In need of assistance on a no-start.

[molly_watah]

I've posted before about this same car, and now that my semester is near over, I'll be spending the entirety of my winter break getting this damn car on the road.

I picked up a US Spec '84 633 5 Speed off craigslist just over a year ago. Car was a non-runner. The guy I bought it from bought it from an estate where the car had been sitting in a garage for almost 13 years. He tried and couldn't get it to run.

Here's where i'm at now.
The car will crank and start for 2 or 3 seconds and then immediately die. It will do this all day long.
However, one three separate occasions, it's started and ran like a champ. It will start with no issue, for a whole day, and then the next day, nothing. The days were all different (one day was damp, one day was ~75 degrees, the other ~80 degrees, so I don't see any pattern there), and it's been entirely random when she does it.
Once I do get it to start, it runs fine for a few minutes until it warms up, at which point the idle bounces between 800-1400RPM. It never dies, just lets the idle bounce.

What I can tell so far:

I'm getting adequate fuel pressure at all times (it never drops). I also had the injectors rebuilt.
I seem to always have spark. (Did new distributor, alternator, plugs, wires, cap and rotor, new coil, etc)
I've tried 3 ECMs. Determined the original was bad, and the other two haven't given me any issue (ran for a whole day on one twice and the other once- both came from running cars, so it shouldn't be causing anything).
Speed and Reference sensors have been replaced and are certainly good.

So bigcoupe, I need a hand. The car has spark, it has fuel pressure, so why won't it run? I've gone through just about every no-start e24 guide on the web, and nothing is poking its head at me. Any advice I can get would be much appreciated.

[BUELLRIDER]

Have you tried shooting starting fluid in to see if it runs with that? Maybe bad old gas, dirty fuel filter, vacuum leak , bad relays . Make sure the oil dip stick is in TIGHT. It will run like crap if it is not sealed Tight.

[molly_watah]

It runs on starting fluid. I changed the fuel filter, and pumped out some gas and it is fresh and good. No water in the tank either. I haven't been able to find a vacuum lea. I'll have to check the dipstick

[JoelR]

Once I do get it to start, it runs fine for a few minutes until it warms up, at which point the idle bounces between 800-1400RPM. It never dies, just lets the idle bounce.

I cured the idle bounce on my car by adjusting the Throttle Position Sensor (TPS) properly.

[slofut]

It runs on starting fluid. I changed the fuel filter, and pumped out some gas and it is fresh and good. No water in the tank either. I haven't been able to find a vacuum lea. I'll have to check the dipstick

Sock filter on the bottom of the in tank pre-pump (if it's dirty then you have evidence of tank contamination), pre-pump itself, main filter, then there's a tiny filter in the inlet side of the main pump that gets overlooked, even though you have pressure it wont maintain flow, main pump.
If it runs on starting fluid you're not getting fuel supply, no magic involved. Have to start at the beginning with the tank and make SURE all is in order. Pull the pre-pump and look well (and CAREFULLY) at the bottom of the tank. Then proceed forward.

[Ralph in Socal]

Fuel pump relay? Not just the relay but check that the contact slots that the relay prongs go into are all solid in the fuse box. I've seen where one of the contacts had fallen out of it's secured location and was barely touching the relay prong.

Also check the connections at the fuel pumps to make sure you have good solid contact. Good luck.

Ralph