will fuel pump prime with 02 unplugged?

[irish44j]

Just doing my last checks before starting up the "new" M42 this weekend. Everything is hooked up EXCEPT the o2 sensor (I'm waiting for it to come in the mail).

In checking electricals (without trying to start it), with the car in the ignition position the fuel pump is not priming (or making any noise at all). Looking at the wiring diagram but not being an electrical guru, it looks like the fuel pump relay may not trigger unless the 02 sensor relay does?

Either that or the DME won't trigger the fuel pump relay if all the other sensors aren't registering.

Sound right?

Easy check would be if someone would unplug their o2 sensor at the socket below the battery tray, and then turn the ignition on and see if your fuel pump turns on or not....but I know that's asking a lot :)

[jlevie]

The fuel pump on an E30 only runs when the DME sees timing data from a rotating engine.

[bmwm42]

^ yup therefore it doesnt prime

[Dj Buttchug]

motronic needs to see a crank signal. When you key on engine off you should hear the pump prime the line.

M42 will run with no o2 sensor. It reverts back to base map.

[irish44j]

motronic needs to see a crank signal. When you key on engine off you should hear the pump prime the line.

M42 will run with no o2 sensor. It reverts back to base map.

That's what I thought. Doesn't do it though. hmm....

[irish44j]

Just jumped the relay and the pump was cranking, so at least I know my M10 to M42 wiring connections are correct. I could hear the fuel coming up into the rail and such...

Still not a sound when the key is turned - no prime, no nothing. Guess I'll just have to see if anything happens tomorrow when I crank the engine.

[ForcedFirebird]

The fuel pump on an E30 only runs when the DME sees timing data from a rotating engine.

Jim is correct. It wasn't until Motronic 3.x.x that the fuel pump primes prior to the key being in the "start" position. M42 runs on Motronic 1.7.

[irish44j]

Interesting....with the M10 and K-Jet, it would prime. (note that I'm still using the original dual-pump setup with the lifter pump, if that matters)

Ok, well I'll go with that being the reason then :)

[Dj Buttchug]

Jim is correct. It wasn't until Motronic 3.x.x that the fuel pump primes prior to the key being in the "start" position. M42 runs on Motronic 1.7.

then how come both my late model cars prime before the engine starts....?

[ForcedFirebird]

then how come both my late model cars prime before the engine starts....?

Late model as in 1990-91 with an m20? No, they do not. I have been hacking Motronic computers since 2008, and they do not prime. Sorry. Take your fuel line off the rail, turn the key in "on" and nothing will come out. Take a video to prove it wrong. The ECU will not kick the fuel relay until it receives signal from the crank sensor. Unplug your CPS and crank your car, nothing will come out of the fuel line, either.

You think jlevie is wrong too?

I work on e30's for a living, not going to say I'm never wrong, but no, Motronic didn't prime the pump with key on until the m50 (Motronic 3.x.x)

[Dj Buttchug]

late model with m42. im not trying to dick up this thread or anything but im also a certified tech... I can hear the fuel pump buzz and prime the line when I key on engine off.

unless this is the sound of something other than the fuel pump.... well sorry but its priming.

[bmwm42]

late model with m42. im not trying to dick up this thread or anything but im also a certified tech... I can hear the fuel pump buzz and prime the line when I key on engine off.

unless this is the sound of something other than the fuel pump.... well sorry but its priming.

Buttchug m42 doesn't prime bro trust me I've tried

[Dj Buttchug]

well fuck me. There is some random ass buzzing noise coming from under my car. Ive got issues.

[irish44j]

Just to follow up on this now that I have the car started and running with the swapped engine:

The fuel pump definitely primes. And since this car has a gutted interior, no soundproofing, etc.....I'm 100% certain of that fact.

Since I still have the original dual-pump setup in the car, I checked both - the lifter pump in the tank AND the high-pressure pump under the left rocker both prime when ignition is turned on (before cranking) - just like it did with the M10 in there.

The reason I didn't hear it priming earlier when I posted this is that I had one of the relays in the wrong slot.....so the main relay wasn't sending power to the fuel pump relay.

Once I fixed that, all works fine.

So not sure what the deal with yours is bmwM42.....maybe you just turn the key immediately before you hear it prime? Maybe you just have good soundproofing or a super-quiet pump back there. IDK?

[irish44j]

but here's the wierder part:

since I'm working on the car, i have the battery unhooked often. When I first hook up the battery and go to start.....the pump does NOT prime.

But once I've started the car and then turned it off, then if I turn ignition on again, the fuel pump DOES prime. I just went out and tested this multiple times, and it works every time.

So maybe the DME has some kind of memory to it, IDK. I don't much care, personally. If the car runs, it runs :)