Well I've got a bit of a pickle on my hands. A little while back my '92 RS started stalling on me. It seems that when the engine is warmed up, and the idle is down low, it will stall when placed under load. Such as when I shift into gear (auto), or use the A/C. It doesn't do it consistently, and every time I shift gears is pretty much a crap shoot.

The car has about 150,000 miles on it, and this is a very recent development. It starts right back up afterward, but when I try to shift from park to drive there's a pretty good chance it's just going to stall again.

Within the last few years I've replaced the EGR; fuel pump; fuel filter; spark plugs, wires, and distributor cap; and had the throttle body rebuilt and cleaned. The car is running really well otherwise, and the computer isn't throwing any trouble codes out.

I've tried raising my idle a bit, but that doesn't seem to have helped much. My next suspicion is the IAC valve, but I don't know a whole lot about how that works. So I'm here seeking the advice of others. Any recommendations?

Thank you in advance.

 

check the tech aritcles thier is a IAC reseting procedure in them. may have a possible TPS problem also Thier is an article for TPS testing also.

 

I tried that IAC adjustment procedure, but it didn't seem to work. I'll give it another shot and try adjusting the TPS too. Thanks for the input.

 

I have the same exact issue did you fix it after all?

 

No luck. My TPS voltage seems to be within the normal limits. And I adjusted the IAC to idle a little higher than normal. But she still stalls from time to time after warming up.

Could it be the IAC getting ready to retire?

 

The IAC may be bad. I'm not sure what you mean by adjusting the IAC. It is not adjustable. I was just dealing with a 91 Buick pile with an IAC fault. I proved it to be a faulty driver in ECM by viewing the control patterns with my Lab Scope. There is no easy way to diagnose an IAC. The test I generally follow starts with using a scan tool to monitor IAC counts while I increase engine load by turning on accessories or shifting into gear. The again while introducing added air by pulling a vacuum hose off. IAC counts should rise to lower idle and should lower to raise idle. If I don't see what looks like normal IAC response, I use my lab scope to view the control patterns. The IAC is a dual coil stepper motor. The ECM uses a 12v square wave (on/off) signal to control each of the two coils in the IAC. With a lab scope you can see the ECM cycle the coils on and off as it commands the pintle in or out. If the IAC is okay, you can hear it with a stethiscope as it "clicks".

One test that you can try without a scan tool is to idle the engine and remove the PCV hose at the manifold end. This will cause the engine to race. In this state, disconnect the IAC. Stop the engine and remove the IAC. It should be fully extended. Use a dial caliper to verify that the pintle length is the same as the depth of the seat in the throttle body. Then, reinstall the IAC and cycle the key on and off. Remove the IAC and the pintle should have retracted to the engine start position. If either of these test results doesn't jive, you can use a DVOM to test for voltage and ground at the IAC connector with the engine idling. You can also test the IAC coils for resistance. They should read 40-60 ohms. Coil A is pins A&B, coil B is pins C&D.

 

A lot of that went clear over my head. So, basically, what you're saying is: go out and buy a carburetor?

Kidding. By "adjusted the IAC" I mean I put the engine in diagnostic mode so that the IAC was fully extended (or retracted, I forget) and then adjusted the idle screw on the throttle linkage to raise and lower the base idle. When put into diagnostic mode, the IAC does make some noise. So it appears to be working on some level. I'll give that test a try.

For the sake of conversation, if it isn't the IAC or TPS what else could it be? All the hoses seem to be in good shape, so I'm pretty sure it isn't a vacuum leak. I may just buy a new IAC and see what happens.

 

The noise from the IAC shows that at least one of the coils is good. Sorry about the hi tech jibberish. I tend to think that everyone does what I do for a living. The IAC on the Buick made noise too. However, the signal from the ECM was also just noise and didn't cause the IAC to move either way. Before you buy an IAC, atleast try the ohm test of the coils. Remember, 40-60 ohms. At least that will tell you if both coils are good. The IAC is a mechanical device and even with good coils the stepper mechanism may have gone south. You really need a lab scope to know if the signal from the ECM is okay. I understand if you don't have a lab scope hanging around in your garage. Most techs don't have them.

 

Hi ASE doc, I have a 1991 camaro 5.0 TBI LO3 auto, it is idling high at 1200-1300 and it stumbles from a start. At 35 mph i can let of gas and it will maintain speed of 35 if road stays flat. A friend said it might be a vaccuum leak and to test with spraying starting fluid on them and the tbi base gasket to see if it revs up. I have new plugs and EGR valve, so can possibly rule them out. Any suggestions would be appreciated a lot!