Hey guys, I've been having problems with my 94 caprice 9c1 that i have turned into a hobby stock dirt track car. This is the first season that i have run the car and it starts and idles well cold but just about the time i go to pull onto the track the idle will start to surge and i will have to keep my foot on the brake and gas at the same time to keep it running. After pulling onto the track and keeping on the throttle the car will run great for the 5 lap trophy dash and the first 8 laps of the next race before it starts to cut out at high rpms under full throttle. By the 3rd and 4th races of the night the car starts to stumble at lower rpms as well with less than full throttle application. I have a mechanical temperature gauge with the sensor mounted in the passenger side block and the car runs between 180 and 200 degrees the whole time I'm racing. I have the electric fans just run to a switch that i kick on as soon as I'm on the track. I thought the idle problem may have been the idle air control but I replaced that and it didn't help. I then thought maybe i had an o2 sensor that was bad since it was only having the problem when warm (in closed loop) so i replaced the o2s with new delco ones and it didn't help either. In reading up on my problems in the forums I then thought it must be the optispark was giving out so i replaced it with a "new" one from oreillys and the car seemed to run worse. I then wired in the obd 1 port directly to the pcm and was able to read code 36 (faulty high resolution signal or extra low resolution) along with codes 11(malfunction indicator lamp), 77 (cooling fan relay control circuit), 78(blank), 95 96 (oil life and level lamp), and 97 (vss output circuit fault). All of these codes were expected since the dash is removed except for the high resolution signal being missing. I then tested the harness at the connection to the optispark and got 5 volts on the first 2 wires, 12v on the third wire, and continuity to ground on the 4th wire so i figured i must have gotten a bad part. So i took the car back apart and took the distributor back to oreillys to get my money back. I then looked at the reviews on the summit optispark and decided to give it a try. The car seemed to run good after installing the summit optispark and the code went away so i took it back to the track. The car ran better than ever for the first part of the night but still had idle issues once warmed up in gear. By the 8th lap of the second race the car again started missing under full throttle at high rpms. By the end of the third race it barely made it to the finish with any power. I replaced the plugs and wires when I bought the car and I replaced the coil when i replaced the optispark. I'm not sure where to go from here. I haven't found any vacuum leaks and the way the car only does it when its warm leads me to think it is something electrical building resistance with heat even after the engine is up to operating temperature. I am now thinking maybe the pcm or ignition control module is bad? I've tried to scan for codes but now it doesn't show a single trouble code in the pcm. What are your guys thoughts?

 

I also forgot to mention that i have tested the fuel pressure and it checks out fine

 

What exhaust are you running?

Try disconnecting the O2s and see of the problem goes away.

 

I cut the exhaust about two feet beyond the catalytic converters. Running stock manifolds. Tried unplugging o2 to put into open loop but problem still present. Also tried unplugging maf sensor with no change.

 

Those codes tell me your Chinese opti is crap. You need a delco or MSD opti. Also don't forget to take it apart and put loctite on everything.

 

Are delco optis made with American parts? I'm very doubtful of that.

 

I don't have any codes showing when I plug into the pcm now that I have the summit opti. Shouldn't the pcm show a code for the fan relays being removed still? I can still plug in to my computer and data log though so does this mean the pcm is good?

 

If disconnecting the o2 makes no difference, I'd look into the ignition side and steer away from fueling (assuming your fuel pressure is still good when its stumbling). ICM can have heat induced issues. Did you do the spacer mod yet?

 

I did the spacer mod on the ICM but it may have been too late for it. I have a new one coming in this week and hopefully this will fix the problem.

 

Most likely not, but time and time again AC Delco units have a proven track record of being better built than any other opti on the market. However, if the unit is not a vented(forced) one, that certainly does change it's reliability....no matter what the brand.

I'd also consider Delphi(seem to be harder to come by now) or MSD at this point as well. Anything else, don't bother. They will give you nothing but grief.

I think the AC Delco units are remans now, but I'd still take them any day over other brands. I've only owned two....and neither have failed. OEM unit pulled at around 95K...no trouble at all...replaced with a new AC Delco unit as I was installing a different cam.

 

Installed new icm today along with plugs and wires all acdelco brand. Still having idle issues only when warm. Not sure if the high rpm/full throttle stumble has been resolved or not since the car hasn't been to the track. Leaning towards just getting a delco cap and rotor since I pulled the original opti apart and the optical sensor was clean and Mitsubishi brand. Fuel pressure is constant with the surging issues. Still no codes. Any thoughts?

 

Listen, it's running different when warm versus cold. That is a closed loop/open loop issue. That has everything to do with your o2 sensors. Unplug them and leave the car running in open loop. Stop throwing parts at it until you do some 10 minute diagnostics, that are free. Let us know how it runs when you leave the o2's unplugged and you get t up to operating temperature.

 

I've hooked into it with my laptop to see that it is in closed loop long before it has idle issues. I've done the "10 minute" diagnostics that you speak of. I've also used my laptop to put it open loop with no improvement.

 

What do the spark plugs look like? Are you losing coolant?

 

I haven't lost any coolant. The plugs were a light Carmel color and all of them seemed to be gapped right.
Checked and rechecked all the plugs/wires to make sure routing/connections weren't an issue. All vacuum lines appear to be in good shape since I replaced the one to the throttle body. Going to try and test the fuel injectors today to see what the resistance is.

 

The last time I had something similiar to this happen, it was on a BMW. Turned out to be a cracked head. Only did it while hot. The heat expanded the metal enough to cause a crack in the head to expand and leak just a bit of coolant. But I also got a misfire code. Being as you're OBD1, your car doesn't have the same level of diagnostics ability. But, none of your plugs look steam cleaned either. Hmmm.

 

Tested all the injectors for resistance they all checked out right around 12 ohms. Did some more reading and came across eehack. I had been using scan9495 and it was not showing any error codes. Data logged with eehack and it showed a code for vss. The idle problem seems to be mostly when the car is in gear and it trys to idle down. Going to try and just set the in gear idle up a little bit and reflash using tuner pro and eehack for now. Won't be able to test high rpm problem until the 6th when I'm back at the track