will fuel pump prime with 02 unplugged?

[ForcedFirebird]

Learn something new every day. I don't work on the m42's often, but had always disconnected the battery when working in the bay or with the fuel system. They wouldn't prime unless the engine was rotating and never gave any more thought to it since the 6cyl cars never do prime.

[irish44j]

Interesting. You do know your DME retains power for several seconds after the car is shut off, though? If you listen to it out in the open, it will buzz for about 45sec after the key is off, so it does in fact retain residual power as you said it might. Does it still prime after the car being off for a few minutes? The e30 fuel pump is not supposed to prime - well using any Motronic 1.x, anyways.

Yeah, it will do it no matter how long the car has been off, provided that the battery is not unhooked (in which case it won't). I just went out and checked it again and it did it - and I haven't turned on the car since I made that vid yesterday.

IDK, whatever. Just an interesting piece of trivia either way, since I don't think it has an effect one way or the other.

[Dj Buttchug]

Here, I just made up a vid. As DJ and me both noticed....it won't prime if the battery has been unhooked, but if the car is started and shut off, and then ignition "on" is turned, the pump will prime for about 1/2 a second. 100% sure it's the pump.

So.....clearly the DME is activating the fuel pump relay (via the main relay, obviously) for a short time. Whether this is some kind of residual power or something else, who knows. Why it only primes for 1/2 a second, who knows.....

maybe it's not actually priming, but can't think of anything else it could be doing. In any case, the DME is definitely sending power to the fuel pump for a short burst....

video is uploading right now. Here's the link. If you wear headphones it's pretty obvious.

http://www.youtube.com/watch?v=35Y7y...ature=youtu.be

thank you. This is what I was trying to get at and this is exactly what all my m42 cars do.

[ForcedFirebird]

Interesting. You do know your DME retains power for several seconds after the car is shut off, though? If you listen to it out in the open, it will buzz for about 45sec after the key is off, so it does in fact retain residual power as you said it might. Does it still prime after the car being off for a few minutes? The e30 fuel pump is not supposed to prime - well using any Motronic 1.x, anyways.

[irish44j]

Here, I just made up a vid. As DJ and me both noticed....it won't prime if the battery has been unhooked, but if the car is started and shut off, and then ignition "on" is turned, the pump will prime for about 1/2 a second. 100% sure it's the pump.

So.....clearly the DME is activating the fuel pump relay (via the main relay, obviously) for a short time. Whether this is some kind of residual power or something else, who knows. Why it only primes for 1/2 a second, who knows.....

maybe it's not actually priming, but can't think of anything else it could be doing. In any case, the DME is definitely sending power to the fuel pump for a short burst....

video is uploading right now. Here's the link. If you wear headphones it's pretty obvious.

http://www.youtube.com/watch?v=35Y7ytLAGAE&feature=youtu.be

[irish44j]

You don't have a true m42 fuel system in your car like I do......we don't have dual fuel pumps.....but who cares if its running

I don't see why that would make any difference. The fuel pumps are still running off the M42 DME and M42 wiring harness/relays. My pump is getting power at exactly the same time as it would if it were an in-tank M42 later pump. The DME don't know what kind of pump is hooked up. So whether I have a 2-pump, stock M42, or aftermarket pump is entirely irrelevant to the conversation.

The fact that there's an electrical load on it remains constant, regardless of what is hooked up......

Again, this is all academic, so whatever. I'd go prove what it's doing....but it's really not worth the effort and getting fuel all over the place, lol.

[Ryann]

Fil is correct. There is a single pump in an original m42 car, and it NEVER PRIMES. The DME will not energize the fuel pump relay until it sees a good CPS signal, which it does not see until the engine is cranking.

[Dj Buttchug]

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 :)

this is what i think mine does... ???

[bmwm42]

You don't have a true m42 fuel system in your car like I do......we don't have dual fuel pumps.....but who cares if its running

[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 :)

[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?

[Dj Buttchug]

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

[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]

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.

[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)