does this sound like fuel starvation?

#1

Hi all,

I was driving at a driving event this weekend at Thunderhill in my 87 325is. There were four 20-min sessions in the day, and in the middle of the third session I started experiencing something that seemed a bit like “fuel starvation.”

Coming around certain parts of the track I would have my foot on the floor and feel suddenly a loss of power, as if I’d stopped applying the throttle. This happened for maybe 3 seconds tops at a few places on the track (after 3 seconds, the power would be back). It would happen in basically the same place every lap.

Every time this happened, it was with a sweeping right turn somewhere in the recent history, though it was not immediately after the turn (for example, one place it happened was about 1/3 way down a long straight following a reasonably fast right turn, and another place was exiting a quick left turn following a moderate right-hand sweeper).

Other relevant parameters: I was 6 gallons into my tank when this started happening. I also had a slight cooling leak, and I was running hot the whole time, but I was hottest when this power cut-out started happening (was about 3/4 between blue and red on the stock temp gauge). On my fourth session of the day, I filled my tank up and came back out and I was ok for about 1/2 of the session, but then the problem came back again. I was again running at 3/4 on the temp gauge, though I think I reached this temperature well before the cut-out problem took place again.

My question is: does this sound like the classic fuel starvation that is often solved by replacing pumps and filters? One instructor suggested that it might be the overheating somehow causing vapor bubbles in the fuel lines.

I’d appreciate any suggestions. Thanks
-Patrick

#2

Starving on the straights means something is likely in need of replacement. I’ve experienced the symptoms you describe due to 2 things:

  • coil mount spot weld failed, coil bouncing around fender by a zip tie (placed in case the coil weld failed). the tach dropped to 0 in this case.
  • bad fuel pump. foot to the floor and the car would stutter and/or die.

Have you checked your fuel pressure on the rail? Do you get a steady stream of gas out of the return side of the fuel pressure regulator?

#3

Hi laz, thanks for the info. I don’t think the first point about the coil spot weld applies to me. No, I haven’t done any diagnostics with the fuel pressure yet. That sounds like a good thing to check. But since my problem only happens on track, do you think I’ll be able to get anything out of that fuel pressure check?

#4

Maybe, maybe not. You could gather no data, swap out the parts, and cross your fingers.

In my case, the fuel pressure regulator wasn’t sending any gas out the return side at idle (the bentley specs a certain volume of fuel should be returned in a certain time period). I picked up a used regulator and pump. The regulator swap didn’t solve it, but the new fuel pump made everything better.

If it all checks out ok at idle and when revving off track, I’ve heard stories of people connecting the fuel pressure gauge with a long length of fuel line and taping it to the windshield, then going out on track to watch for dips in pressure :slight_smile:

p.s. I think I’ve seen a good used fuel pump for sale on www.baye30.net. dunno whether it’s late or early.

#5

Mine would do it on the exit of a sweeping turn (VIR T1) with anything around a half tank or less of fuel. Transfer pump in the tank was dead. Replaced it and all is well.

#6

Your problems sounds very similar to mine at Watkins Glen last year. Bad in-tank pump ended up being the problem.

This pump was a year old, so I was concerned. Upon further investigation, I found the tank had rusted pretty badly. So, like GRM, I replaced the tank with one from eBaY. New tank, pumps, filter, damper, and connecting pipe later, I can run it almost empty without fuel starvation problems.

#7

Mine was doing the same thing on a really hot day at Barber. Replaced the pump with a Summit Racing pump and replaced all of the relays on the drivers side wheel well. Not a single problem since. My fuel pump was original.

#8

Thanks everyone. I’ll replace my in-tank pump. Is it worth replacing the main pump as well? (I have a 5/87 car with two pumps… one under the car.)

#9

Run the fuel system pressure and delivery tests documented in the Bentley manual. That will reveal what needs to be replaced.

#10

and don’t forget about the fuel pump relay - it could be loose or getting old.