after tune up high idle (Page 1/1)
canamscott JUL 31, 07:39 PM
I have always admired Fieros and now have my own '85 GT 2.8.

I changed spark plugs, cap, rotor, air cleaner. Noticed throttle cable was not riding in the guide channel so slipped it back in. CC was disconnected from throttle so reattached it.

The car definitely has more pep now, but idle is about 1800 rpm. I slipped throttle cable back out and disconnected CC, but did not change idle.

Is there something I should adjust after a tune up? Did I knock something loose?

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Scott
Pontiac lover, next to Wild Turkeys
1985 Fiero GT, Raven
1977 CanAm, Power Tour survivor

Gall757 JUL 31, 08:41 PM

quote
Originally posted by canamscott:
Did I knock something loose?




Easy to do with a car this old. If your engine is all original, check on the EGR tube. If it has a crack the idle will go up.
armos JUL 31, 09:41 PM
A vacuum leak is likely. Gall mentioned the EGR tube which is a common issue.
Are you getting any check engine codes now that it's doing this? Before you say no, make sure the check engine light actually works - it should turn on with key on, then turn off when you start the engine.

Take off the air intake hose and clean up the throttle plate and bore. Make sure it isn't sticking.
While you have this exposed, you could also remove the Idle Air Control valve and clean out the air passage that leads to it. Get in there with Q-tips or something from both sides. This passage gets gunked up over time. That won't explain a high idle, but it typically needs to be cleaned out so might as well do it if you already have access.
Reinstall the IAC and electrically plug it in before you turn the key back on, otherwise the ECM will lose track of the IAC position (but it can be reset so this isn't the end of the world).
With the IAC back installed and the engine idling (preferably warmed up), put your finger over the IAC inlet. The engine should quickly stall or at least drop to a very low RPM (something like 600rpm would be okay). If you block that passage and it continues to run above 900rpm then you either have a vacuum leak or the throttle rest position is misadjusted.

Is the throttle set screw exposed on the top of the throttle body? From factory it was sealed in the correct position. If it's still sealed then leave it alone, it's not the problem.
If somebody has messed with that screw in the past then it's a potential problem. There's a procedure for finding the correct position for it, but that only works if you're sure there's no vacuum leaks.

[This message has been edited by armos (edited 07-31-2018).]

canamscott AUG 13, 10:19 PM
Here is an update on the high idle issue after a tune up.

It seemed reasonable I had damaged the EGR tube while changing the distributor cap and rotor. I looked and found the intake manifold end of the tube broken and laying on the lifter valley. I ordered a replacement. Removing the old part I found EGR was already blocked off, so clearly not the issue. Replaced EGR anyway then went back to examine throttle cable.

When mounted correctly, throttle was not fully closed, about 1/8 inch open. Slipping throttle cable out of groove to back of pulley at least closed throttle. The cable bracket had been heated and bent to try to correct the geometry. I found the throttle cable mount clip had broken from the sheath. The sheath wires would not let the broken clip end back into place effectively making the cable too short and always tugging on the throttle.

I swiped a throttle cable bracket from my nephew's Fiero. I trimmed the wires back on the sheath, pushed the clip end back into position, and thoroughly taped the clip/sheath together. Throttle works great and I can re-connect the cruise control.

P.S. does anyone have a throttle cable bracket for my nephew's '86?

------------------
Scott
Pontiac lover, next to Wild Turkeys
1985 Fiero GT, Raven
1977 CanAm, Power Tour survivor

armos AUG 14, 08:55 PM
Thanks for posting back about the resolution.


quote
[B]It seemed reasonable I had damaged the EGR tube while changing the distributor cap and rotor. I looked and found the intake manifold end of the tube broken and laying on the lifter valley. I ordered a replacement. Removing the old part I found EGR was already blocked off, so clearly not the issue. Replaced EGR anyway then went back to examine throttle cable.


A bad EGR valve could cause a rough idle but probably not a high one, unless it was running so badly that it provoked the ECM to raise the idle speed intentionally, and then you'd probably still notice a lot of roughness to how it was running. However, a leak in the tube that runs to the intake plenum will still represent a vacuum leak, even if the EGR itself is blocked.


quote
P.S. does anyone have a throttle cable bracket for my nephew's '86?


If you haven't already, try checking the Mall section of the forum. There's usually some cars getting parted out there.

[This message has been edited by armos (edited 08-14-2018).]