E28 Fuel Issues

A few weeks ago I picked up a 1985 535i with probably about 200k on the clock (odometer quit at 189300). The seller bought it as a project, drove it down the street and saw smoke, and parked it. When we went to buy it, there was no problems with smoke or overheating, but the issue we're having seems to be with the fuel system. The original owner dumped $20k into this car, and that included replacing the fuel filter and pump Bosch OEM within the past few years. Here's the problem: The car seems to quit getting fuel when it comes under any load. It will idle all day, but if you try to accelerate, it cuts out and sputters. In addition, the inline fuel pump makes a very loud whining noise at all times. I would suspect it was going bad, but since its a fairly new oem pump, that doesn't really make sense. I'm thinking there's a blockage somewhere in the system that's causing the pump to work extra hard, and the same blockage is preventing the engine from getting the fuel it needs, thus causing the cut out issues. Does this reasoning make any sense? To diagnose, couldn't I unhook the pump from the line to the engine and see if the pump will make them same noise just trying to pump fuel into an empty containter? I don't have much experience with fuel systems, so please school me. I'll start a build thread this weekend, and maybe you guys can help me with the million other issues I have

There is also a fuel pump in the tank. Lack of power and a noisy main pump are common symptoms of a bad siphon (in tank) pump.

In reply to Cone_Junkie:

Is there an easy way to diagnose this before I order one?

Rmon wrote: In reply to Cone_Junkie: Is there an easy way to diagnose this before I order one?

It will run better with a full tank. I think there is a check valve in play there too. If the fuel filter or check valve are clogged, this could cause the issue. Also you can plumb a fuel pressure gauge up front at the fuel rail to see if you're losing pressure at higher RPMs.

In reply to Tyler H

I'm pretty sure the tank was at least 3/4 full when we bought it. Bad gas couldn't cause this, could it? It wouldn't make sense, but we siphoned some out and it was browinsh.

Remove the inlet fuel line from the main pump and crank it over. If you have fuel pouring out, siphon pump is working. Can also listen for the buzz in the tank, but a loud main pump might overwhelm the sound of the tank pump.

Probably useless info but I had a 75 530i. When the battery was about to go out, the computer got really stupid & made for puzzling drivability problems.

Another thought is to look at the airflow meter. Sometimes the flapper valve in there will stick, and it throws the computer for a loop - the car will start and idle, but will sputter and cut out when you give it gas. Try thumping on the airflow meter with your fist a couple times to see if it makes any difference.

Can you get to the inlet for the cold start injector easily on m30's? It makes a convenient place to put a fuel pressure gauge to check static pressure (engine off, no vac reference). If it's too high, a clogged fpr or filter would be next guess. Too low points toward faulty fpr or pump/supply line.

I'm not as familiar w/ m30's as the m20, but how difficult is it to get at the plugs? Wouldn't hurt to check for overall combustion condition and if any localized cylinder issues (injector problem).

Why not just pull the in-tank pump and bench test it? You can inspect the pre-filter while you're in there (it's attached to the pump). If you need a pump, and you probably do, order one for a Chevy Vega; it's exactly the same unit, about 25% of the cost last time I checked (which was admittedly a while ago) and should come with a new filter. Very easy to change.

So little update,

After reading everyone's recommendations, we decided to test the in tank pump. We hooked up some hose and and cranked it over. Fuel came out pretty quickly. I guess there's no way to tell if it really is operating at 100% using this test, but the pump didn't seem weak to me. Next, we decided to replace the fuel filter, and we were in for a surprise. We got it unhooked and a rust red mixture poured out. We also tried to blow thru the filter to see if there was any clogs/stoppages, and it was very hard to blow through.

Yeah...

Surely, this was the problem. The car started and we could rev it as much as we wanted in neutral with no fuel issues like before. The fuel pump was still whining very loudly however. It drove maybe half a mile (but it felt awfully slow) and then tried to cut out. it was idling very low and very roughly, but we managed to limp it home. As soon as it went in to the garage, it started to idle fine.

Also, we tried whacking the AFM, and it didn't do anything.

What do you guys say? The fuel pump noise (it's pretty bad) makes me think its either on its way out or is under a lot of strain either due to a bad in tank pump or a blockage somewhere. I'll probably try to test the fuel pressure today. Hopefully I can get to the cold start injector.

What fuel pressure would I be looking for?

Either 2.5 or 3x bar (36.25 or 43.5psi). It probably says 2.5 or 3.0 on the fpr. Thats engine off, no vac signal.

Thats interesting that it went to a good idle after driving a bit. Would you say that the change was concurrent w/ the engine reaching warm operating temp? Anyone know what motronic version this runs? If its like the e30, it switches from an uncorrected to o2 corrected idle when warm.

So I drove it around some more and the fuel problem seems to be completely random. It doesn't seem to have any issues when its cold, but once it warms up, it will either sputter or run perfectly fine. I took some video, hopefully you can see what I mean. The video starts with the car already warmed up. When I pull back into the garage, it revs fine with no problems.

E28 Fuel Issue

Could there be dirt/water in the gas thats coming through every once in awhile? Hopefully I'll be able to do the fuel pressure test tonight and that should tell us more.

In reply to bluej:

It does seem to be dependent on temperature, but it doesn't always happen when its hot.

EDIT: I just drove it again, and when it starts to sputter under acceleration, if you push the clutch in, it idles rough for about 5 seconds and then goes back to normal idle.

That rust isn't a good sign. What's the tank look like? If it's rusty, I would replace it or coat it. It's going to keep giving you issues and kill fuel pumps.

Running with clean gas and a new filter, you can check the fuel pump flow rate by running the supply hose in the engine bay into a container, key on, and manually pushing open the flapper door on the AFM. There is a specification for how many L of fuel in 60 seconds, but I can't recall the exact number.

Other than the fuel system issue, the E28 is known for weak solder joints in the DME and other electrical issues. Clean up all the fuse connectors and rebend them to tension the fuses better. Clean up the grounds, etc. The coolant temp sensor can cause running issues. Do the usual Bosch electrical baseline stuff.

Could it be operating in the limp home mode?

spitfirebill wrote: Could it be operating in the limp home mode?

E28s are old enough that they don't have a limp home mode.

stuart in mn wrote:

spitfirebill wrote: Could it be operating in the limp home mode?

E28s are old enough that they don't have a limp home mode.

E28 Limp home mode is what happens after you kick it in anger, just as it begins to rain.

I started a build thread for this, here's the link: E28 Build Thread

You can see pics of the inside of the tank there. The rust was pretty bad.