Non Revving and stumbling 944 - Thoughts?

[ChasR]

My 944 has bee quite dependable as of late. It is a good job really since it became a daily driver with me putting on more miles in 6 months than the car covered over 5 years.

However it seems it may have taken its toll. A short while ago now and again on rare occasions (it used to do it more when cold ) the car will idle OK (if I am honest, it would stumble a bit more, and hunt slightly), yet when it came to revving the car it would not rev beyond 2krpm. This would soon clear itself with business returning to normal (idle is generally very steady on this car, and it pulls well), although sometimes it would idle at closer to 2000rpm for a short period of time before settling back to its normal idle within 15 seconds or so.

Today this problem came back to haunt me twice, but regardles of engine temperature. This time the car was barely idling, with revving it causing no change in RPM but for it to attempt to cut out, and at one point it did just stumble and die. I restarted it and within seconds the issue went away. When it was close to dying with no throttle I noticed that the boost gauge went close to 1 bar, indicating that very little vacuum was present (at idle it tends to set at or lower than 0.4 IIRC).

I was thinking along the lines of AFM or TPS, but I cannot see how it would tie in with a change in vacuum.

What are your thoughts? There seem to be no vacuum leaks present, although I guess you never know.

 

[Mike_Pollock]

Hi Chas

Think FPR - thats linked to vacuum and would give stumbling engine.

I'd pull off the vac line and smell for petrol in it.

 

[DC911SC]

I would start with the airflow meter, also worth seeing what the fuel pressure is like.

 

[924nutter]

Just so that we understand how to read the boost gauge, the 944 turbo gauge gives a reading of manifold pressure measured from a perfect vacuum so I bar on the gauge is simply atmospheric pressure, so there is the possibilty of a loss of electrical integrety in the gauge circuit or the is a leak in the manifold, and there is nothing wrong with the gauge reading. Also from what I have read on this forum FPR (fuel pressure rergulator) is an equally valid diagnosis, and to be honest an engine barely idling wouldn't produce a lot of manifold depression.

If the engine rpm drop below 200 the engine will stop, because the motronic system needs to detect 200 crank rpm to enable fuel pump and ignition.

 

[944 man]

FPR is now what the DME relay was in the past.

 

[ChasR]

ORIGINAL: A9XXC

Hi Chas

Think FPR - thats linked to vacuum and would give stumbling engine.

I'd pull off the vac line and smell for petrol in it.

Cheers, I did pull the line off yesterday and it seems to be fuel free. With the engine running it made no real difference pulling it off (vacuum was present at the end of the tube though).

ORIGINAL: 924nutter
Also from what I have read on this forum FPR (fuel pressure rergulator) is an equally valid diagnosis, and to be honest an engine barely idling wouldn't produce a lot of manifold depression.

If the engine rpm drop below 200 the engine will stop, because the motronic system needs to detect 200 crank rpm to enable fuel pump and ignition.

After reading about FPRs going bad recently I also suspected the FPR bar the issue being intermittent and with a lack of vacuum. Although now you state the obvious about a stumbling/dying engine producing less vacuum than it should it would make sense. But would an FPR not simply fail altogether or have they been known for becoming intermittent (You have to love intermittent issues).

Saying the above I am tempted to change the FPR regardless due to the recent spate of failures as Simon has pointed out. The one I get however will depend on the chips I have (which are still unknown; a job for later).

 

[Mike_Pollock]

Mine was intermittent, getting steadily worse, until it would hardly run.

It was also running very rich - sucking fuel into the inlet from the FPR!

 

[George Elliott]

FPR is now what the DME relay was in the past.

[][][][][]

FPR is pure mechanical, and I find mechanical things are generally consistent in behaviour.
Which is why I would be surprised it it was that, but you are right Simon - if the heater wont work..........its the FPR some-how
The DME Relay has been really quiet of late.......

The symptoms are electrical IMO Mike. The 5v signal in the AFM is routed thru a 4 pin mouthorgan connector which faces forwards into the damp air and between corrosion on that and the wear on the resistor track, the ECU is getting rubbish information. At tickover the resistor wiper is in its home position where it does not wear, so its giving a relatively accurate ECU input. If you touch the throttle, the butterfly opens, engine vacuum opens the AFM flap, this moves the wiper into a worn area, the ECU signal goes wild, the engines runs badly.
If one wanted to argue - the AFM is mechanical, I agree, but the electrical part of it is worn.

Not sure what to suggest, AFM wiper repairs are tricky, and they need to be reliable.

Good luck
George