I just bought an 87 firebird with a 2.8L V6. Every sensor that feeds back to the ECM has been replaced by my son (former owner) or by me except the IAT. The only performance mod that was made since I bought it was an Accel Super coil. That's when the problem started. Here's how it goes.... When the engine is cold it idles as it always has. Once it comes up to peak temp, around 220, the idle shoots up to 14-1500 and falls to approx 500. What could cause this?

 

That's an idle surge....not usually common on unmodified motors though..

So the IAC valve has been replaced? What about the EGR valve?

Since it only does this when it's warm, that makes me tend to think that it's an emissions component at fault since the car only starts using them once it gets up to temp.

My cures for an idle surge (since my cammed 302 does it ALL the time) are as follows:

a) Up the idle speed to around 850-900 rpms. At this high of an idle, a surge usually won't appear.

b) If it starts surging, rev it up to 1500-2000 and slowly release the gas pedal to bring it back to a normal idle. This only works sometimes...but if you get lucky it'll stabalize itself. Or worst case scenario, shut it off and start it up again. Thats always stops mine from surging.

 

F-Boid,

Welcome Aboard!

That Accel Super Coil should have had an instruction sheet. The sheet should have listed an average primary current in a 12V system with no ballast resistor. Chances are, the primary current exceeds the capability of the factory HEI amplifire/switching module in the base of the distributor. The recommended "fix" is to install a stock coil or replace the stock HEI module with a high-current design, like the one Accel should have suggested.

The problem could be that the primary current is so high that teh HEI module is not able to send accurate switching data to the ECM (distributor reference pulses). With weak pulses, the ECM may have a problem establishing the engien RPM, and may actually be missing injector firings as a result, as well as being unable to control idle speed at the target idle RPM (since the reported idle RPM would be essentially a moving target).

If you have the stock coil, reinstalling it would be the easiest way to verify the hypothesis. You can aslo perform an archive search for "Accel" and "HEI" and see if the problem has surfaced before.

 

My 2.8 did it when it was cold - exact same description - super high rev, then drop to the point of almost stalling. Two words - "vacuum leak" - lean in and listen, all it takes is a small leak.

 

Thanks for the help! Looks like I've got a few things to try. I'll keep you all posted. One additional note. I think the suggestion on the hi-perf ICM is going to be a winner. The ICM that was in the car (I think original) got toasted right after the coil install. I assumed it was "just old" and needed a rest. Replaced it with a new OEM. I'm going to swap the coils back first and see what happens.

 

You poor man!! Welcome to the boards! And welcome to the world of Mr. computer and all his little sensor BUDDIES! I bought my Daughter-in-laws F-bird a couple of yrs. ago and It's drove me crazy trying to figure things out that were beyond my yrs. of experience. This web site has been more than helpful to me and I have enjoyed it alot. Good luck with your problem and don't give up! There's no shortage Of ideas on these boards! Thanks to everbody that's helped me My baby is well again!

 

Vader...I've got a nasty random idle surge on my Mustang. And I just realized I have an MSD Blaster coil and a STOCK HEI module. I might just replace my module after reading that! I KNEW there was a reason I still cruise these boards!

 

Bad news guys. Put the stock coil back in and same problem. Idle races up to 2k! There was mention of emission devices coming in at 200 to 220F. Could someone give me a list and how to check them? I did check all vac hoses and can't find a leak.

 

Had a thought that I need help disecting. Could the hot coil have damaged the new ICM?

 

Here's a partial list of temperature dependent functions:

Coolant Temperature Sensor Related Parameters
BLM enabled between 50°C (122°F) and 140°C (284°F)
Cold spark advance disabled above 56°C (133°F)
Hot spark retard begins above 116°C (240°F)
Highway Mode spark advance > 59.8°C (140°F)
Knock sensor disabled below 66.5°C (152°F)
Power enrichment at base A/F ratios > 56°C (133°F)
Target IAC idle RPM >80°C (176°F)
IAC multiplier at 1.0 (base) > 32°C (90°F)
Knock Control enabled > 67°C (153°F)
EGR Duty-cycle enabled at 56°C (133°F)
EGR Duty-cycle at MAX >80°C (176°F)
TCC lockup enabled >50°C (122°F)
SHIFT light enabled >50°C (122°F)
Diagnostic communication enabled at 70°C (157°F)
DTC 43 enabled > 90°C (194°F)
Cooling fan #1 enabled at 107°C (226°F)
Cooling fan #1 off at 104.7°C (220.5°F)
Cooling fan #2 enabled at 115.2°C (239.5°F)
Cooling fan #2 off at 110°C (230°F)
Cooling fan duty cycle at 100% at 80°C (176°F)
Acceleration enrichment multiplier at 1.00 below 80° C (176°F)
Acceleration enrichment multiplier at 0.75 above 80° C (176°F)
Acceleration enrichment multiplier at 0.50 above 104°C (220°F)
Acceleration enrichment decay factor at 25% below 80° C (176°F)
Acceleration enrichment decay factor at 40% above 80° C (176°F)
Acceleration enrichment decay factor at 50% above 104° C (220°F)
Acceleration enrichment decay factor at 60% above 128° C (262°F)
Fuel limiting factor timer at 217 counts below 80° C (176°F)
Fuel limiting factor timer at 169 counts above 80° C (176°F)
Code 13 (oxygen sensor fault) enabled above 70°C (157°F)
Code 14 (CTS high fault) enabled above 130°C (266°F)
Code 32 (EGR fault) enabled above 30.5°C (87°F)
EVAP canister purge enabled above 70.3°C (158.5°F)
Hot closed-loop timer enabled above 70.3°C (158.5°F)
Rich/Lean O2 offest at 16 counts between 20°C (68°F) and 92°C (197.5°F)
A/C clutch disabled above 150°C (302°F)

 

Yikes! Which of these can cause idle problems?

 

Vader, You need to give up the intergalactic travel, and go to work in a automotive field somewhere on the Planet Earth! You sure could make alot more money working on ordinary earth cars than warp drive mechanisms! Besides, you can only go as far in space as your air supply will last!

 

try just unpluging the purge soleniod at the charcole canister, if the canister is loaded w/raw fuel and the ECM comands a vapor purge, the raw fuel will be sucked in, the ECM will see the O2 go rich and increase the IAC some to compensate, then the idle will shoot up, I have seen it several times before

 

Thanks Dave. I'll check it out. I'm glad to see that I might not have to worry about any matter/anti-matter reaction.

 

If unplugging the EVAP purge works, you have other problems. The ECM should not command an EVAP purge at any RPM near idle. If you hack out the program, the EVAP should be enables only above 1,500 RPM or so.

 

Exactly where is the EVAP? Is it present on the 1990+ V6 models?

 

Well..... I've disconnected the vapor storage system, solenoid and vacuum. No change. At a loss as to what to do next.