Hesitating and Lurching

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528i-1981
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Hesitating and Lurching

Post by 528i-1981 » Tue Apr 24, 2018 5:16 am

I'm getting a violent hesitation under load at full throttle (i.e., hard acceleration) under these conditions:

1. Below 1/2 tank of gas exclusively
2. Warm engine
3. Car will idle fine and will hold 4K RPM when parked without stalling
4. The hesitations don't usually have time to register on the tach, but will sometimes last up to 2 seconds and will show then
5. Never stalls or cuts out
6. If the car sits for a few minutes with the engine off, the symptoms resolve

More details:
- M30B28, 178K
- This E12 is new to me (Jan 2018) and past behaviors are unknown; it was running very poorly
- Plugs/wires/rotor/cap and voltage regulator all replaced Feb 2018 and runs great
- Fuel level sending unit was not functioning when I got the car; I was able to clear the float of sediment and return it to operation, but I reused the old O-ring and the smell of gas is strong after yesterday's fill-up
- This is only the second fill-up since symptom emerged
- Added bottle of Heet at last fill-up (yesterday)
- underside is noisy near rear of car, but when I put my ear near the filler tube with the cap off, I don't hear anything coming from inside the tank

My hypothesis for peer review: in-tank pump problem and the main pump is whining under the strain. When fuel level falls below 1/2 tank, the suction required exceeds pump capacity. Warm engine means warm main fuel pump and less pumping efficiency. Could also be a crack in the rubber tube connecting the pre-pump to the line on the hanger. I don't think the o-ring is the culprit, but it needs to be replaced.

I read that I should go through six tanks of gas before making any decisions, but wanted to put this out there in the event I'm off and it's something else like the combo relay or ECU - but neither of these know the fuel level so that seems unlikely to me.
(oo=00=oo) Eric
1981 528i Manual

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Mike W.
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Re: Hesitating and Lurching

Post by Mike W. » Tue Apr 24, 2018 12:01 pm

An educated guess, but by all means not an absolute, would be the AFM. They can get cranky and do things like you have going on.
Mike W.



1980 528i, 3.5 euro, 5 speed conversion
1981 528i, 3.6, Recaros and more. Project
1998 328is, quick and efficient, but not satisfying
2000 528iit, Vacation mobile/wife's grocery getter

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528i-1981
Posts: 211
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Location: Boston North Shore, MA

Re: Hesitating and Lurching

Post by 528i-1981 » Sat May 05, 2018 8:14 am

Update - down to 1/4 tank with no symptoms yet. This is the lowest the fuel level has been since the fuel gauge sender was rehabbed. Weather is finally OK in New England so I can hopefully use the rest up this weekend and see what happens. If she's back to normal, it was probably just water in the gas.
(oo=00=oo) Eric
1981 528i Manual

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528i-1981
Posts: 211
Joined: Wed Feb 07, 2018 5:50 am
Location: Boston North Shore, MA

Re: Hesitating and Lurching

Post by 528i-1981 » Sun May 06, 2018 6:55 am

Got the tank down to the reserve, and the indicator came on (first time I've ever seen it, glad the sender is working there). Only one very brief hesitation while cornering to the right - otherwise resolved. At this point I'm concluding it's water in the fuel and downgrading this issue to inactive. I'll replace the sender's O-ring for good measure.
(oo=00=oo) Eric
1981 528i Manual

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Robert Bondi
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Location: Austin, TX

Re: Hesitating and Lurching

Post by Robert Bondi » Sun May 20, 2018 8:24 pm

I think you got it. I had a bad gas experience last year from just a 4 gallon top off . It took 5 or
so tanks of premium and a can of Berryman's B12 Chemtool on almost every one to apparently
clear the problem. I had a drain plug present on the tank added years ago, but considering this occurred on the
interstate about 700 mi from home and the car continued to run with intermittent spasms, the show just went on.
Problem was pretty much cleared by that time.
Robert
77 530i
77 Euro 528

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