Weird fuel issue, falling on it's face

[UNCC Grad]

88 325is, stock fueling components to my knowledge. Recently I've had an issue pop up where the car will start and idle fine although a little high 1100-1250, however it will fall on its face like the fuel has been cut off while underway. This happens at random, more so when cold but does happen occasionally after warmup and only happens a couple times each day. I was told the fuel filter was recently replaced but it seems a clogged fuel filter would cause idling issues and be a constant problem. The plug wires are new and appear to be seated correctly.

My thoughts are vacuum leaks, faulty fuel pump, partially clogged fuel filter or something else fuel related. One thing I have noticed is the mpg gauge swings to 0 when the issue happens so that's why I thought it could be a vacuum leak.

Anyone else have this issue or know what it could be?

[BMWManiac]

First, I would make sure your battery cables are tight! I had the issue where the car would stop and go (everything else worked fine) and i thought it was a fuel starvation issue myself or poor spark...

Came back from Afghanistan, reinstalled the battery, and the issue went away. Turns out I had been doing some work and forgot to reinstall the battery tight!

[jlevie]

The econometer driving towards zero says that the DME has greatly increased injector on time in an attempt to achieve stoich combustion. Which in turn could mean a fuel deliver problem. You can confirm that by tee'ing a gauge into the rail supply line, zip tying the gauge to a wiper, and seeing what the fuel pressure does in an event. It is a plus to hook a test light to the pump power circuit in order to tell if a loss of pressure is due to a loss of power.

[UNCC Grad]

The econometer driving towards zero says that the DME has greatly increased injector on time in an attempt to achieve stoich combustion. Which in turn could mean a fuel deliver problem. You can confirm that by tee'ing a gauge into the rail supply line, zip tying the gauge to a wiper, and seeing what the fuel pressure does in an event. It is a plus to hook a test light to the pump power circuit in order to tell if a loss of pressure is due to a loss of power.

Thanks for that tip. It almost seems as if all power is lost for a split second and then it recovers and it seems to be no common theme as to when it happens.

When changing out the timing belt and water pump I did notice some suspect wires on the crank position sensor and have another one ready to install when my new fan clutch comes in this week.

Could a faulty CPS signal cause an issue like this?

[mpowerful]

The cps does signal the dme to fire the injectors in batch so yes it could definitely cause it.

[UNCC Grad]

We've had some cooler weather lately and it seems to be worse the cooler the temps are in the morning. Once the engine temp got over 1/4 it only happened once in a 30 mile drive.

I tried the stomp test to get any codes but I can't even get the CEL to come on when the key is turned to ON. Wondering if I have a bad bulb or gauge cluster.

One thing I have noticed is when it acts up the econometer goes to max (40+) then settles back to its correct level when the power comes back. I will be replacing the CPS soon to see if that fixes the issue but I'm doubtful. It seems temperature dependent.

[UNCC Grad]

Update....replaced the coolant temp sensor since it was cheap and could be a possible culprit. Still no change.

Also changed the CPS and swapped with another 173 ECU that had a Conforti chip. Still did it a couple times.

I have a spare fuel filter so will look into that as well.

[Mthrizzle]

check fuel filter...check fuel tank for rust/crud...update us...

[UNCC Grad]

Last night I pulled the main and fuel pump relays and found this...could it be bad contacts in the main relay? The fuel pump relay had the same brownish color to a caouple of the contacts. I cleaned them up and put them back in and the issue wasn't as bad as it has been but that could be coincidence.

I've also noticed that when it cuts out, it sometimes backfires....possibly a faulty coil?

[jlevie]

The oxidation of the relay contacts isn't likely to be the problem. Run the fuel pressure test I described earlier.

[Earendil]

Run the fuel pressure test, that'll eliminate a lot of things one way or the other.

Do all you gauges drop to zero as if the car had been turned off? Could be the ECU relay. I had this issue where the relay would cut power to the ECU for a split second, but it wouldn't kill the car dead but maybe one in five times.

[UNCC Grad]

I replaced the AFM with a cleaned used unit (original was missing the factory adjustment plug so it had likely been tampered with). Also replaced the fuel filter. This didn't seem to do much so I pulled the fresh plugs and found that they looked fouled.

I must have forgotten that I had done a seafoam treatment through the vacuum line AFTER I had replaced the plugs a few weeks back. I think the old AFM was making it run pig rich on open loop, slowly fouling the plug further where they were having issues with the added fuel in open loop.

I replaced the plugs with a fresh set of NGK ZGR5A. Since this it has been problem free the last couple of days. If it continues I will test the fuel pressure and try to catch it when it stumbles and report back.

Thanks for all the help!

[purrfektrs]

I had that issue. You check the timing?