Bosch fuel system in a P38

discostew

Well-known member
Sep 14, 2010
7,706
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Northern Illinois
I'm hoping somebody here has seen this. Working on this truck for a friend and it's got a strange problem I haven't seen before. It's a 2000 P38 with Bosch Engine management.

Truck runs out of gas when the gauge says 1/2 tank. I ran the pump with a jumper until it starved at 1/2 tank. Pulled the tank and it looks to me like as long as the fuel level is above the white housing in the pics the pump is fine. But once the fuel is below that white housing, the pump can't pull fuel off the bottom of the tank. I'm hoping some BMW guys might have seen seen his before.

The fuel pressure regulator dumps off its extra fuel into the same area the fuel pump inlet sits. So could the regulator be the problem? It's replaceable .
 

squirt

Well-known member
Nov 13, 2008
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Los Angeles
I'm more familiar with the actual BMW setup, but that's your fuel bucket. The idea is that this bucket stays full and feeds the pump regardless of how fuel is sloshing around in the tank. There should be a 1-way valve setup at the bottom of the bucket that allows fuel in but not out. It sounds like that valve is not working on this truck, if it's starving after the fuel can no longer spill in over the top. Depending upon your level of investment, you can replace/fix the one-way valve, or simply drill holes in the bottom of the bucket to allow fuel in and out - for 90% of users there would be no noticeable difference.
 

discostew

Well-known member
Sep 14, 2010
7,706
1,015
Northern Illinois
I'm with you on all that. But I'll post another pic of the bottom of the canister. I've also heard it called a swirl canister. Looking in the bottom of it I see the flapper valve your talking about. And truck not running I think your right that the fuel around the canister would slowly reach equal level inside the canister. But I think when the truck is running it needs the fuel coming out of the regulator to maintain enough fuel to run under load. The black tube in the picture is from the regulator, the hole opposite it is the passage into the canister. Look at how restricted the fuel flow would be thru all that plastic.
 

discostew

Well-known member
Sep 14, 2010
7,706
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Northern Illinois
My fix is going to be a genuine Bosch pump assembly for $246 from CARiD. Land Rover pump I was quoted $780. I figure it's not my truck and it's been starved about 100 times.
 

squirt

Well-known member
Nov 13, 2008
824
13
Los Angeles
You could be right. Can you observe the behavior of the fuel in the bucket by running the truck with the cover ajar? (sorry for my ignorance on P38 fuel delivery) Or run it in a 5 gallon pail with the outlet dumping into the pail, outside the pump bucket, to check the return behavior and see if the pump eventually gets starved.

I think you're on the right track, but I've been unable to think of a way that a regulator failure would cause starvation without other noticeable side-effects, like abnormal rail pressure or drivability issues regardless of tank level. Assuming the pump is running within spec, where is the return fuel going?

For what it's worth, I eliminated the bucket on my 335i when I swapped in a higher capacity pump, and have had no issue with it, even when trying to challenge it under hard launch and cornering with <1/4 tank of fuel.
 

discostew

Well-known member
Sep 14, 2010
7,706
1,015
Northern Illinois
I was thinking about doing some of what our saying. Like run it in a bucket. But the thing has been run out of gas so many times it's gotta be close to done. If it was mine I would have drilled a hole in the side of that swirl bucket and sold the bitch.

And like you said about the no drivability issue till it gets to that level. You would think you would feel something from a pump that was that weak.