chevelless502

I will try asking again after changing some parts....
Ive got a 1998 Z28 and it has a high idle problem ive been fighting for some time. Sometimes i will start it and it will rev up for 2500-3000 RPM and just stay there. Other times i may start it and it will run normal for a few minutes then slowly start to creap up higher and higher till its around the 2500-3000 point again. I had been advised from other forums it was a vacuum leak so i replaced the main vacuum lines that go to the PCV valve and the rubber 3-way connector thats on the back of the intake as well as the hoses gong to and from that connection. I also took the intake off and found the plug that the MAF sensor plugs into to be loose so i siliconed it to the intake as well. When i put the intake back on i put new gaskets on it also. After getting it all back together i started it and it was excatly like it was before. I then took carb cleaner and sprayed it around the intake and vacuum lines and the throttle body and saw no fluctuation in the way it was running. Im at a loss on it at this point. Anyone here have any ideas for me?

Thanks in advance.

RPM WS6

iTrader: (3)

Have you tried scanning it during the high idle condition? At the very least, you should have a P0507 code if the idle is hanging that high and the TPS is reading ~0.4v/0.x% - if you don't have a P0507 then it's possible that the TPS voltage/percentage is higher than ~0.4v/0.x% due to either a failed sensor or something actually causing the throttle blade to stick open.

So then the next question is, have you physically checked the TB/blade/cable?

Also, I'd want to look at IAC readings on the scanner. The IAC may or may not be bad, but first I'd want to see if it was at least being command closed (count at 0) when the idle is hanging.

I would check these things before going any further with parts replacement.

murphinator

iTrader: (2)

I had the same issue with one of my 98's and it was the IAC , I unplugged it I recall and it idled normally unplugged then I cleaned it well and it came out of it , its been a while but I think I hit it with pb blaster and it freed up - been awhile sorry I am foggy on it.

I was concerned it was going to break something going into gear , it was harsh car had been off the road before I got it must have been gummed up...

chevelless502

There is no SES light on at all. Thats a bit strange to me as well. I recently bought a HP tuners but i havnt learned how to run it yet. Will it tell me what the readings are that you are asking me about? If so maybe i can fumble into it enough to figure that out. Yes ive actually tried 3 different throttle bodies on the car and all had IAC in them. I even went and bought a new one and it didnt help either. The blade and the cable all move very freely as well.

RPM WS6

iTrader: (3)

Well, the air to produce ~3000rpm has to be coming from somewhere. Sounds like you've already tried several IACs so it would be interesting to see if the PCM is in fact commanding it shut (closed is 0 - and yes any comprehensive scanner should show IAC counts).

If the IAC isn't letting air in, and the TB blade is closed and cable isn't binding, then the only other possible sources of air (other than all the hard vacuum lines/PCV lines/intake gaskets/etc. that you've already replaced) would be the EGR and EVAP systems. But, for a leak big enough to produce such high rpm, I would expect it to just kill the engine with a super lean A/F and unstable idle. At the very least, you would have some extremely high LTFT (long term fuel trims) to compensate for all the unmetered air entering the system, so that's another thing to look at with the scanner (though if they were maxed out, you should see lean DTCs as well as the P0507 high idle DTC.....of which you are seeing neither.)

I'd still be interesting in seeing IAC counts and TPS percentage and voltage before going any further. I wonder if the IAC isn't following command, or if the PCM isn't sending the proper command. Without more data, it's really all just speculation at this point.

chevelless502

Thank you for that info. That all does make sense. My car does still have the AIR system on it. Could this be causing my issue possibly if it is malfunctioning some how? I will hook up my HP tuners to it tonight and see if i can get some info out of it.
Thank you for your input.

chevelless502

I did a scan with my HP tuners and if im reading it right there is no signal at the TPS....it was blank the entire time of the scan. I started the car and it instantly went to just under 3000 rpm. I let it scan for roughly a minute and the idle was solid right around 3000 the entire time.

RPM WS6

iTrader: (3)

If there is no reading for TPS voltage or TPS percentage then something is definitely wrong there. The issue could be with the harness, the PCM and/or the sensor itself. You can try reading TPS voltage manually from the harness with a multi-meter, it should read about ~0.4v at idle with the throttle plate closed.

What did the scanner show for IAC counts? I wonder if the PCM is commanding the IAC to full open due to erroneous readings (or no ability to read) from the TPS.

chevelless502

I did a scan and ran it for about a minute. This is a screen shot to what it was like for the entire time. As you can see there was no TPS reading. Also notice the 2900+ rpm level its stuck at.

RPM WS6

iTrader: (3)

Looks like that chart is showing TPS percentage and not TPS voltage. The percentage should be at 0 or 0.x% with the throttle closed, so that reading may not be a mistake. Did you try pushing the pedal down a bit to see if the TPS reading would respond?

I would be more interested in seeing TPS voltage. Should be about 0.4v at idle with the throttle closed.

I'd also be interested in seeing IAC counts. If they are 0 then the air is coming from somewhere else (or the IAC is simply not responding to, or receiving, commands.) If they are high then you'd just have to figure out why it's open or being commanded open.

LTFTs look decent at 4.7 for both banks, so probably there is no major vacuum leak.

I don't use that scanning software myself, so I'm not familiar with all the settings or how you get IAC or TPS voltage information.

750Credit

As the title states I have the same issue as the previous guy. Was it ever resolved? I have a swapped 96 impala with basic stocking mods ls6 cam and springs on a 5.3. Out of the blue the idle climbs as the car warms to operating temperature. And it will redline in park after a while. I changed the pac with a new ac delco. Scanner in hpt shows tps reading %'s I sprayed all over the vacuum lines and TB and intake with brake cleaner no noticable hesitation. Car starts up ok then the idle continues to surge as the car warms. If I press the pedal the idle will hang at 2-3k and then throw the code p0507 before it drops back to 1k and climbs from there. Fuel trims look no different from my previous logs so I'm lost. No play in the TB blade. Any ideas?