My '91 3.3 has been in the shop for a while now because the mechanic is stumped. The car is stock except that the secondary muffler is bypassed. To his credit, the mechanic doesn't want to spend a lot of my money by changing out parts haphazardly, but at this point the repair has kind of ground to a halt. The problem is that the engine will barely run when it warms up. When cold, it starts easily and the car drives normally. After just a few minutes of running, however, any pressure on the accelerator brings lots of coughing and backfiring - or the engine just quits. If you pump the gas pedal, you can get the revs up, but there's no power. This problem began after the car hadn't been driven for a few weeks in the winter. The mechanic says that fuel pump pressures are OK. Fuel filter was changed. Spark is good and we've put in new plugs. The mechanic found a mouse house in the engine compartment, but he didn't see any chewed wires.

Does this problem sound like a WUR issue? How about the alarm? Any other suggestions? Does anyone know a step-by-step diagnostic procedure?

Thanks!

 

I don't understand why a Porsche mechanic wouldn't check the warm control pressure in this situation. Have him do that if he knows how.

 

Maybe he did. I don't really know. In any case, can you explain this control to me?

 

hi,

i assume that the engine runs fine when hot.

so -beside control pressure/WUR- i would check the auxilary air valve (must slowly close while the engine gets warm), cold start valve ( must shut down after cold start) and the temperature-/timeswitch (controls the cold start valve ).

also possible, that the failure is caused by lambda son/lambda control unit.

fritz

 

Hey would start by disconnecting the lambda sensor to force the car to run open loop all the time (how it runs when cold). This would eliminate if it's an issue with the lambda control which only works when warm.

 

Fritz - The car only runs when cold. It won't run properly when warm or hot.

Francisco - Thanks, we'll give this a try.

 

hi,

does not realy alter my guess. should be more probable if it smells of unburned fuel or if you have a bit of black smoke out of the right tip while the engine is running.

fritz

 

I've seen this problem before on the forums. Do a search. I remember it being recent within the year as well.

 

Check both control units under the drivers seat (turbo and boost control units), one has most likely failed,====$$$$$$$.

 

The mechanic was making no progress at all, so I took the car back and am now tackling the repair myself. Sure is lots to learn, but there's lot of good info on this forum and Adrian Streather's piece about the engine systems is also very helpful.

Here's what I've done so far:

1) Cleaned the contacts on all of the controls under the driver's seat and inspected for damaged wires.
2) Took the circuit board out of the turbo control unit, visually inspected (with strong magnification) all solder joints, and confirmed that the two relays on the board are functioning properly.
3) Checked voltage at the Auxiliary Air Valve (it was 12.8).
4) Took the circuit board out of the acceleration enrichment control unit and visually inspected (with strong magnification) all solder joints.
5) Tried plugging and unplugging the oxygen sensor connector (near the fuse/relay carrier in the engine bay).
6) Checked the ground connection near the overboost pressure switch.
7) Took off the air box and checked for free movement of the air flow sensor.

I haven't found anything wrong so far, and the car still barely runs. Idle is OK, but opening the throttle (even slowly) brings coughs, sputters, backfires, and sometimes a stall. In my original post, I related the problem to temperature, but that was incorrect. The engine behaves the same whether cold, warm, or hot.

The mechanic told me that he checked the fuel pressures, but I'd like to be sure and do this myself. Where's the best place to attached a gauge?

Any ideas what I should do next?

 

Recommend checking your fuel filter and pump..

 

Changing the fuel filter is easy and cheap so do that first as suggested above. Also, make sure that the fuel pump in the engine compartment is working.
You need to check your cold and warm control pressures. If your mechanic is experienced with K-Jet he can easily do this. Otherwise you will need the Bentley manual, a fuel injection pressure gauge (available from any auto parts store) and fittings (not sure which ones) to do your testing. If control pressure is the issue the WUR will need to be rebuilt. Brian Leask is the best source of this service as he can make the WUR adjustable (stock ones aren't).
Here's the link to his site: http://members.***.net/930wur/

Also, post your issue in the Pelican Parts turbo/supercharging forum. Lots of experience with K-Jet over there and how to test it.

Good luck.

 

my 964 turbo did what yours did. i took it to three shops before mb euromotors fixed it. i was some sort of computer box with a bunch of plugs that had to do with cold starrt. The part wasn't cheap. I'm sure its common.

 

Here's an update on what I've accomplished since my last post.

1) Checked both fuel pumps and verified that each is operating properly.
2) Checked the resistance of the heating element in the WUR.
3) Connected a pressure gauge and verified that control pressures are within spec. Also, there is no change in pressure when the engine starts misbehaving.
4) Drained the gas tank and flushed the lines with fresh fuel.

No change in performance; something is still very wrong.

I noticed that there is considerable pressure in the fuel tank after the engine has run for a few minutes. There's a big 'whoosh' when you crack the cap open. Is this normal?

There is a plug with two brown wires dangling from the relay panel in the engine compartment. I don't see anything that it would plug into. What's its purpose?

On top of the air pump is a bracket that holds the oxygen sensor connector and another electrical connector (with three conductors). What is this connector for?

 

The plug, if next to the EZ69 loom is likely the low octane jumper. I answered a question about that not so long ago and included pictures.

Is there another 91-94 turbo in your locale? I found swapping parts, particularly the control unit to be a good way of diagnosing.