No Start

I'm working on a 2000 Impala with the 3.8L engine. The engine won't start. We started diagnosing it and found a bad fuel pressure regulator (replaced it) and then notice it had coolant coming from the TBI. Knowing the 3.1L/3.4L/3.8L are notorious for bad intake manifold gaskets, we tore in and replaced them (yes, they we're cracked like usual). After we pressurized the coolant system (post gasket replacement) we found the upper intake (phenolic) was broke. Replaced it (to the tune of $298.00). That's the long story. We know it dumped/pumped a lot of coolant through the system (pulled half a coffee can of coolant from between the upper and lower intake manifolds), but we still can't get the thing to run. We checked and it doesn't have any engine error codes. We have good spark (25kV), we have gas (noid lights say injectors are firing AND we got wet plugs - gas wet, not coolant wet), the thing won't run. Replaced the ICM (Ignition control), won't start. Replaced the PCM (computer), won't even crank! Engine error code on new PCM says security code mismatch P1631. Questions: How do you reset the security system. Old (original) PCM has security light flashing. "New" PCM has security code. Any clues as to why this thing won't run? HELP!

For starters did you program the new PCM?

Did you perform the VTD relearn procedure?

here is the circuit description for the P1631
The PCM controls fuel injector and starter operation based on a vehicle theft deterrent (VTD) password from the vehicle body control module (BCM). When the ignition is first turned ON, the BCM sends a programmed VTD password to the PCM. The PCM acknowledges the password and responds to the BCM so that normal fuel injector and starter operation will continue. If the PCM detects an incorrect password (VTD system failure or attempted vehicle theft), the engine will not start or crank as long as the condition is present and DTC P1631 will set.

Well, I'm not sure what you mean by "program" the new PCM...BUT, with the "new" (junkyard) computer, we performed the procedure where you turn the key on for 10 min, then off for 5 sec, then repeat 2 more times (supposedly this resets the VTM and clears the DTC). Still nothing. No crank. Put the "old" (original) computer in and she'll crank all day, plenty of gas (noid light every injector and can smell it) and good spark (18-25kV on all cylinders). Compression on the front bank is 165-175 psi on all cylinders. It really seems like the system is out of time. If you crank briefly and let off it kinda backfires or tries to fire. What affects the timing? If it had a distributer I know what I'm doing. The crank doesn't even have a mark that we can view! We installed new crank sensor last night, no luck. Scan tool says that Cam sensor signal is present. Next step we may try a BCM and possibly a different PCM. I am stumped (and more than a little frustrated)!

PCMs are vehicle general (1 part number will fit many cars) BUT..the internal calibration is vehicle specific.....Swapping a PCM between vehicles with out programming the PCM to the vehicle will only cause you a ton of headaches...Plus the fact that GM PCMs are covered under a 8year/80k warranty.

Put the original PCM back in the car, perform the VTD relearn, remove all the plugs and diconnect the injector harness...spin the engine over a few times to blow out the liquid in the cylinders, install new plugs and then try to start the engine.

I see where you're going with this but I am still curious. With the original PCM, the engine turns over and has fuel (and no error codes - i.e. NO P1631). I was under the impression the the VTD relearn was only if the security feature was enabled, and if it was, the crank relay would be disabled and the fuel system would not function. Do I still need to do the 10 min key on 5 sec key off procedure with the original PCM if the car cranks? Also, I'm still concerned about the timing. It just seems like we aren't firing at the correct time, like on the exhaust stroke or something...what controls the timing? ICM, PCM, BCM???

I see where you're going with this but I am still curious. With the original PCM, the engine turns over and has fuel (and no error codes - i.e. NO P1631). I was under the impression the the VTD relearn was only if the security feature was enabled, and if it was, the crank relay would be disabled and the fuel system would not function. Do I still need to do the 10 min key on 5 sec key off procedure with the original PCM if the car cranks? Also, I'm still concerned about the timing. It just seems like we aren't firing at the correct time, like on the exhaust stroke or something...what controls the timing? ICM, PCM, BCM???

Even though the PCM has been swapped in and out, it is a good practice to perform the VTD to verify the theft system is working properly.
Since in your erlier post, you stated that there was a large amount of coolant in the intake, some of that gets into the cylinders...removing the plugs, disconnecting the injectors and spinning the engine over a few times will force out the remaining coolant.
A small amount of coolant on the plug tip will prevent the spark from travelling...therefore you get a buildup of gas on the plugs.
So it may not be that there is a timing problem, but the engine is just flooded.
This is common on that engine to have a difficult time starting after the upper intake has been replaced for coolant intrusion...sometimes you will have to remove the plugs and clean them several times before the engine will start and run.

Thanks for your help. We will try new plugs. FYI, we have had the plugs out and cranked it over to see if we got spark in a pattern (1, 3, 2 etc.) or all at once and the plugs fired nice white/blue spark. Also, we have let the thing sit for 24 hours at a time 3 nights in a row and tried it so I would have though if it's flooded it would have cleared by then. I won't take long to pull the plugs and turn her over then put new plugs in so it's worth a try. I'll let you know how it goes. Thanks again!

Well your suggestions paid off, pulled all the plugs last night. Removed ignition and fuel relays and cranked it over several times. Installed new plugs and she fired right up! Man what a nightmare, thanks a ton for your help. I would have felt terrible if I would have had to send him to GM after all the work we did just to have them install plugs. Once again, thanks for your help!

Glad to be of help...Good Luck