ThunderC4S

Hi Guys,

I am helping a local Porsche 964 3.6T owner with an issue he is having relating to an off idle stumble. The vehicle is totally stock standard.

The information that I have so far is that the car idles OK but when you start to accelerate it starts to stumble and then higher in the RPM range it comes good again.

The car has some miles on it - so could it be a tired WUR or electrical (EZ69) module related ?

Any suggestions appreciated.

Rgds,

ThunderC4S (John).

X-1

Definitely check the injector stacks. Mine was doing exactly this until I pulled the manifold, installed new gaskets and reinstalled. The stacks are made of phenolic and shrink with age... and my manifold nuts were hand-loose! This was causing major air leakage under vacuum, and boost leakage under pressure. I had to reset my idle and CO after fixing as the problem had pre-existed the last tune-up.

Staffan

Could be o2 sensor, WUR, frequency valve, many things really.

Personally (no expert yada yada) I would start by disconnecting the o2 sensor, if that makes things better, it's probably something wrong with parts that regulates fuel.
When you disconnect the o2 sensor, the system goes blind (kind of "limp home" mode), the fuel system is then "true" mechanical.

If disconnecting the o2 sensor doesn't make any difference, I would do a CIS test (fuel delivery, fuel pressure, control pressure, spray pattern etc.), that would most probably give valuable information.

I think it's very likely a fuel injection problem, but you know, it wouldn't surprise me it would be something else.

Edit:
Just talked to a friend, he has a similar problem on his, car.
It works fine on full throttle, but stumbles otherwise.
They just found that his alternator is producing 16-17V, we think that could cause the problem (confusing or damaging the "computer boxes").

paul_23

Hi

I had similar problem especially when cold and warming up. After loads of checks, injectors being replaced/cleaned, HT leads tested etc etc the main problem was the black plastic trumpets on the inlet manifold that were leaking so no proper vacuum and then loss of boost on acceleration. All the gaskets were done at the same time and it made it much better.

However, there are numerous areas for vacuum leaks (even like the headlight angle adjustor on the dash) so all need to be checked. As others state, could also be the WUR, O2 etc.

There is a good fault finding chart on Pelican Parts web site I think.

Cheers and good luck

Turbohead

Sounds like it could be the acceleration enrichment control unit under drivers seat.

Elliot

ThunderC4S

Still checking nothing found as yet. The Problem will be found eventually !

Turbohead

BTW,
I have complete 3.6T CIS sytsem .

John McM

John, any developments?

ThunderC4S

Problem found. It was Internal engine related. Timing chain apparently.

Engine is under full rebuild currently.

Rgds,

ThunderC4S (John)

John McM

Interesting result, I would've put my money on an air leak. Please keep us informed as these topics are a great resource for later searches.

tjr

Good luck John. What are you having done - full or top end? Any upgrades "while you're in there"?

ThunderC4S

Not my 3.6T - A fellow Porsche Club members ride.

I am keeping mine stock (it's in too nice of condition to modify). I am looking for a higher mileage car that I could mod though but all I could find was a low mile 993 TT S.