Stall Out Question --Update

[rlomba8204]

Sorry if this is a touch long. Wanted to give as detailed a description as I could so you guys could chime in with ideas on a problem that started as a routine e28 issue, but with a weird twist.

Tonight on the way home, it was very hot -- probably 95 to 100 degrees here in DC. I was driving the 535 and all of a sudden, a few miles from the office, it stalled out. I rolled (didn't want to block a main road) onto a side street, popped the hood, and had a look. No steam or what-not, and the engine temperature had been indicating on the gauge slightly to the left of center (blue side), and I was actually pretty pleased at this given the high temperature and the fact the AC was on. Maybe a few moments before I had noticed what I can best describe as a sluggishness or hesitation.

So, I proceed to swap out the fuel pump relay and main relay with spares I had in my trunk. Changed them both out, crossed my fingers, twisted the key and it popped right back on. So, I figure I got lucky and that the very high temperature probably finished off one or both of those relays. Put the AC back on to see if that was OK, and it was fine -- cold with no strange engine noises or vibrations. When I restarted the car, I also noticed the fuel gauge was now reading a little lower than a quarter, so I figured I should stop and fill up, as it is probably not the best idea to run an older car around in 100 degree weather with an almost empty tank. So I stopped at the gas station that was probably less than a quarter mile up the road, filled up, and then restarted the car. On the way out of the gas station, the car briefly stalled again. "Oh shit," was my immediate thought. But, I turned it off, restarted it and drove the next 10 miles home in traffic with the AC on. Car ran fine. Engine temperature was fine. Even noticed that the idle was a little smoother and the motor slightly more responsive to throttle.

If not for the second stall out episode, I would not give this any thought as anything other than a standard e28 fuel pump/main pump relay "flake out." But the second stall had me thinking that perhaps the initial stall disturbed the fuel flow or something that caused the engine to stall out.

The car is in the garage now and I went out to start it, and it started like it always does -- a little longer crank when it's hot out. But the idle is actually a little smoother. But other than that, it's exactly like it has been since I owned it.

Any thoughts here? My primary suspect is some sort of fuel disturbance having to do with the intial relay failure and subsequent fill up. An additional fyi is that the in-tank fuel pump is new -- I changed it before 5erFest as it was not working, and I also checked the main fuel pump at the same time -- seemed fine.

Any thoughts are appreciated. TIA.

[Martin in BellevueWA]

I'd pull the DME & have a look with a magnifying glass. Look for cracks in the solder joints on the underside of the board. Several joints may need to be re-flowed to improve continuity.

[I'm the DRIVER]

It could be vapor lock. That just happened to me last week. I was on the freeway and the car started stumbling. I got off and at the light it stalled and it wouldn't start for a while. Pop the hood and switched main and fuel pump relay with spares and still won't start so I let her rest for 10 mins. and she started right up.

This happened to me last summer too with temps in the 100s. That time I knew my transfer pump was weak and failing. This time I tested it again by jumping the relays and all I can hear was the main pump making a "whurrrring" sound and I couldn't tell if the transfer pump is working. I think I weakened the already old battery with the test because now it cranks really slow. Everything else seems to be ok and I even cleaned all the gunk on speed and reference sensors. Next I'll check the ECU and see what's up. This week will only be in the 90s so I think I'll be okay.

[rlomba8204]

Update:

I drove to work this morning, my typical 12 to 13 mile drive, temperature outside was 80 degrees. Car started more quickly than normal, idled a touch smoother (a relative thing for an M30), and I drove in the whole way with the AC on and no problems.

I am tempted to look at the ECU, but I generally don't like to play with older electronics -- more than likely I will disturb a 22 year old circuit by playing with it rather than accomplishing anything useful. This is the same reason I am not a fan of chipping at this age -- general unease of prying on old circuit boards.

The vapor lock could very well be. High temperatures, a fresh refill and the fact that the fuel delivery may have "hiccupped" in some way could have done that. I was surprised at it though, given the fresh in tank pump. You can run these cars without the in tank pump working, but besides overworking the main pump you make vapor lock and other driveability issues more likely.