High Idle

[TimboSHO]

I know this has been discussed, but I can't seem to find anything quite like what I'm experiencing. When I first start the car, idle is about 1500 for a minute or 2, then will come down. If you hit this gas, it stays high for another couple of minutes. When driving it, it does the same thing. If you mash the pedal to the floor, it will stay at high RPMs until you turn the key off, and restart it. Then it will idle at 1500 or so for a couple of minutes and come down. I see there is a TSB, but that is for 98-99s and mine is a 96. I have taken the throttle body off and cleaned it, but the plate wasn't sticking anyways. Now it's just cleaner . Scanner verifies correct voltage at TPS (.94V at idle and 4.75V at WOT and smooth transition in between). I was also thinking IAB, but it doesn't just do it at idle, so I don't think that would be the problem.

Background on this car: This was Beth's. I drove it around at her place and it was fine. I drove it back to my place (~200miles) with a leaking front transmission seal and the problem seemed to develop about 20 miles after leaving her house. I got it home, took the transmission out and rebuilt it, noticed the IRMC was stuck closed, and had to pry the linkage to open the butterflies. The little plastic wheel in the box was broken from trying to open the secondaries. I now have them wired open, and it runs rough for a couple of minutes at startup. I realize this is normal, and that's ok. I hope that taking apart the intake and cleaning it and the butterflies will help somewhat. But it still has the high idle. That's what I'm worried about.

I haven't seen any of the high idle problems like this on a 96. Should I still buy the wiring harness for the TSB and see if it fixes it? Thanks in advance for any ideas/suggestions....

 

[NebraskaSHO]

A start:

http://v8sho.com/SHO/High_Idle.html

 

[TimboSHO]

Yep, that's the TSB I'm talking about that is for 98s and 99s only.
http://v8sho.com/SHO/TSB00-3-5.html
I'm just wondering if installing this harness would help on a 96, or if there is anything else to check. I've gone through all of the articles on V8SHO.com about high idle, and none of them say anything about the revs sticking after you are on the gas either. Probably not a very common problem, as I haven't gotten many suggestions yet..... keep 'em coming!

 

[SHOZ123]

Try tapping on the IAC when it is idling high to see if it drops down.

 

[TimboSHO]

Try tapping on the IAC when it is idling high to see if it drops down.

I was thinking of trying this (even though it's not just an idle problem) just to make sure. I will do it tomorrow when I see the car.

 

[TimboSHO]

Tried tapping the IAC, and no change in idle speed. Should I just throw the recal TPS harness in it? I just don't like throwing parts at stuff if there is anything else I can test..... .Thanks.

 

[SHOZ123]

If the TPS voltage at the TPS and at the PCM is the same then I would suspect the actual IAC valve. Disconnect the electrical connector when it is idling high and see if there is any change. It should stumble and die.

Or you could try manually reseting the idle.

On a warm running engine screw the throttle stop screw in until the engine is running about 1500 rpms. Then disconnect the IAC connector, screw the throttle stop screw out until the engine is idling at around 400 rpm. Stop the engine, reconnect the IAC, disconnect the battery to drain the brain then start the car and let it relearn idle.

 

[TimboSHO]

If the TPS voltage at the TPS and at the PCM is the same then I would suspect the actual IAC valve. Disconnect the electrical connector when it is idling high and see if there is any change. It should stumble and die.

Or you could try manually reseting the idle.

On a warm running engine screw the throttle stop screw in until the engine is running about 1500 rpms. Then disconnect the IAC connector, screw the throttle stop screw out until the engine is idling at around 400 rpm. Stop the engine, reconnect the IAC, disconnect the battery to drain the brain then start the car and let it relearn idle.

Any wiring diagrams telling me which wires to test TPS voltage at the computer? Or would that be what the scanner reads? I'll try checking it right at the TPS with a DMM and also unplugging the IAC. I'll also try reseting the idle as you described as well if I have some time. Thanks!

 

[SHOZ123]

The scanner should be reading what the PCM is seeing.

 

[GreenSho99]

Mine is actually doing the same thing, I found the IAC to be bad, you really gotta hit that sucker if its acting up.

 

[TimboSHO]

I'm sorry I haven't updated this in awhile, the shop got busy all of the sudden. I went to the dealer, and they told me the harness was $105!! That's a far cry from the $17 it used to go for (as I found on v8sho.com). When I have more time, I guess I'll just go through and carefully inspect all of the wires going to the TPS.

 

[JoesSHO]

Timbo, try unplugging the TPS and see what it does. When I unplugged mine, it did that, not returning to idle, or very slowly. When I replaced the TPS, it was fine.

 

[TimboSHO]

I figured I would update this and just let you all know what fixed it. I spent yesterday cleaning out the intake (my, what alot of carbon buildup in 60K miles!). I also inspected the TPS wiring harness, and found alot of corroded wires. All of the plastic shielding on all 11 wires in there were browned up, and the 3 going to the TPS had evidence of corrosion inside. So i cut them open, only finding slight corrosion, but no clear breaks. So I repaired them all with some butt connectors and heat shrink, and continued to diagnose the problem. I noticed on the scanner that the IAC value wasn't changing much at all when it was revved up. So I unplugged it and TADA! Idle went back to normal, even when blipping the throttle. So I had a drive all the way across town in my 89 to pick up the IAC, and on the way back, the 89 breaks a shifter cable. I guess that's just my luck. At least I still have one on the road! Now I have to figure out some of the radio problems.....

 

[Mr Anonymous]

LOL! That's classic SHO ownership for 'ya!

Glad you got it all figured out and solved.