The MJ has been sitting for 3 months. I'm buying it from a good friend and a very honest person and he tells me it ran great but was "hard starting". It could be that something was on the way out.
Ive been told that if you have spark then the ECU is good, I'm leaving it to the last thing to try. I have a "known good" RENIX ECU that was given to me by our local Jeep dealer, it was their test ECU.
The battery is brand new, I cleaned the grounds, rechecked the index on the new distributor.
What I'm confused by is that I have fuel pressure with the key in the "run" position but nothing in the "crank" position. If I prime the TB it will fire and run for a few seconds but it cuts out before the fuel pressure comes up on the rail. The fuel pressure is 0 until it cuts out no matter how long it runs(I tried keeping it running for 30 seconds or so one time and no fuel pressure until the engine cut out)
I'm still leaning towards the starter relay but I will test the wiring/connectors as well.
It has been a long time since I had a starter relay apart, but if I remember correctly the solenoid and the fuel pump connectors are joined by a copper strip inside the relay, basically the same connection. If the starter solenoid is working, the power should be getting to the fuel pump (BAL) outlet (spade connector) on the relay. Easy enough to test for voltage at the "BAL" connector on the starter relay while your cranking and see if you are getting any power *into* the fuel pump circuit. Whether the power is making it all the way to the pump is another question.
The ECU closes the fuel pump relay for a few seconds when the key is turned to run on the way to start, this primes the fuel rail. Then when the starter relay closes, this supplies power to the fuel pump, once the CPS sends a pulse signal the ECU starts spark and closes the fuel pump relay. I'm not exactly sure when the fuel pump relay closes, likely an RPM thing.
Could be as simple as a partially plugged fuel filter. Be careful of the fuel filter, the fuel may siphon on when it is disconnected. The filter also often comes off of the hoses hard and is a good spot to get a face full of fuel. Empty the fuel filter out the inlet end and see what comes out. If chunks of rubber come out it is a sign the hoses are deteriorating on the inside.
You may be getting some pressure, but very little volume.
I had a 87 with a fried connector, it tested 12 +/- volts (or available voltage) while I cranked, but didn't have enough amperes to run the pump very well. Only seen it once, it drove me nuts.
Just for the heck of it, turn the key to run then off, then run then off, about three or four times. This gets you a good prime to the fuel rail. Some Renix loose prime in the fuel rail pretty fast because of a leaky check valve at the pump. Though extended cranking will usually allow enough fuel to get pumped up to the fuel rail for a start, even if the check valve is leaking, it just takes longer cranking to start.