A week ago my '94 945t developed a condition where any more than slight throttle opening will cause it to bog down and lose all power. It will rev fairly freely in Park, but under load it runs worse the more throttle I give it.
The conditon came on overnight, except that it has had a hesitation after cold start that usually cleared quickly. CEL light came on one day before the driveability problem appeared. Initially code 121 (AMM fault) then 113 (incorrect mixture) also.
I've done a fair bit of testing and maintenance in the last week (see below for more wordy details) and I believe I have the source narrowed down to two main faults:
1. My fuel pressure tests at 24psi. Removing the vac line to the FPR had no effect on pressure, and pinching the fuel return line had no effect on pressure. According to Bentley that points to the main pump.
2. I pressure tested the whole intake system (excluding crankcase breather vac line) and found a significant leak at the egr valve. Blanking the egr valve's connection to the intake manifold stopped the leak, but I'd like to leave it working as installed for CA smog check. I did test drive the car with the egr intake manifold port blocked though, and it still bogged down under throttle.

I'm thinking the casue is the low fuel pressure. I picked up a spare pump at the JY from a non-turbo 940, but I'm not sure if the spec is close enough to the turbo pump to substitue in a pinch.
I'm also still open to the possibility that both of the AMMs that I have tried are bad (my original and one JY unit.)


Thanks for reading this far, and thanks for any thoughts.

Joe



What I have done to trace this condition so far:

I was getting codes 113 (incorrect mixture) and 121 (AMM signal fault) when the condition started.
I swapped out my AMM for a JY unit without any change in the condition.
Both AMMs pass the Bentley resistance test.
My O2 sensor gives about .5v steady at idle, no fluctuation. Sensor heater wires were not connected at time of this reading. Sensor is about 3 years old.
I ran the OBD DTM mode 3 test routine. The main fuel pump did not run, even though the FAQ says it should during this routine. I did confirm that both the main pump and pre-pump were working with the key on(engine off) and with the engine on, though.
If I recall correctly, the voltage to the main pump connector was almost 1v under battery voltage. While operating the main pump drew less than 3 amps - the max draw in Bentley is much higher, 5 or 6 amps.
I replaced the sparkplugs and plug wires. Wires were in bad shape, but plugs were neither wet/fouled nor burned.
I cleaned up the cap and rotor electrodes.
I tested the fuel pump flow output and got maybe 30% less than the expected 1 liter in 30 seconds at 12v.
I installed a new fuel filter. This appeared to make some improvement in the condition - giving me a slightly wider range of usable throttle position.
I pressure tested the intake system and found several small leaks and one big leak at the egr valve. I haven't removed it yet, but the leak seems to be in the valve, not in the vacuum servo, and leaking to atmosphere, not to the exhaust manifold.
I finally pieced together enough adapters to attach my fuel pressure gauge to the fuel rail and got 24psi - way below the 45psi spec in Bentley. The reading was not affected by removing the FPR vac line, nor by pinching the fuel return hose tightly.

One more diagnostic note that makes me question whether the main pump is the cause - throughout my testing the bogging condition has been intermittent. It is usually present, but occaisionally completely absent and the car runs strong throughout the throttle range. On one test trip (after fuel filter/plugs/wires intstallation) the car ran fine for 30 min, including 20 min at ~75 mph, only to return to bogging down as soon as I exited the freeway. When the condition is present I can drive by the stock boost gauge - if I keep the needle to the vacuum side of 11:00 on the gauge I am fine.

PS - Thanks to RSTARKIE for a post about using his homemade pressure brake bleeding resevoir cap as a plug for pressure testing the intake system. I had made the cap many years ago for pressure-bleeding (probably following a tip from him at the time) and it came in handy again today.