alfisti

Hi!
I'm new to the forum but not to Audi ownership. I just bought a 1993 V8Q that has been converted to a 5-Speed, for a very low price as a result of a strange misfire that it has at 5000rpm and occasionally under heavy load while cornering at lower rpms. It pulls evenly and strongly right up to 5000 and then starts to misfire and buck quite violently. I have replaced the spark plugs with Bosch Platinum, one of the distributor caps (the other is in the mail) and both coils, with no significant change to the behavior. Every once and a while it will pull through 5000 no problem, but I'm reluctant to push it past frequently to test this out. The PO replaced the crankshaft position sensor recently, and did the timing belt change in the last 500 miles. The only other time I've had symptoms similar to this was when my fuel pump for my mechanically fuel injected Alfa Romeo was going south. Any help on this unusual behavior would be very much appreciated!
Thank you!

a-talk

Quick thought - have you checked the timing to make sure the TB was installed correctly?

alfisti

That is definitely a good thought. I was thinking the same thing, but I passed emissions today with numbers below those of my 2005 S4 so I think the timing is probably fine. (I found out that small timing problems mean BIG HC levels on another car).

Update though: Driving it 100mi today I discovered that it bucks and misfires also on RIGHT hand turns only at much lower rpm when accelerating out of the turn, not on left hand turns. This makes me think it may be fuel delivery related. Thoughts?

V8BOB

Based only on the above, possibly the fuel pump in the tank is loose and causing the flow issues on turns and when high demand is needed? Let the tank get to near empty, in the Red Zone remove the float and see if you can grab the pump and move it, turn it by hand. The pump should be be mounted in place firm and tight. You gain access in the trunk, pull the carpet back from that upper shelf right behind the back seat.

https://audi.7zap.com/en/usa/audi+v8...0/2/201-24010/

audi_marv

Also....don't forget to change the fuel filter, if you have not done so already.

alfisti

Well, the fuel system checks out. I replaced the fuel pump and there is good pressure, but I think it was good before. I asked someone else to drive it and they are 98% sure that the problem is in the rev limiter kicking in early. It's almost like the engine tries to shut down after 5100 or so rpm. Could this be a problem with speed sensor and/or crankshaft position sensor? They seem to be wired in parallel into the ECU. The tachometer also seems to bounce a bit when the engine gets up to those rpms. Thank you!

toml99

If you haven't done it, blink out the fault codes and see what, if any are there. People have had issues with a similar stumbling at more like 2500rpm due to bad hall sensor. Wouldn't explain the right/left thing. Tach bouncing leans me toward RPM sensor which should show code if wonky. Maybe when PO did reference sensor, didn't get RPM sensor plug clicked all the way/have a bad wire thats swaying and grounding. I use the cheap ebay sensors(rpm and reference the same sensor even though Audi shows 2 part #'s) for 5000/100/200 that are way longer, so maybe PO needed another zip tie and wire is rubbing on exhaust. Only a sadist would go to the trouble of replacing just one sensor(in same holder) as it's in a difficult spot. Good luck, hopefully it's something simple. Do you have any idea if Bruce or Keith from German Auto Connection did the 5-speed conversion? Late 90's they were the masters of that. Tom