Opti, IAC, ICM, Coil, or worse?
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Joined: Dec 1999
Posts: 1,791
From: Upstate New York,USA
Re: Opti, IAC, ICM, Coil, or worse?
Do you have a volt meter? With the engine off, ignition key in the "run" position, test the voltage of the TPS sensor at closed throttle and at WOT. If memory serves, .6 volts at Closed throttle and 4.6 volts at WOT.
Next, pull off the vacuum tube to the Fuel Pressure Regulator (FPR) Run your finger tip across the end of the tube, then smell your fingers. You shouldnot be able to smell gasoline. What you're testing for here is a ruptured FPR diaphram.
If you have a scanner, hook it up and start the car, drive it until you get into closed loop. Once there, look at the scanners, IAC counts at idle(where you're encountering the rough idle). Tell us what counts you're seeing. Back when I had issues with my own cars idle quality, I knew off the top of my head what the counts should be. I'll have to look it up now though and get back to you.
Next, pull off the vacuum tube to the Fuel Pressure Regulator (FPR) Run your finger tip across the end of the tube, then smell your fingers. You shouldnot be able to smell gasoline. What you're testing for here is a ruptured FPR diaphram.
If you have a scanner, hook it up and start the car, drive it until you get into closed loop. Once there, look at the scanners, IAC counts at idle(where you're encountering the rough idle). Tell us what counts you're seeing. Back when I had issues with my own cars idle quality, I knew off the top of my head what the counts should be. I'll have to look it up now though and get back to you.
Registered User
Joined: Dec 1999
Posts: 1,791
From: Upstate New York,USA
Re: Opti, IAC, ICM, Coil, or worse?
Scanned the good 'ol 'net and found this from 2004....
Injuneer04-27-2004, 10:11 PM
Both the IAC counts and the TPS voltage are within the "normal" readings.
IAC should be in the range of 20-40 counts at idle. With an aftermarket throttle body, you typically find the stop screw is out too far, so you have to back off on the stop screw slowly, until the RPM comes down to spec (or your reprogrammed value), then us a scanner to read the IAC counts and make sure they are in the required range.
After you have the IAC counts under control, you need to verify the TPS voltage. This can be read with either a scanner or a voltmeter. The LT1 PCM accepts any closed TPS voltage between 0.20-0.90V. Some people feel they get better results with values in the range of 0.50-0.67V, and just about every stock setup I haver ever looked at was around 0.67V. If you are not within range, or you feel you want to adjust the TPS voltage, you need to slot the screw holes in the sensor so it can be rotated.
The "reset" procedure for the IAC counts is outlined on Shoebox's website. I personally have never heard of the "short the pins" procedure.... that sounds like something that was used in the 3rd Gen ECM's. Shorting the pins on anything after 1993 accomplishes nothing that I am aware of, but I would like to learn more if there is such a procedure.
Both the IAC counts and the TPS voltage are within the "normal" readings.
IAC should be in the range of 20-40 counts at idle. With an aftermarket throttle body, you typically find the stop screw is out too far, so you have to back off on the stop screw slowly, until the RPM comes down to spec (or your reprogrammed value), then us a scanner to read the IAC counts and make sure they are in the required range.
After you have the IAC counts under control, you need to verify the TPS voltage. This can be read with either a scanner or a voltmeter. The LT1 PCM accepts any closed TPS voltage between 0.20-0.90V. Some people feel they get better results with values in the range of 0.50-0.67V, and just about every stock setup I haver ever looked at was around 0.67V. If you are not within range, or you feel you want to adjust the TPS voltage, you need to slot the screw holes in the sensor so it can be rotated.
The "reset" procedure for the IAC counts is outlined on Shoebox's website. I personally have never heard of the "short the pins" procedure.... that sounds like something that was used in the 3rd Gen ECM's. Shorting the pins on anything after 1993 accomplishes nothing that I am aware of, but I would like to learn more if there is such a procedure.
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