ran into a problem on my project '60 Cadillac with a 2006 5.3 swap. it has worked fine up till now. went to start it an cranks fine but no start. has fuel pressure but noticed that the MIL light does not come on. played with everything and once in a while the MIL light comes on and it will fire but not keep running and when it dies the MIL light does not come back on. thinking the PCM is dying and not waking up or staying awake. any ideas

Check the fuses and grounds, did you de wire a factory harness, if so I would check the pins at the computer .

yes it is a modified factory harness. I did all the work so if there is a problem there it is on me. all fuses are good. power to the coils etc when keyed on. I have not driven it yet as it is a project that is nearing the end. I have started it a bunch of times and let it run so I know it did work. every time the MIL comes on with key on and engine off. now it only comes on once in a while and that is the only time it fires up. engine only has 65k miles but was in a pretty good wreck (hit front and rear) but did not reach the engine. I assume there is an internal relay that wakes up the PCM when it sees the key on? thinking that may be dying, that or a voltage regulator in the PCM?

Haven't had a computer die like that before but I have had the pins at the computer plug slightly back out and fail to make contact. I use a vom to check the wire at the plug for input ( 12v key on ) and output (5v sensor power). Can you look at the data stream with whatever tuning software you're using? I haven't looked at a wiring diagram but I would be looking for wires in common that are acting up ( voltage to check light and other sensors)

scanner says "no communication" which also tells me PCM is not waking up. cant say that a pin did not back out or some similar issue. guess I will have to pull the inner fender out so I have good access to the PCM and plugs.

Do you have 12v to the aldl plug?

Does it have fuel pressure when cranking and no start condition? Is the fuel pump relay triggered from the computer?

spent all day with an uncooperative '39 Ford Coupe and just checked a couple things before leaving the shop. have 12v and good ground to the aldl plug and 12v to coils but still no waking up of the PCM. hard to tell on the fuel pressure as by the time i put the gauge on it lit up the MIL light and tried to start but then nothing else and at that point it had 58 psi and held steady as there was no injector pulse after that. yes pump relay runs off the PCM. looks like this weekend I am going to pull the inner fender to get good access to the PCM and plugs to make sure a pin did not back out. this is the car

If the computer is triggering the fuel pump, and you have fuel pressure, then I would say the computer is "awake". Do you get an RPM reading from the computer while cranking? I once had a similar issue with my 5.3L swapped jeep wrangler, Sometimes it would start, sometimes it would just crank. Turned out to be a bad connection at the MAP sensor,
If your coolant temp sensor is unplugged or not connected properly it will also prevent the motor from starting.

only time i know I got fuel pressure was the one time the MIL came on and it started. all the rest of the time no MIL and can not connect to the PCM so no way to tell on rpm. all sensors are hooked up and it was running a couple weeks ago and nothing was touched in between so the problem is new and came up while sitting with the battery disconnected. that is what annoys me.