Hey guys,
Since I got the car, it has had an issue with the idle. When it was cold, it would idle at about 1500 on startup and climb to ~2800 RPM. After warmed up, the idle would stay constant at about 1200 in park and ~1000 in drive/reverse. I replaced the IAC motor because it was a bit gunked up and figured it couldnt hurt but that made no real improvement. Yesterday, unplugged TPS and car idles smooth at around 800 in park and 700 in drive. When plugged back in, idle starts to rise again. Car idles fine with the TPS plugged in and off the car as well. When bolted up, the TPS% is reading between .8-2.4% at idle and never sees 0%. My dad bent the tab inside the TPS back so it would start at 0% when bolted up. It idled well with this setup but threw a TPS circuit voltage low code. I assume this is because the tab is bent slightly too far so when MAF reading is increasing with throttle but TPS voltage is not, the car assumes the TPS is not functioning. The code resets immediately after cleared with this setup. The code also causes a harsh shifting condition. We cant get a happy medium with this sensor where you can read both 0 and 100% TPS. I tried a new TPS from Autozone and had the same issue with the TPS never settling to 0. Has anyone else had an issue with this. Is it possible that its the sensor and Id be more likely to solve the problem buying one from the dealer?

Thanks in advance!
Dave

 

tps should read .5 volts when throttle is at idle. if its jumping around when the throttle is moved instead of steady rise or falling then tps is failing.

 

Just a FYI throttle position sensor reading high would not cause high idle. So it sounds like you started down the wrong path to start with.

As you drive the IAC closes off. If the computer thinks you are pushing on the gas it will start the handoff from the IAC to the throttle body. So pressing on the throttle or the computer thinking someone is pressing on the throttle will not make the IAC open more if everything is working as the throttle opens the IAC should start to close. For the idle to go up the engine needs air. The only place the engine should be getting air from is the IAC or the throttle body.

For high idle you what to figure out where the air is coming from. When there is not obvious issues I would remove the IAC and plug the idle port. If the car still idle its getting air from somewhere it is not suppose to be. If the car will not start then the issue is coming from the IAC itself. With better computer analyzers you can home the IAC with out taking it out you can also open it the max. I really do not think you can get enough air through the idle port to hit 2800 RPM. Which would mean air is coming from someplace else.

 

Have not had much time to work on the car but discovered Ill need to replace the LIM gaskets. Visually leaking and discovered oil in the coolant. Will break down the intake Saturday and see if I can get it back together as well depending on how well I want to clean everything up and how difficult it is with that motor being tucked under the cowl. It was a quick and easy job on my FWD S/C 3800 Riviera but it is a much easier setup to work on.

As far as the idle issue is concerned, i will have to test the IAC. I am puzzled because the car idles smooth and normally when TPS is unplugged. The TPS itself is not the issue. I have replaced IAC but not tested at all with it pulled. I have checked all vacuum hoses for leaks without finding any. If the LIM gasket is bad enough, could that cause a bad enough vacuum leak to create the idle issue? And if that were the issue, does it make sense for the idle to be perfectly normal when the TPS is unplugged? Also possibly worth mentioning is that I had several issues resolved by fixing bad grounds so Im wondering if a computer issue from the bad grounds could possibly cause it. I may be able to have the dealer reflash it for no charge. Possibly worth a shot if I can have it done for free? But first things first, I will get the LIM gaskets taken care of and then test With IAC off and port blocked to see how car acts. Thanks for the replies and I apologize for the slow feedback. Have been very busy these past few weeks.

 

Coolant in the oil can be VERY bad. It is the number one cause of engine failure. You need to get the coolant out of there asap. If you get any engine noises in the next few months its not even worth trying to fix that motor.

Bad ground can cause all kinds of issues.

I would be curious if the car was going into "limp home mode" when you disconnect the TPS. LHM is when the computer gives up running your engine based on sensor inputs and just runs on a predefined values. A scan tool would show the status. That information would just tell you that one of any of the sensors that are being ignored is messed up or the PCM is not interpolating something right. Disconnecting the MAF sensor should have the same effect.

 

So, a few updates. Car is not mixing coolant and oil after all. There is no coolant in the oil but there was a very little bit of oil in the coolant. I am guessing previous owner accidentally added oil to coolant, or the Trans cooler is leaking trans fluid but it definitely looked more like oil. Car currently has water and degreaser as coolant. Will drain that and verify that there is or is not a leak.

I took a guess and replaced the PCM with a "known good" and reprogrammed to my VIN from SpareECM.

I read that I had to do a "Security Relearn" after installing the new PCM but am not sure the procedure I completed was correct.

My car now idles normally and TPS% reads at 0% at idle which is a relief but the car stalls out without warning. Sometimes it will crank and crank and not turn over, and I can try again and it will turn right over and run for any where from around 5 seconds to a minute and then shut off as if I turned back the key. While its running, I can drive the car and it all seems okay, and then it will just die, again, as if I had turned the key back while I was driving. Has anyone every replaced a PCM in a '96 V6 and done a "relearn"? Am I missing something. Also worth mentioning, there is no Security light when the car stalls. There were no cutout issues before the new PCM.

Dave

 

tps should read .05-.07 volts at idle

 

That doesn't seem to be true according to several runs with 94 or 95 F-car LT1's. The graphs below are typical of those and they show the IAC rising 10-20 steps when the throttle is opened, and then ramping slowly down when the throttle is closed. I think this is done to prevent the engine from stalling after a sudden closing of the throttle.

 

The issue ended up being caused by a faulty ECM. Not sure exactly what was wrong with it but the symptom was the high idle on start which would rise even higher as the car warmed up. The TPS was always giving the right voltage but the PCM was not reading it correctly and a scan would indicate the throttle as always open.

 

Are you saying the "known good" PCM actually wasn't?