318i Intermittent No Start Issue, fuel related

[rallyvvagon]

Hi! A couple months ago I bought a 1984 318i 5 speed, and recently I've been having a very specific no start issue that I can't figure out. I'll start it in the morning on my way to work, and it fires up with no issues. But when i come back to start it again after sitting for a couple hours, it will crank but wont fire at all, like it has no fuel pressure. The only way to get it running again is by pulling the fuel pump relay, bridging the 87 and 30 slots with a wire and then starting it, at which point it will run but then die after touching the throttle, and then plugging the relay back in. After doing that, it will start up again without any issues. it only ever happens if its been sitting for less than 12 hours, because it always starts up pretty much immediately if I've let it sit for a day or two. additionally, it doesn't have any issues while driving, no bogging, fuel cut, etc.

When I first got the car in June I pulled the engine and replaced all the vacuum lines, fuel lines, filter, fuel injectors, MAF sensor, plus plenty of other things unrelated to the fuel system. And since this issue started about a month ago, I've replaced the fuel pressure regulator, fuel pump and fuel pump relay. At this point I'm fairly certain it's something electrical, but have no idea what it could be beyond that. Any help would be much appreciated!

[Sunnyledge]

Need to confirm whether or not there there is spark when you experience your no start. There are several things that can affect fuel delivery causing a no start. If the fuel relay is not getting a pulse from the distributor telling it that the engine is turning over the fuel pump won’t run. But it will if you jumper it. The thermo time sensor can also affect whether the cold start valve works. Those two come to mind off the top of my head.

[Sunnyledge]

This is a good checklist. Hopefully you can read it.

[packratbimmer]

Car will not work with a bad coolant temp sensor. 1984 L-Jet is different from 1985.

[rallyvvagon]

Thank you guys! there is consistent spark, I pulled one of the plugs and grounded it while cranking to make sure. The distributor and rotor, along with plugs and wires were also replaced when I pulled the engine. How would I check to see if the fuel pump relay is getting a signal from the distributor? I'll also check the coolant temp sensor and connector, although I think the issue is inconsistent enough that it isn't that.

[Sunnyledge]

The 7 pin fuel pump relay is a little more complex then most. Terminal 30 is alway on and should show battery voltage. Terminals 87 is output for the fuel pump and 87b for injector control. Terminal 15 powers the relay from the ignition and should show battery voltage when the ignition is in the on position. Terminal 31 is the ground and so you should see continuity with an ohm meter. Terminal 50 is a signal from the starter so you should see battery voltage when the key is turned to activate the starter. Terminal 1 is pulse input from the distributor. I think you just check for continuity from the negative terminal of the coil to terminal 1 (obviously with the ignition off).

[packratbimmer]

The coolant temp sensor will work intermittently. I had a 1984 318i car towed home one day and it started after it cooed down. It consistently cut out when warm and started perfectly when cold. I suspect the sensor completes a resistance circuit that is essential for the primary functionality of the ECU. The "problem" was corrected for the 1985 318i by design. A little trivia here: in my experience, a 1985 ECU will start and run a 1984, but a 1984 ECU will not start and run a 1985.