I've been fighting with this stupid PCM for a couple months now and not sure where to go from here. The car is a 95 with LT4 hot cam, LT headers, high flow cats, AFR 190 heads, 52mm trottle body, and SLP intake. I also have LT1 edit and a WB O2 sensor. Now for the problem - the car fires right up cold or hot and idles fine with slightly rich mixture, just where I wanted it. I start to drive off and get maybe 1/4 mile and the mixture goes extremely lean, like 20 plus. It's like the computer just shuts down the injectors. The car spits and sputters but doesn't die. If I push the throttle past 60% for WOT to kick in the mixture goes right where it's supposed to, around 12.5-13.0. But then when I let off it goes lean again. However, about 1/2 to 1 mile later when the car is good and warm the mixture straightens out and goes to 14.5-15.0 where it's supposed to be in closed loop. I have let the car idle and go into closed loop and it still does it, so I don't think it has to do with closed loop or open loop. It appears to be more related to temp. Also, the O2 sensors are new and look normal on a scan tool. I was thinking temp sensor, but it also appears normal on a scan tool. I'm now leaning toward replacing the PCM, but thought I would check with the resident experts here before I just started throwing money at it.

Thanks for your help guys.

 

You probably have your coolant to fuel mixture/tables messed up.

I would go back to the stock tune, and work it out from there. You can insert your timing table you have worked out so far into the stock tune, and then adjust your part throttle fuel from there.

In other words your starting from a good known tune, that shouldn't have another other parameters your not aware of screwed up.

 

Thanks for the reply ski. However, I put the factory program in this morning and it is still doing the same thing. Really frustrating, because I don't think it is the program, but I don't see any indication of a hardware problem either.

 

Have you checked the fuel pressure with a known good gauge going down the road. Monitor the pressure when this happens.

Also what are the stock O2 sensors doing? Do you have a datalog to look at during the event? I have seen stock O2s drive the system full lean because they are bad. I had one tell the PCM that it was lean and that caused one bank to go full rich. Double check your O2 sensors and see what they are doing. The PCM uses the O2s as a final control element, if they tell the PCM its rich the PCM will drive the trims lean and in extreme cases it will drive it to the lean limit.

Also check the sensor connectors for any damage or contamination. The sensors breath throught the wires and the connectors to get its reference air, if it can't breath then the signals are skewed. Another thing to check is all your grounds. Use an ohm meter to check all your PCM grounds from the connector to battery negative. It should be well below 4 ohms. Check the rear engine ground above the left (drivers) side cat as well. Grounding is very important in these cars.

 

Sure your temp sensor is reading right? Might be flaking out, sending a bad signal to the ECM, which is dumping in a ton of fuel or pulling it out.

Just a thought

 

Thanks guys I appreciate the suggestions. I'm about to go out of town for the next week or so, but when I get back I will go through and double check everything again. I have checked the fuel pressure and it is rock solid before, during and after its psycotic episode. Also, as I said previously the O2 sensors are new, I replaced them not to long after the problem started. The grounding issue very well could be a problem as well as the temp sensor. I will check it out and let you know.
Thanks