Faithfull readers of the Brickboard are probably tired of my posts on this subject, but maybe someone who hasn't read one of my previous posts or hasn't suggested a diagnosis could have an idea about this. I have also updated the list of things I have tried.

It has also returned from two weeks at my reliable local foreign car shop where it has stumped the experts.

THE PROBLEM: If you try to accelerate more than the slightest amount, the engine starts bucking violently, until you get to about 4,000 RPM. RPMs drop down some and then back up (a few hundred RPM). If you back off, it stops. It does a less violent stumble version at cruise when going up an incline (a rhythmic 4 or 5 hicuups).

** The problem starts at about 1,500 RPM and stops when you get to about 4,000 RPM.

* It stopped doing this at the end of a 200-mile trip, but then returned over a couple of days.


I have tried by category:

Fuel Delivery:
replaced fuel filter.
filled tank with gas.
put in large bottle of techron.
resoldered FI relay.
swapped fuel pump relay.
swapped fuel pressure regulator.
checked fuel pressure at rail. (18 psi with ign switch on; 38 at idle; rises with load and drops to 38 when it stumbles)

Electronic/Electrics:
swapped AMM.
swapped coil.
replaced distributor cap and rotor
replaced plug wires.
replaced plugs
swapped power stage.
swapped RPM sensor.
Swapped alternator.
Cleaned grounds in trunk, passenger and driver side footwells, behind headlight on driver’s side, at engine block, and at fuel rail.
Swapped ground wire from alternator to intake.
Swapped ballast resister pack.

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.

Other:
Checked exhaust for individual bursts (to see if blocked).
Unbolted cat. converter.
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

Help!