Hesitation/jerking gone at about 4,000 RPMs -what does this

[baughpd]

I continue with my quest to diagnose a severe hesitation at load on a 1995 940 Turbo. If I try to accelerate with any but a light feathering of the gas pedal you get it. Also when cruising at a steady speed on the highway and you go up any kind of an incline, you get a slight rythmic hesitation.

I NOW HAVE SOME NEW EVIDENCE THAT MIGHT MEAN SOMETHING. I DISCOVERED THAT THE HESITATION GOES AWAY AT ABOUT 4,000+ RPMs! So what does that mean in my search? That is NOT a turbo hose leak? That is is NOT a fuel pump problem?

I have (by category):

Fuel Delivery:
replaced fuel filter.
filled tank with gas.
put in bottle of techron.
resoldered FI relay.
swapped fuel pump relay.
swapped fuel pressure regulator.
checked fuel pressure at rail.

Electronic/Electrics:
swapped AMM.
swapped coil.
replaced distributor cap and rotor.
replaced plug wires.
replaced plugs.
swapped power stage.
swapped RPM sensor.
Cleaned grounds in trunk, pass, and driver. footwells, and at fuel rail.


Air/Vacuum:
inspected all and replaced some vacuum hoses.
inspected turbo hoses (replaced one, swapped one).
pressurized system at turbo inlet (after hearing air under intake manifold, but not finding a hose problem, I replaced intake manifold gasket).
replaced two idle motor hoses.
cleaned throttle body, with new gasket.
Swapped IAC.
Plugged intake to EGR pipe.

Also:
Checked exhaust for individual bursts (to see if blocked).
There is no black smoke from the exhaust when the hesitation happens.
There are no codes (only 1-1-1).
I then tried to replicate the problem on my 1994 940. I disconnected several air hoses off the intake manifold to create air leaks. It did not produce the same effect.
It stopped doing it at the end of a 200 miles trip and returned a day or two later.

Any ideas would be greatly appreciated.

Philip

[Kmaniac in California USA]

This might be a long shot, but with what you have done so far, it is worth a try.

Twenty years ago, my father had an Audi 4000 5+5 that exhibited similar symtoms. After wringing out the car on the back roads one afternoon, I determined that the problem was mostly confined to the fuel system, a fuel injection system similar to the Volvo. Being in my early twenties at the time, my fuel system experience was limited to the Carter AFB on my big block Mopar. At my suggestion, we took the Audi to the dealer for them to sort out.

Even though I told the mechanics to look at the fuel system, they knew better than me and proceeded to swap out all of the ignition hardware first. Having exhausted all ignition possibilities without success and wasting two hours of labor at $60+ per hour, they then decided to look at the fuel system. They opened up the interior kick panel and exposed the fuel computer. They discovered that the electrical connection to the computer was loose, disconnected it completely, the plugged it back in. The car ran perfectly after that. My father was rather upset having to pay $200 for the mechanic to replug an electrical connection.

Anyway, I would suggest looking at the connection to your fuel computer. This is located behind the passenger front seat cowl panel ahead of the door, or at least it is in my 740. A long shot, but you never know. And what an incredibly cheap fix if this is the problem and you fix it yourself. And please, let us know what you find.

[baugh]

Thanks for the suggestion. However, I already tried un-plugging and re-plugging the ECU. There was no difference afterwards.