Fuel delivery problem

[Raven Tooth]

hi everyone, just bought an 89 325i and i'm having a weird problem. the car will crank but won't start unless I disconnect fuse #11 (fuel pump) and give it full throttle. it will idle just fine once it burns off the excess fuel. as soon as i reconnect fuse #11 the car cuts out. i can rev the engine all the way up with no problems when in neutral but if i try to drive it with fuse #11 removed the car starts to stall. If i hold the throttle at about 1500 rpm and reconnect the fuse then i can drive and accelerate with no problems but it will cut out once i come back to idle. I have already replaced the injectors (an erroneous diagram said that fuse #11 was the the fuel injector electronics, which led me to believe leaking injectors were causing the problem), and the guy i bought it from said he changed the plugs, cap and rotor. any help would be greatly appreciated.

[Raven Tooth]

forgot one thing, my friend has an 88 325i and when we disconnect fuse #11 on his car the engine dies after a few seconds. mine keeps running which is a bit strange

[jlevie]

It sounds like fuel rail pressure is too high that could be from a bad FPR or a blocked fuel return line.

[Raven Tooth]

so after borrowing a fuel pressure gauge, i was getting 70 psi from the pump when cranking, swapped out the pressure regulator with a known good one and no change, disconnected the return line from the regulator and finally got normal pressure (about 40 psi when cranking). So i have a pinched or clogged return line. with the return line disconnected, I'm still getting a problem with the motor flooding when cranking and fouling out the plugs. Possible problems i'm thinking is the mass airflow sensor, incorrect injection timing from the dme (could it have adapted to compensate for the high fuel pressure so much that now it floods with normal pressure?), coolant temp sensor? o2 sensor? Is 70 psi enough pressure to damage the new injectors and make them leak? any ideas would be greatly appreciated

[jlevie]

70psi should not damage the injectors, but it would probably flood the engine.

[treff-7]

the engine still runs the same way with normal fuel pressure? runs for a while with fuse #11 removed then stalls?
if your timing is off (which should not be possible) it should not run at all or run with the fuse in.
the oxygen sensor should not cause the engine to fail. i think if it sends crazy data the computer ignores it. but you can also unplug it and the engine should still run fine. it only burns a bit more fuel.
what happens if you try to start the engine with full throttle? does it start?
i keep thinking and you should have someone check the signal going to the injectors or unplug one of them and connect it to a used one you pulled out and hold it in your hand while someone cranks the engine. if you hear/feel it clicking it's fine. also you might be able to feel it with your hand when it's in place but i'm not sure.

[Raven Tooth]

We pretty much think it must be a blocked fuel return line now. We go an extra piece of hose which we ran from the fuel pressure regulator fuel return line into a bucket, and the engine seems to run fine. At least at idle. The more we try to experiment, the quicker the engine gets flooded to the point where it won't start and we need to let it dry out over night.

Now we're trying to find out where the blockage is. We think its either in the metal fuel return line under the car, or somehow in the fuel tank. Whats the best way to test this and find the exact area of blockage? When we connected the engine into the line, and disconnected the line right before it goes back into the fuel tank (letting the fuel drain into a pan) it seem to spray out of there fine. But I don't know what the pressure is. Maybe it's still too high. Is the fuel return line supposed to be under any pressure? When we set up the gauge out of the engine into the bucket, the pressure reading is 0psi. But the engine runs well.

[Raven Tooth]

Also, when looking into the gas tank through the level sender hole (under the backseat), I can see a hose inside the tank. Could this be what is blocked? What is this hose and how do I access it? (I would need very tiny hands to fit through that hole.)

[Raven Tooth]

Ok, so we figured it out!! Woo hoo! Well not THAT exciting because we need to replace the fuel tank, but that's GOOD news after the 4 weeks we've spent solving this mystery. There is a blockage (of probably rust) in the fuel return line pipe inside the fuel tank. We tried blowing it out with an air hose and doing whatever we could with pipe cleaners (which wouldn't go in very far.)

We checked the fuel pressure all along the way and drained the other end of the return line into a bucket. The pressure is only high if we have it hooked into the fuel tank. If we drain it at any point in the line into a gas can, the pressure is normal. So we know the blockage is not in the line before the tank. Thanks for everyone's help. Will be ordering a new gas tank next paycheck.

[Lurker27]

I just found the same problem on my car - any alternate solutions to a new tank? A fuel cell might even be cheaper.

[jlevie]

By the time you are done with it, a fuel cell will be more expensive than a replacement tank. A new tank is the easiest solution. But a used tank will work and if you acid clean and seal the used tank you'll never again have a rust problem. Acid cleaning the tank you have might save it.

[Lurker27]

Jim,

As I understand it, the clogged return line happens in the bottom of the tank, where the return runs a sort of siphon system to transfer fuel over to where the pump pickup is. As a temporary fix, can one simply cut or relocate the fuel lines? I've seen your clever fix for starvation in sweeping turns, but it seems like a fuel cell setup has some natural advantages too...(Weight further back, probably less starvation, cost isn't too bad.)

[jlevie]

If the return fuel doesn't go through the siphon in the left side of the tank, fuel that gets into the left saddle will be trapped there. So feeding the return fuel in a some other point w/o making provisions to pick up fuel from the left side isn't an attractive idea.

The stock location is pretty good. It is only a little aft of the car's CG and thus has little affect on the car's balance as fuel burns off. Unless you get a custom fuel it would need to be mounted in the trunk, which will increase polar moment and have a greater affect of balance as fuel is consumed. Mounting the fuel cell aft of the rear shock towers places it in the crush zone. If you'd seen my car up close after the wreck, putting anything important aft of the shock towers would not seem like a good idea.

There's more to installation of a fuel cell than just hanging the cell where the spare goes and running some lines. If you want a gas gage, you'll have to mount an gage somewhere. And for safety the swiss cheese rear bulkhead and shelf under the rear window have to be covered with sheets of metal and any penetrations sealed. Then there's the recurring expense of periodic bladder replacements.

My guess is that you could probably clean the tank you have. It is a job for when the tank is out of the car, but if the tank isn't leaking now acid cleaning and sealing it should give it a long life. You should be able to rod out the plugged return with small wire pulling snake.

[Lurker27]

Jim,

Thank you for your detailed and knowledgeable response. People like you truly make the community a joy to be a part of.

We were able to snake out the clog and return fuel pressure to normal in my case. When the fuel pump is swapped we'll investigate how much rust/sediment/water/crap is in the tank. If it's bad we'll drop and clean/seal the stock tank as per your recommendation.