Strange Stuttering Problem

[J.DeFeo]

1992 535i 5-speed, 158,000 miles.

At extremely low throttle (<5%, foot barely resting on it, purely to maintain speed on a flat road), the car bucks/stutters softly as if the fuel injectors are being shut off/turned on. It feel like it does when the throttle is released and the injectors shut off while coasting in gear, only repeatedly on and off. It feels almost like the clutch slipping, but at even a tiny bit more throttle it completely vanishes. From 5% or so to WOT, there is no issue, loss of power, and it runs perfectly. It occurs when the MPG gauge is between 20 and 25, the closer the gauge is to 25 while still having non-zero throttle the worse the problem gets.

I have checked the alignment of the TPS, and it is set so that when the throttle is fully closed, the idle switch is enable (moved it around until it just barely clicked with the throttle closed).

I did a 24-hour ECU reset (unplug all the connectors). No change.

It is not temperature Dependant, the problem occurs with the engine cold and warm and feels the same.

The entire clutch system is more or less brand new (<15,000 miles), and it is definitely not slipping because WOT in all gears is perfect.

It happens at all gears, although it's much harder to reproduce in 1st since its so short.

Brand new IAC, idle is perfect, no other issues. New spark plugs.

Oh yeah, brand new driveshaft, guibo, center bearing. No vibration issues.

Any ideas?

[Ross]

s s sounds like a f f fueling issue. P P Perhaps too lean. V V Vacuum leak?
Recheck TPS

[e34.535i.sport]

Have you been able to check the fuel pressure? Prob couldn't hurt to change the fuel filter either if you don't know when it was last changed... but that's my cure for everything! LOL! :D

[Ross]

I think this will turn out to be twitchy TPS, sorta- kinda sending the idle, ooops off idle signals intermittantly. Perhaps re-adjustment will cure it. That TPS is just a switch me thinks with no mid range signal, I've read about disassembly and cleaning them.

[J.DeFeo]

I think this will turn out to be twitchy TPS, sorta- kinda sending the idle, ooops off idle signals intermittantly. Perhaps re-adjustment will cure it. That TPS is just a switch me thinks with no mid range signal, I've read about disassembly and cleaning them.

I'm going to give this a go. It is just an on/off switch, probably needs to be cleaned. I threw a can of seafoam into the gas tank and see if that makes any difference. Vacuum is perfect, no leaks.

[JD525IA]

1992 535i 5-speed, 158,000 miles.

At extremely low throttle (<5%, foot barely resting on it, purely to maintain speed on a flat road), the car bucks/stutters softly as if the fuel injectors are being shut off/turned on. It feel like it does when the throttle is released and the injectors shut off while coasting in gear, only repeatedly on and off. It feels almost like the clutch slipping, but at even a tiny bit more throttle it completely vanishes. From 5% or so to WOT, there is no issue, loss of power, and it runs perfectly. It occurs when the MPG gauge is between 20 and 25, the closer the gauge is to 25 while still having non-zero throttle the worse the problem gets.

I have checked the alignment of the TPS, and it is set so that when the throttle is fully closed, the idle switch is enable (moved it around until it just barely clicked with the throttle closed).

I did a 24-hour ECU reset (unplug all the connectors). No change.

It is not temperature Dependant, the problem occurs with the engine cold and warm and feels the same.

The entire clutch system is more or less brand new (<15,000 miles), and it is definitely not slipping because WOT in all gears is perfect.

It happens at all gears, although it's much harder to reproduce in 1st since its so short.

Brand new IAC, idle is perfect, no other issues. New spark plugs.

Oh yeah, brand new driveshaft, guibo, center bearing. No vibration issues.

Any ideas?

I know you said there are no vacuum leaks, but this can be easily overlooked. This loss of power can come from something as simple as a small hole or crack in the boot.

JD

[genphreak]

CHeck MAF output is smooth across the range, spikey curve = confused ECU and wonky fueling... etc. etc.