chevelless502

I have a 98 z28 and it has a fast idle problem i cant seem to get fixed. I have done everything i can think of and it still is an issue. I will get in it and start it and it may idle fine for a minute or two and then all the sudden its like you put your foot on the accelerator and it revs up to 3000+ rpm and wont back down for anything. If i shut it off it will then rev right back up when i restart it. If i take the battery loose and rehook it back up it sometimes will then idle back down to normal on the next start up but it only lasts for less than a minute then goes crazy again. Anyone have any ideas where to go next with it.

Thank you

_JB_

iTrader: (33)

What have you checked? Vacuum leaks, IAC, loose throttle body ect? Did it start randomly doing this, or did it start after a mod or two?

LS6427

iTrader: (2)

I would say you need a new IAC valve. Seeing how you disconnected the battery and it seemed to help for a little bit longer....IAC.

You could do what I do with sensors and valves and little **** parts like that.....just go buy a new one real quick, pop it in and see if you're good to go. If not......clean it and return it in a few days, just tell them it was the wrong part and you NEVER EVEN installed it.

Then.....start looking for the vacuum leak....because if the IAC valve is good.....the ONLY thing that can INCREASE rpm's is MORE air getting in somehow. There is no other way an engine can rev UP. Air only.

But, if it were a vacuum leak it should shoot UP in rpm's the second you start it, not after one minute. Vacuum leaks are there or they are not there. And yes, sometimes they do show themselves AFTER the engine is warm and seals/gaskets/plastic can get soft and then the vacuum leaks can start. But they are not just BAM....3,000 rpm. They will be all erratic and different each time they start to leak.

I'm thinking since it happens when you hit the throttle, and it always goes up to 3,000..........IAC valve.

But with that said......it can still be 10 other damn things.......lol....
---could also be a worn TPS that's telling the PCM wrong info, causing the PCM to open the IAC valve. That has happened to my car years ago. So maybe after trying the IAC valve, try a new TPS. Again.....return them if they don't fix the issue. Free diagnosis......

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chevelless502

Thanks guys for the ideas. I will give that a shot with the iac and see if that's it.

chevelless502

I swapped the iac and still no luck. Any more help or advice?

thechef

iTrader: (13)

look and make sure the throttle body blade is closing all the way, start the car and touch the throttle bracket and see if it is closed all the way when it should be at idle....

i would also look for a vac leak

BADD SS

iTrader: (4)

... no

It's electrical, I've seen it before. Since it stops when you disconnect the battery, the pcm resets.... different things help different people... tps sometimes works. Usually iac is ok, since it's being told to open and does so....

chevelless502

That was one of my first thoughts that maybe it was hangingup on something as well. I tried closing the blade manualy and it didnt move. Also i took carb cleaner all over under the engine bay checking for a vacuum leak as well and i never could get the idel to change up or down like it was sucking it in some place.

Thank you for the ideas on things to try thoughi really appreciate them.

chevelless502

Any ideas where to look?

BADD SS

iTrader: (4)

1 car I tried was tps... My car, still no luck. Happens every 3-4 months and I shut the car off with my kill switch, start back up and she runs fine.... For me it's not a big deal, have not investigated further... But the root problem is your IAC opens all the way. Causing high idle. If you use a scan tool and close the iac, it will return to normal idle. Therefore, no vac leak. it's being commanded to open.

chevelless502

OK.just yet another update. I switched out to a completely different throttle body and. MAF. the car ran well for about 30 minutes then it started to idle about 3000 rpm again. When it did it it Sounds like a massive vacuum leak. Help........

chevelless502

Anyone?