Intermittent Fluctuating Idle

[compyelc4]

This has probably been beaten to death but here goes.....

'95 Stock Lt1 with 67,000 miles. Since I can remember engine has had intermittent higher-than-normal idle on a hot engine after cold start; around 900 rpm when hot. Turning ignition off and back on solves the problem until 20-30 cold starts later, then occurs again. Because I could always correct the idle with a restart I never much bothered to fix it. Lately, however, It started to bug me because after I researched the problem and replaced the IAC valve, it stopped doing the high idle thing but began up-ticking and down-ticking engine RPM about 200 RPM or so when hot, but not always...maybe 70% of the time. Sometimes when the tach ticks up it almost overrides the brakes! When it ticks down it feels like a misfire. Idle is pretty stable before and after. I can't say it is rough. When I floor it, no misfires to high RPM. Gas mileage on interstate is good at about 24.5 mpg.

Here's what I attempted to do without fixing:
1. New plug wires, plugs, and optispark.
2. Replaced the new cheap "made in China" IAC valve with a brand name one.
3. Thoroughly, and I mean thoroughly, cleaned IAC passageways, throttle plate, etc.
4. Scanned for codes but none present. Oh, last year an EGR code was set once. I cleaned and tested EGR valve and that has not returned.
5. Checked PCV for proper operation.
6. Checked throttle cable for correct operation.
7. Checked optispark vacuum control circuit for leaks. Optispark pulls modest amount of system-restricted air flow where optispark vacuum hose attaches to intake plenum.
8. Because it is intermittent I doubt a vacuum leak, but who knows.

I just returned from a 1500 mile car show in Myrtle Beach and car ran flawlessly, except when coming to a stop at a light, then that danged fluctuating idle thing! It might bounce that tack 5-6 times before the light turns green. I am at the end of my knowledge here. Any help is greatly appreciated.

 

[compyelc4]

Really? No one has an idea out there in C4 land?

 

[Hib Halverson]

Do you have the 95 Service Manual?

 

[compyelc4]

Do you have the 95 Service Manual?

I do. What should I be referring to in there? With my specific symptoms and no DTC codes there's not much in the book that can point me in the right direction. I've been pouring all over it. No joy! BTW, nice to hear from you. Actually, a couple of years ago you helped me correct a fuel pressure leak-down problem with my '07 C6, to the tune of $1600! All is fine with that now.

 

[Hib Halverson]

In the section of the 95 on engine controls there are several pages of troubleshooting instructions on driveability. Read all the stuff on unstable or surging idle. That will give you a start.

Also, do you have a scan tester which supports a 95?

Lastly, to confirm...after you changed the IAC the problem remained but its characteristics were different, ie: first it was high idle speed but after the IAC replacement, its an unstable and surging idle. Is that correctd?

 

[compyelc4]

Thanks.
I do have access to a scan tool.

After IAC change from original OEM (65,000 mi.), characteristics were diff. in that idle speed would drop from high idle to normal idle when hot, this first replacement resulted in fluctuating idle from about 600rpm, instantaneously, to 800rpm and back again, interspersed with intermittent but instantaneous drop in rpm from 600rpm to maybe 400rpm. This downward deflection of the tach feels like a misfire. In between episodes she idles nicely. While sitting at a light it might go through this routine 3-4 times. Sometimes not at all! This mess typically does not occur until water temp reaches about 180 degrees +/. If I accelerate casually or full-bore there is nothing that feels like a misfire to redline. Thing just happens at idle whether in drive or neutral. A 2nd replacement using a GM OEM IAC unfortunately produces same result as the Chinese-made 1st replacement.

 

[compyelc4]

Fluctuating idle solved. After weeks of troubleshooting with my GM manual, with some luck thrown in, I discovered an intermittent "open" wire in the circuit connecting the ECM with the coil. The break in the wire was buried deep inside the wire bundle carrying signals between the ECM, ignition coil, and optispark. I had to peel back about 5" of insulation to get to the break. Amazing I even found it. The problem was throwing codes all over the place for different engine and accessory systems, so none of that made sense. Call me lucky.

 

[JT ZR-Won]

Wow, great job in finding that. That sounded like a tricky one!

- Jay

 

[Robertwav1]

Nice job on the find!