in the service manual that one way to test whether the problem is the IAC is to unplug the electrical connection to it when it's idling rough. If it doesn't change the situation, then it's not the problem. Which is exactly what happened. No change. I did use Seafoam through the manifold intake--but not as much as I would've like for fear of the neighbors and since I kept cutting the engine out--and it SEEMS not to be as stuck in the rough idling mode. In fact, if I gun it in gear from a dead stop once the ABS light goes out (about 10-15 seconds), you know, a jack rabbit start, then I don't have the problem at all. If I drive off a little too late or don't have the chance to gun it and it goes into the rough idling as I'm driving, then I'll gun until it clears its throat (about 2-3 seconds). Then it's smooth sailing.

The car starts up just fine. No problem whatever. Just after about a minute it will go into the rough idling, until a rev it a lot or gun it. No black smoke.

Thanks for the ideas. The service manual has a few more leads for me to follow.

Just a thought but you might consider checking the vacuume lines.

Final update (I think!). Cleared the problem (hunting idle at cold start) with a new O2 sensor and PCV valve. Don't know for sure which was the culprit, but metering pointed to the oxygen sensor. Now on to ironing out other little wrinkles in this gem of a car. Thanks again for all the help!