I have a 96 SS/LT4 auto. After a recent engine rebuild I have a high idle that I can't figure out. I suspected I had a vacuum leak. I used a smoke machine to pressurize the intake and found a couple of small leaks. One at the PCV hose. One at the TB gasket. When the car starts up cold, it revs to 2000 rpm and eventually settles down to around 1100 rpm. Once it is warmed up, the idle stays up around 1500+ all the time. After I replaced the PCV hose and the TB gasket the car started at 1300 rpm and stayed there. But I soon remembered that I had forgotten to reconnect the throttle cable. When I reconnected the throttle cable, it did exactly the same as it had done before. IAC? TPS? Or could I still have vacuum leaks that I haven't found yet? Help please. Thanks.

 

Use data logging software to figure out what the PCM is doinbg with the IAC counts in response to the excessive idle speed. If the PCM hac closed down the IAC valve to the 0 counts position, and the idle is still high, either the IAC valve is not moving to the position commanded by the PCM, or the engine is getting air elsewhere.

My choice for OBD-II scanning software was always AutoTap, but appears they have dropped out of the scanning software business. Not sure what everyone is using now, but there is a lot of OBD-II stuff available.

Test IAC valve, courtesy of Shoebox:

http://shbox.com/1/iac2.jpg

IAC reset procedure (you may have to scroll down a few pages on this one):

4th Gen LT1 F-body Tech Articles

Have you verified that the throttle blades are fully closing, and are not sticking due to problems with the throttle body of the cable?

Make sure that at closed throttle (after verifying it is fully closed) that the TPS is reading between 0.20 - 0.90V.

 

Thanks for the info Injuneer. I tested the IAC according to your test procedure and it tested fine. But I do have a question about it. When I checked it today the pintel was rusty but everything else looked fine. I cleaned the pintel with 400-grit emery paper. Is that OK? I don't have an OBD II diagnosis tool. A friend has one but I don't know how much detailed info it can deliver. I'll try to borrow his. Visually, the throttle blades certainly seemed to be closed completely. Does the OBD II usually read the TPS voltage? Also, I do intend to check for additional vacuum leaks with the smoke generator since the TB gasket leak wasn't visible until I fixed the PCV hose leak. Path of least resistance.

 

You can read the throttle position sensor position (TPP) and typically the voltage with an OBD-II data scanner. As long as the voltage is between the limits I mentioned, the PCM baselines the closed throttle voltage as 0% throttle position percentage (TPP), and then prorates the TPP by adding 4V to the closed throttle voltage to get the 100% TPP.

You can also use a simple volt meter. Key on/engine off, check to make sure you have 5V reference between the gray wire and the black wire at the TPS connector. Then measure the TPS volts between the blue wire and the black wire. The 0.20V - 0.90V reading should be seen at closed throttle. Some people seem to feel that a reading of 0.50 - 0.67V produces better results, but I've never found anything to support that preference. The stock/factory build reading is typically 0.67V.

Then, slowly rotate the blades from fully closed to fully open, and the voltage should increase to about 4.0V above the fully closed voltage, and increase smoothly with no spikes or dropouts.

I've never heard of rust on the IAC pintle. There's a good chance the IAC motor is seized up. Normally you will see some carbon on the pintle, and it can be cleaned with throttle body cleaner, holding the pintle pointed downward to avoid the liquid getting into the electronics of the stepper motor. The pintle should never be moved by hand.

And to clarify, the test procedure is not mine. It is from Shoebox's (Rob) website - the absolute best LT1 reference on the 'net.

 

When I was putting this motor together, I mixed TB parts between the melted non-stock LT1 that was in the car when I bought it and the LT4 motor that I bought off the internet. The stock motor had been so overheated that the heads were gold colored and the temp sensor in the head was completely melted. The LT4 was supposed to be perfect but somehow had been underwater by the time it got here. I don't remember which IAC I used but one didn't pass the test that shoebox detailed. One did. And since it did have rust on the pintel, it probably was the one that had been drowned. I think I used the original TB. No telling which TPS I used. Now I have some things to check. I think this car will be a pretty strong runner, once the bugs are worked out. Should be fun. Thanks for the input.

 

Quick fix. I have a friend who used to be a Chevy line mechanic at one of the local dealerships. He alternated between unplugging the AIC, TPS and MAF connectors. Based on what he heard when he unplugged each one, he deduced that they were all working fine. In the end, my friend decided that someone had jacked with the TB stop in a previous life. We adjusted it and that restored the car to a normal idle. Too simple. The check engine light is gone and all seems well. Now on to other problems.

 

That's why I asked if you had verified the blades were closing completely, and suggested that you verify that the TPS sensor voltage was correct. Did you do that?

 

Yes, sort of. I took the intake plumbing off and I visually checked the blades. They appeared to be fully closed. At least to my untrained eye. Someone with more experience might have recognized that they were not really closed far enough. I put the air cleaner assy back on. Then, with the motor running, I checked to make sure that the throttle blades were against the stop and that the cable was not holding them open. I had borrowed a set of special diagnostic clamps that only put a pin hole in wires when testing. I was just starting to hook them to the TPS wiring when my friend stopped me and went through his check. I wound up not testing the voltage on the TPS.

I do appreciate your patience in helping me solve this problem. If my friend hadn't interrupted the process, I would have found the problem on the next step. My friend was really opposed to putting any holes in the wires but I don't think it will be a problem. My plan is to go ahead and test the TPS voltage when I have a chance to work on the car again.

Thanks for the help and guidance.

Greg

 

The nice thing about the scanning software is it lets you read things like the TPS voltage directly from the PCM.