2.8 Erratic/high-idle Issue (Page 1/10)
ITALGT JAN 07, 08:42 PM
So here's the deal: I have a 1986 Fiero GT (155,000 miles, stock 2.8, 4-speed manual) with a persistant idle issue that has me a bit stumped.

When I bought this car a couple months ago, it had a high/erratic idle. No check engine light, but the computer did show the infamous code 32. While idling, it revved up and down between about 1800-2800 RPMs. I've spent some time fixing a few things on the car since then; the idle is now a little smoother and has come down to about 1400-1600 RPMs, not nearly as erratic but still not 100% right.

When the engine is cold, the RPMs stay at about 2300+RPMs, then they slowly come down after the engine gets to operating temperature. It seems to run decent otherwise, with the exception of a very light, sporadic sputter (or miss) while cruising.

The engine always starts immediately with no hesitation.

Catalytic converter is fairly new from previous owner.

I'm missing something... somewhere.

So far, here's what I've done and replaced:

EGR valve
EGR solenoid
EGR tube (Rodney's stainless unit)
Stainless steel vacuum lines (from Rodney)
Zip-tied/clamped all vacuum connections
TPS sensor
MAP sensor
MAT sensor
CTS sensor
O2 sensor
IAC valve
PCV valve
Cleaned throttle body
New fuel filter
New gas cap
New vapor canister filter

Also:
Distributor is new
Plugs are new
Wires are new
K&N air filter freshly cleaned/oiled

I also cleaned several grounds and added a 4 gauge engine-to-battery ground cable, which did help the idle stabilize a bit. I've tested the intake and vacuum system for leaks by spraying carb cleaner around possible problem areas, but no luck (idle doesn't change). The idle air control tube going from the lower intake to the throttle body is leak free, and idle doesn't change with the brake booster nipple blocked off... same with the two lines running to the trunk wall; no change after the supply lines were capped.

I'm a bit stumped at this point and looking for any suggestions as to what I might look at next. About the only thing I can think of that's left is the possibility of having a bad connector on one (or more) of the sensors... before I bust out the Ohm meter and start diagnosing everything, does anybody out there have any other suggestions or ideas as to what I'm missing here?

Thanks in advance!

[This message has been edited by ITALGT (edited 01-18-2015).]

Gall757 JAN 07, 10:02 PM
Do you have cruise control? Does it work?

You have looked at just about everything in the vacuum system and you are still 400 rpm too high...so that's all I got!...
ITALGT JAN 07, 10:12 PM
Yes, it does have cruise control, however it currently does not work.

For diagnosing, I capped the supply lines off at the engine that go to the trunk wall to see if the idle would change... but there was no difference. Also disconnected the vacuum supply line to the can and capped the supply nipple... no luck. I believe that the cruise control and vapor canisters are leak free.

Not sure why the cruise isn't working though... haven't checked into that yet.

[This message has been edited by ITALGT (edited 01-07-2015).]

countach711 JAN 08, 12:10 AM
your post reads almost identical to mine, I'm sure there are many people on here thinking we are idiots for 'throwing money at the problem' but since parts are so cheap, I figure it's not such a bad thing, especially considering how old my parts were. The main difference is I had somewhat different symptoms, rough running at low speeds, but my idle was kind of high as well. What finally fixed it was new injectors, $125 shipped from Rock Auto, and about 4 hours time.
ITALGT JAN 08, 08:17 AM
That's pretty much been my mentality too.

I don't typically throw parts at a problem, but with 155,000 miles on the clock, I wanted to replace the sensors and vacuum lines anyway. And as you said, the parts were all fairly cheap so I went ahead and replaced them first. I'm glad I did, because it's running a lot better now, and all the sensors I replaced looked like the factory Delco originals.

Fuel injectors... yeah, that does sound like a possibility, especially with the miles and symptoms I'm having. That may be next.

I did run a can of Seafoam through the gas tank, but didn't seem to help.

[This message has been edited by ITALGT (edited 01-09-2015).]

ITALGT JAN 08, 06:01 PM
Any more ideas floating around out there?
fierofool JAN 08, 08:21 PM
I had been experiencing high idle of about the same rpm on my 87 but I also had a hesitation when accelerating immediately after changing gears. I replaced the TPS and both problems went away. Fortunately I had about a half dozen on hand so it didn't cost anything.

Then, because I had a coolant leak, I pulled the thermostat to reduce some of the pressure so I could get the car back home and the idle went back up. That later went away when I fixed the leak and replaced the thermostat. Apparently the ECM wasn't going into closed loop.
LornesGT JAN 08, 10:42 PM
Did you reset the IAC? I had a high idle reset it and now I have to workout my rough idle.
ITALGT JAN 09, 10:49 AM
Thanks for the input guys.

I installed the new IAC after cleaning the throttle body, making sure the pintle was protruding under 1-1/8" inch from the mounting surface per the instructions (it was about an inch or so when I put it in). I also thoroughly cleaned the pintle seat on the throttle body before installing it. Then I drove it for a few miles over 40mph to set it.

A couple nights ago, I pulled the IAC out to check the pintle length, and it was right at 1-1/4", which is what I've heard from others is correct. My understanding is that the IAC valve reseats itself every time you turn the ignition on, so I'm not sure what else needs to be done to "reset" it.

Or am I missing something?

[This message has been edited by ITALGT (edited 01-09-2015).]

ITALGT JAN 09, 02:53 PM
I spent some time on my lunch break today listening a little closer to what the engine is doing.

RPMs are still high when the engine is cold, around 2500 RPMs then drops while warming up to temperature. I know it's normal for it to come up some when cold, but this seems too high.

Once warmed up and idling, it fluctuates randomly between 1350-1550 RPM's.. It's much more stable than before I fixed the vacuum leaks and replaced the sensors.

When holding the throttle above idle, the exhaust randomly "pops" like it has an occasional miss; it's not nearly as noticeable at idle, just a small random shake as the engine runs. It almost acts like an ignition issue, or as previously mentioned, maybe some bad fuel injectors.

The previous owner replaced the distributor with a new one, and also installed new plug wires. I changed the spark plugs to NGK V-Powers just a couple weeks ago. However... the wires look like generic cheapies and I didn't install the distributor, so there may be an ignition issue here I've been overlooking. Also, the coil is the factory original.

I've been wanting to replace the cap, rotor, coil and wires with MSDs and Taylors; now might be a good time to do it.

I'm SO looking forward to when the engine is idling like it should... this has been bugging the sh!t out of me since the day I bought it the car.

[This message has been edited by ITALGT (edited 01-14-2015).]