I know this isn't C5 related but i'm posting it here because i know there are a lot of ENGINE EXPERTS on this board.

MODERATORS: please let this post stay....for a little while at least.....PLEASE!!! :blueangel:

Ok here's the deal. I'm in the process of completely restoring my '84 Fiero (4 cyl) and i'm having an idle issue, and i've tried just about everything i know but still can't figure out what's causing it.

Here's a brief background, everything in the engine compartment is new (less then 100 miles and less then 3 months old) except the following: Intake/Exhaust Manifolds, TB-Unit, Tranny, Crank Pulley, Thermostat Housing, and Fuel Vapor Purge Canistor. The engine wiring harness is the original but was refurbished.

Here's the symtoms: On start up (engine cold) it cranks quick and easy, and idles high at about 1800 RPM's. The idle is smooth with no problems. Then afer about 45 seconds or so the idle drops to about 900-1000 RPM's.....this is when the idle issue appears. What's happening is that that idle seems to be "pausing" sporadicly(sp?)....almost like a popping sound but it's not a poping sound but rather a pause, it's almost as if there's a random weak/no spark occuring....thus causing the idle to "hunt" +/- 150 RPM's.

What i've checked/done (not necessarily in this order):
- Checked all vaccuum hose for leaks
- Checked fuel pressure
- Checked timing
- Checked spark plugs
- Checked plug wires
- Checked for distributor/rotor
- Checked ECM for codes (only Code 12 appears)
- Pulled IAC (Idle Air Control Valve) harness and idle smooths out (no more "hunting") but the pausing sound is still present.
- Disconnected each plug wire at the cylinder (one at a time) and cranked the engine....engine runs great on 3 cylinders with NO noticable "pausing" sound & no "hunting" occurs (almost as if it was meant to be running on 3 cylinders all along), and this was true each time i pulled a cylinders plug wire.

So my question is what could be causing my idle issue if all the above checked out good and the idle problem seems to go away when a plug wire is pulled?

Thanks!

 

Try a compression check. I don't see that in the list of things you've tried.

LOL

:cheers:

 

Possibly a bad ignition coil?

 

Compression Test

I haven't done this since it's a brand new GM long block, and i assume GM did a good job at assymbling it. Plus if it was a compression issue it would be cylinder dependent (unless all 4 are bad), and since it's a 4 cyl if i pulled the plug wire on a good cyl then that means that the engine would be running on 2 cyl or 2.5 cyl.....thus would idle realy rough.....but it doesn't.


Ignition Coil

I checked each wire & plug for a spark and they all get a good solid blue spark. Also i would think if it was an Ignition Coil issue that the problem would still be present even if i pulled a plug wire......and by doing so, thus making the engine run on 3 cyl, that the engine would run worse...but neither is the case.

Is my logic wrong in one or both cases here?

I'm at wits end on this matter....nothing seems to fit the bill as far as symptom diagnosis goes. :confused:

 

Sometimes coils fail under load or when they heat up, hence pulling one lead makes things better; often the easiest check without equipment is to borrow a known good one. Your hunting could also be a timing issue but you say you checked that too. Sorry, but I don't know the Fiero well enough to do anything else but generalise.