High idle issue

**chevelless502** · 09-13-2015, 07:47 PM

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.

**Naaman** · 09-13-2015, 10:04 PM

Have you checked the IAC yet? I had this issue when I replaced my throttle body. The IAC o-ring broke and I didn't know it. Got a new o-ring and it solved the problem.

**pajeff02** · 09-14-2015, 03:26 AM

It does sound like the IAC (Idle Air Control Valve) may be acting up. Could need cleaned or it may have an issue and is hanging up. Any excess air into the engine will idle it up, so also check your PCV hoses, especially where it crosses around the back of the engine from the driver side to the passenger side and also the hose between the intake and the PCV valve.

**RichWS6** · 09-14-2015, 05:29 AM

just remember when you replace the IAC you need to drive it 20 to 30 miles so it can fully adjust. after first start it will run a little rough but after the drive it will be fine.

***SMWS6TA*** · 09-14-2015, 08:39 AM

That's a little too high for a vacuum leak.

I know it sounds crazy but have you been having any battery or alternator issues? If the ecm detects a low current it will go into limp mode to get the alternator to charge the battery. Can't hurt to test them.

**chevelless502** · 09-14-2015, 01:14 PM

Thank you for the ideas so far i appreciate it. Ive replaced all of the vacuum lines with new as well as the T's and Y's that connect them all together so that should take care of that issue. Ive also tried two other throttle bodies and also a new IAC on it and still am having the same issues. As far as i know the battery/alternator is functioning fine. The car always snaps right off when starting it and the gauges read that its charging. I can throw a meter on it just to make sure though and rule out that potential problem.

**pajeff02** · 09-14-2015, 06:35 PM

Stock PCM or aftermarket engine controller? Do you have access to a scanner?

**chevelless502** · 09-14-2015, 07:42 PM

Originally Posted by pajeff02

Stock PCM or aftermarket engine controller? Do you have access to a scanner?

Its got a tune in it from a Hypertech but thats it. Ive got an HP tuners i bought recently but i havnt learned how to use it yet.

**pajeff02** · 09-15-2015, 03:05 AM

HP Tuners has a great scanner function. I highly recommend that you sign up on the HP Tuners forum and watch some of the instructional videos and such. I bought HP Tuners Pro a few years back and it is kind of intimidating.

As noted above, high idle almost has to be excess air making its way into the intake. It has four possible ways to get there: 1) the throttle blade is opening too far; 2) the Idle Air Control Valve is stuck or not functioning properly; 3) a vacuum leak; or 4) a bunged up PCV system that is split open.

(BTW: When I answered above I was looking at your sig and saw "1970 Chevelle". I see that we are playing with a '98 Camaro now. My bad...)

**0rion** · 09-15-2015, 03:13 AM

also check for any leaks between the maf and throttle body. Usually the line that likes to split is the pcv where it turns to go into the intake but it sounds like you've already replaced that. IAC is new so most likely not that. Any mods on the car?

***SMWS6TA*** · 09-15-2015, 06:12 AM

Two other possibilities:

1) Idle set screw on the Throttle body.

2) The hole in the Throttle Body blade has been open up.

Those above are small in chance.

**0rion** · 09-15-2015, 11:40 AM

when you changed the iac did you do a reset?

**chevelless502** · 09-15-2015, 02:58 PM

Originally Posted by 0rion

when you changed the iac did you do a reset?

Yes i did. After i realized i had to. lol. another learning experience as i go. lol.

**chevelless502** · 09-15-2015, 03:00 PM

Originally Posted by SMWS6TA

Two other possibilities:

1) Idle set screw on the Throttle body.

2) The hole in the Throttle Body blade has been open up.

Those above are small in chance.

Ive had multiple throttle bodies on it as well and all have the same problem of the high idle. All of the holes in them appear to be the same size so i am "assuming" all are stock.

**chevelless502** · 09-15-2015, 03:01 PM

Originally Posted by 0rion

also check for any leaks between the maf and throttle body. Usually the line that likes to split is the pcv where it turns to go into the intake but it sounds like you've already replaced that. IAC is new so most likely not that. Any mods on the car?

Not to many mods on the car, cat back exhaust and SLP lid, changed rear end to 3.73 gears.

**chevelless502** · 09-15-2015, 03:03 PM

Originally Posted by pajeff02

HP Tuners has a great scanner function. I highly recommend that you sign up on the HP Tuners forum and watch some of the instructional videos and such. I bought HP Tuners Pro a few years back and it is kind of intimidating.

As noted above, high idle almost has to be excess air making its way into the intake. It has four possible ways to get there: 1) the throttle blade is opening too far; 2) the Idle Air Control Valve is stuck or not functioning properly; 3) a vacuum leak; or 4) a bunged up PCV system that is split open.

(BTW: When I answered above I was looking at your sig and saw "1970 Chevelle". I see that we are playing with a '98 Camaro now. My bad...)

No problem on thinking it was on my 70 chevelle. Im hoping to do an LS swap on it this winter....maybe.....

***SMWS6TA*** · 09-16-2015, 06:55 AM

I was just tossing out the less known possibilities. Have you checked the valve cover gaskets and the "port" on the top of the covers for the pcv?

**35th-ANV-SS** · 09-16-2015, 07:01 AM

Is it possible the throttle cable itself is hanging up not allowing the throttle body to fully close?

What about a crack in the intake itself?

**kingls1** · 09-16-2015, 06:24 PM

Originally Posted by 35th-ANV-SS

Is it possible the throttle cable itself is hanging up not allowing the throttle body to fully close?

What about a crack in the intake itself?

I was also thinking about a crack in the intake myself...especially on the back where the vacuum fittings are. When you reinstalled your intake did you follow the proper tourqe sequence? There is a certain sequence and increase in pounds. When I changed mine when moving in sequence some of the bolts would feel really loose after already torquing them. After several passes all was good. Just a thought..