Won't start, what am I overlooking?

[BlackVic_P71]

A few weeks ago on the way home from work, the fuel pump died, I have replaced the pump and it hasn't run since.
I put in the same pump and have good pressure at the rail, I have spark(even put in a new coil just in case the other one wasn't strong enough).
I've swapped relays and still it's not starting, fuel pump is turning on as it should

[kishy]

Air: nothing changed, your throttle blade is still set the same as it was before, so should be no issue here.

Fuel: 1/2: pump pressurizes the system, OK there. 2/2: are the injectors actually putting any fuel into the cylinders?
If fuel pump is priming, ECM is triggering the fuel pump relay, therefore the ECM power relay is working and the ECM is powering on.

Spark: 1/2: it sparks, so coil and mechanical parts good. 2/2 is the spark happening at the right time?

PIP is the only thing I can think of that could affect both failing to put fuel in and failing to spark at the right time. But even if both were way off but still happening I'd think it would at least fire and sputter a bit.

If the system is pressurized even if the pump has no power, the engine should start, then stall almost immediately. If it never did that, it does prime every time you turn key to run, and the system is still pressurized (40PSI+) then I don't see it being an issue with the pump or electrics feeding it.

[sly]

Check for power at the injectors. They should all have 12V with key on. The ECM is a low side trigger (meaning it connects the circuit to ground to pulse the injector). If the injectors don't have 12V, check the wire harness to the mess around the brake booster and make sure nothing is disconnected or otherwise messed up there.

[BlackVic_P71]

Yeah, injectors were the next step.
Gotta do some testing when the weather is clear.

LOUD EXHAUST IS FUN! AM I SHOUTING?

[BlackVic_P71]

I tried to start again this morning and same result, then I checked a few plugs and they didn't smell like any fuel had been in there.
I had a spare computer so I put that in and the car started right up, I guess there's a reason why I keep extra parts around.

[DerekTheGreat]

Hrm, I have yet to see a computer fail in one of these. I'm chasing gremlins in our Mark VII. Got it back from shop, said they replaced the TFI harness and that "the timing was waay off!" So they advanced it wayyy too much, I put it back at 10 BTDC without the spout connector and now it's got hesitation/miss at part throttle or really any throttle beyond 30%. As they had it, it was pinging but no dead spots. Idle is smooth as silk in or out of gear. Leads me to think TFI or PIP. Balls cold so I quit for today.

[kishy]

Yeah, that's really odd.
The ECM commands the fuel pump relay on, so if you had priming fuel pump, you had the ECM at least powering up. So if there's a main internal fuse or something, that shouldn't have been toast.

Electronics can and do fail, though. I wonder if any of our resident board-level-electronics-repair guys might want to take a stab at figuring out what failed.

[87gtVIC]

I have no idea what failed but you can open the computer up Tom and see if there is anything visibly bad in there. If so repair kits/part numbers are flowing around the corral.

[BlackVic_P71]

I have seen plenty of people who are looking to replace a computer being told that they never(or rarely) fail, I find it odd as well.
I don't know what happened, but the car didn't run with the old computer and does run with the new one...it may not make sense, but it's what happened.

[BlackVic_P71]

I have no idea what failed but you can open the computer up Tom and see if there is anything visibly bad in there. If so repair kits/part numbers are flowing around the corral.

I was hoping to find something melted or brown/burnt, but at first look I didn't see anything.

[87gtVIC]

Id also be curious to see if you swap the now labeled "bad" computer in and see if it runs. Maybe contacts just needed to be brushed up?

[sly]

The Mad Marquis was running wonky right before the ECM let the magic smoke out. Which reminds me... I need to update my for sale threads. I still have an 86/87 ECM I had as a spare.

[gadget73]

the ECM does rarely fail, but that doesn't mean they never fail. These things are pushing 30 years old at this point. I re-capped mine, they were leaking magic juice and starting to corrode the board.