Hello all!

I have a 96 SS with original engine and OBDII PCM. I have a problem with the engine stalling and will not restart when driving in warmer weather. When the engine stalls, the only way to get it restarted is to let it cool down. The engine temp is not high when this happens. It seems the fans do keep the coolant from going too far over the third hash on the temperature Guage. I also get code P0335 when this issue occurs.

Now what I suspect is happening is the PCM board has a crack that causes the ignition to fail when the PCM gets to a certain temperature. I was able to determine this by trial and error. I was able to replicate the condition by running the engine until the PCM became warm to the touch. I shut down the engine and it failed to restart (it will just turn over). When the engine failed to restart, I put a portable fan over the PCM for 5 minutes to cool it down and the engine fired up.

I have devised a temporary workaround by wedging a DC fan with thermostat control probe connected to the PCM that will kick on when the probe senses the PCM shell reaches a predetermined temperature I set with a thermostat dial. The fan will kick on and cool the PCM to help minimize the issue.

However, I find blowing hot air on the PCM can drain the battery over time, so it may be time to replace the PCM.

This is where my question comes in. I am shopping for a replacement PCM and found one place online that will sell the PCM and flash it with the VIN number. My car is a 6 speed so I assume no transmission, gear ratio or wheel size is required? I also see some comments online that the PCM programming should also include the security codes for the theft deterrent system. Also is mileage info supposed to be programmed? There are also programming tweaks that can be added (fan temp setting, rev limiter, etc.).

Would I be OK to just go with the PCM with the VIN programmed? Or is there a better source to obtain the PCM and will provide proper programming that is needed?

Thanks!

Richard...

Assuming your analysis is correct, and the PCM has heat soak problems, a correct part # 96/97 PCM programmed with your VIN, SS program differences if any (I honestly do not know) and T56 program you will be OK. If you are still running the stock 3.42:1 rear axle and the stock 275/40-17 tires you're OK. I good PCM programmer/tuner typically has a programming data sheet that would ask these questions.

A common tweak on a relatively stock engine can pick up 10-15 HP simply by leaning out the power enrichment (PE) mode target (roughly WOT) A/F ratio. Stock tends to be too rich and leaner will make a bit more power. What temperature t’stat are you running? Swapping a 160°F t’stat, and lowering the fan on/off temps can pick up a few HP.

The “theft deterrent codes” (assuming you mean the PASS-Key II (VATS) key pellet resistance) is not controlled or read by the PCM. It connects to the body control module (BCM). Or did I misinterpret your comment? There is a diagnostic code set in the PCM if it does not receive the fuel enable signal from the BCM, but there is nothing to set in the PCM, other than turn the signal diagnostic requirement off or on.

Curious about the “thermostat control probe”. Is that simply an additional coolant temp sensor screwed into an available port (e.g. - passenger side head between #7 and #8 plugs)? Or is this some sort of sensor integrated into the thermostat?

Hey Injuneer!

Thanks for the info! This is helpful. I am dealing with a sales rep at one of these online PCM stores who seems to not know what part number to quote for a price. I informed him the original part number was 16214399 which was replaced with part number 88963801. On their web site they show the cost for 16214399 is $349 with programming but he quoted me a price of $579.99. So I may need to look for another source and someone who can program the module. If you have any suggestions, please PM me.

Regarding your comments. This car is stock so it has the original wheels and rear end. The tire profile is a little smaller (245 vs 275). I have not looked at the thermostat but it appears to be stock 185.

Regarding the thermostat/fan with probe, I am attaching a picture. The probe is attached to the top of the PCM case.

Thanks again!

Richard...

Is your SS a convertible? I don't think GM put 17” wheels with 275/40-17 tires on the convertible, only the coupe. The 16” tires on the convertible were 245/50-16. But both tires have the same outside diameter (~25.6””), and that's what determines the speedometer calibration.

I don't know the PCM part #, but I believe there should be anther number on a sticker that reflects the programming.

PCM failure is a fairly rare problem, so I don't remember too many discussion about them.

I guess the “thermostat” switch is actually a stand-alone “thermostatic” switch.

Yup! I failed to look at the wheel size - the car is a convertible and the wheel size clearly shows 16.

I got a response from the online seller regarding the part numbers. He confirmed my research and is now quoting the $379 price. I think it may be worth the risk to try the rebuilt PCM. I will look for the part number to confirm before I buy.

The thermostat switch is stand alone. I bought the fan and thermostat online. Pretty loud when it kicks on - lol.

Thanks again!

Have you bought it?

Yup I bought it and installed it. I ran the car at idle for 20 minutes in 90 degree heat until the temp guage approached the first red line. I then turned A/C on and brought temperature down to normal (mid mark). I ran it an additional 10 minutes and shut the car off and it started up fine. So next step is for me to drive it around where it will be OK if I broke down. So fingers crossed. I will take it out some time this week.

Best of luck

Well the issue remains. Ran it for 30 minutes on the streets around my neighborhood but when I pulled into the driveway, it stalled and would not restart. After sitting for an hour it restarted. I removed the cooling fan when I installed the new ECM. Last thing I can try before moving on to Optispark is to replace Ignition Control Module. Ordered AC Delco ICM today so we will see what happens next after swapping ICM.

I didn't anywhere if you checked to see when the car dies, and you are trying to restart it, is the tech showing any rpm's while cranking?

I do try to restart after it stalls. Most times it will restart briefly but dies within one to a few seconds. I do see the tachometer needle go up a couple hundred rpm when restarting. Once it dies it will not start - it will only turn over.

does the tachometer needle go up a couple hundred rpm, when it won't start?

That is correct - when it turns over I do see the RPM jump from 0 to a couple of hundred RPM. What are you thinking?

That means the PCM is receiving the low resolution pulses from the Opti. That's what the PCM uses to drive the tachometer.

Thanks Injuneer for that info. I replaced the ICM this evening. I made sure to use the grease on the back of the new part. I bought the ACDelco part ($130). I pulled the old one off which also had the grease applied but the old part does not look like ACDelco. I did clean the grease off to check part numbers.




I will again run the road tests to see what happens. If it dies again it looks like I will need to change the opti and wiring harness. Hope it does not come to that as that will be expensive and beyond my capabilities.