Hesitating and Lurching
Posted: 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.
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.