Got my 750il all back together. Swapped in replacement cams, etc. Very, very sure timing is right (had the right tools) and there were no issues getting the job done. The oil bar had come off both sides, hence the cam replacement.

Anyway, its all ready to roll and I was so happy to get this project this far but it wont fire up. Cranks over and will sputter a few times if its been sitting. Then all I have is wet injectors. Spark was checked a few times and it good. Even changed to larger tipped copper plugs on one side. Some said platinum tips are small and can be an issue in this engine. Fuel pressure fine too.

What I found is constant power at injectors. Some 7v, some 12v. There should be no voltage. Torn apart most of the harness and found no major issue. Pulled relays, reset computer, pulled battery for a night, etc.

Can not find reason for voltage at injectors. Next I will pull the rails and see if they are indeed constantly firing (the 12v ones). Or its its just a stray voltage with no real amperage behind them - although that still would not explain why it floods so fast.

Note: When it floods there is no sputter and flooding happens fast. If I let it sit over night I will get a subtle sputter again then nothing. If I pull the injector connections and crank it, it will start to sputter again as the extra fuel is blown out. Rinse, repeat.

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1991 560 SEL / 185k miles
1992 750il / 17k miles - project car

Sounds like something to do with cam position sensors, could they go backwards?

There is no cam sensor per say, instead a sensor for both sides that loop over the spark wire for a specified cylinder. When spark occurs its detected and the car knows which cylinder is in sequence. Its a rather odd setup.

And being that there are literally two of everything in this engine, both sides would have to be bad for it to not run. The car is designed to run just 6 cylinders. There are even two computers to do this.

My problem affects both sides. Which is very odd and apparently very unique as I am getting to clues on the BMW forums.

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1991 560 SEL / 185k miles
1992 750il / 17k miles - project car

Copper plugs good, platinum bad. Might as well get all twelve the same. As for the injectors, they should have 12v when the key is on, the DME(s) ground the other wire when it wants to fire the injector. Not sure where 7v would come from, you might want to look into that.

MV

Im talking 12 at the connector itself. Not one lead to ground, etc. I know one side is always hot with 12v+. And that is the case. Problem is when on I get 12v across terminals unless I pull the computer for whichever bank I am testing.

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1991 560 SEL / 185k miles
1992 750il / 17k miles - project car

With the injector plugged in, you will have 12v on both sides, and since the injectors are grouped in threes (the DME fires three at a time), even if you unplug one, you'll have power feeding through the others. You'd have to unplug all of the grouped injectors to get an accurate reading. Not sure without looking at a wiring diagram if you can unplug the connector at the DME, since other wires will be included. Unplugging those might upset you readings (particularly if they go to the main relay which feeds the injectors).

Do you have a wiring diagram to look over?

MV

The intake plenums got taken off along with the injector rail right?
Could you have missed a ground or pinched a wire?
Or swapped the throttle position sensor wires side to side?

As you said, 2 of everything, what's swapped? It sounds to me like one thing's backwards.

Yes in the injector rails came off, then both intake plenum. But the harness is completely clear and shielded from any pinching.

There are no throttle position sensors. And it impossible to swap connectors unless I rip the harness apart.

__________________
1991 560 SEL / 185k miles
1992 750il / 17k miles - project car

Yes, all injectors are unplugged and wires inspected for damage. If I pull the DME, the voltage goes to 0 as expected. I even clipped a group ground and it lost power telling me wiring is okay from DME to connector. If it still had voltage after being clipped, there would be an issue with wires for sure. And like you said, they are grouped yet all show voltage issues.

This is a pain. I am going to pull the rails with injectors and see if they really fire non-stop.

__________________
1991 560 SEL / 185k miles
1992 750il / 17k miles - project car

Might be continuous with a stop in flow when they are supposed to be firing?
The computer's getting reverse or negative signal?
That's the only way this makes any sense to me.

Are they original injectors? I read somewhere the computer needs to know what injectors it's running.

Also read broken wire in the harness, check for continuity?

And it was running before?

Its suppose to be 0 volts with key on. The computer grounds a group of 4 injectors to pulse them. At that moment, there would be a reading.

Not sure how the computer could be getting a reverse signal. Especially both. And all the other car electronics would. But that will be something I will be testing anyway at this point.

Found one wire worn but no others. Just the housing was worn on one side. Enough to raise red flag but with the wire isolated nothing changed.

Yup, running before. Drove it right into my garage! Replaced cams and other top end work. No work on electronics, etc. Thats the frustrating part!

__________________
1991 560 SEL / 185k miles
1992 750il / 17k miles - project car

Does you harness look like this?

At the end of the injector tree, out of the 4 connectors, the 2 longer ones look like they could be swapped. In the pictures, the air flaps open in opposite ways, maybe they're wired mirrored?

Attached Thumbnails

 

Ok, so with the injectors all unplugged, one wire (red/white/yellow) should have 12v with key on. It does, correct? Injectors 1,3, and 5 (bank one, other side similar) should have a brown/white/yellow wire, while injectors 2,4, and 6 should have brown/yellow/white. Those wires should run to the DME (pins 32 and 3 of X6010 respectively), and with the injectors unplugged, should have 0v. You're saying that with the injectors unplgged, you have either 12v or 7v on those wires? Do you have the ability to remove those wires from the DME connector while being able to plug the rest of the DME connector back in?

Note also that the odd injectors also share their signal wire with the EML unit for that bank, that may be something to look into once you can isolate the wires from the DME.

Voltage to both sides of the injector will not cause the injector to fire, or do anything really. Power on one side, ground on the other will make a fuelly mess of things.

Do you have decent diagrams to work from?

MV

Yes to the first paragraph. And I did snip one ground at the DME itself to verify that the harness, at least on that one branch, was good. What I may do next is snip all the grounds to see if one ground is the sole issue somehow. Such as in some odd way its getting 12v back onto itself and contaminating the other grounds. if that makes sense.

Have diagrams now. Looking them over. It really must be something simple!

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1991 560 SEL / 185k miles
1992 750il / 17k miles - project car

Injectors probably have voltage to them constantly and the computer switches in the ground side of the circuit. You get weird voltage ramping and spikes if you switch on the positive side. Some of them might work this way, don't know.

I have thought of buying a 12 cyl BMW. If you had it to do over, would you?