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964 not revving under full throttle

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My car since new is a 964 C4 Cabrio from 1991. Since it is a bit of a challenge to stay faithful to any object for this long, I have had quite a few upgrades installed during the years. Most relevant to my cry for help are the HotFilm air meter system seven years ago and a couple of years later a Supercharger kit with appropriate ECU upgrades. I have been very happy with both of these and for a few years the car was running great a had a stunning performance.
A year ago the car developed the following fault; It runs fine in "normal" circumstances but if I accelerate with full throttle - any gear - it only revs to c.5000 and that's it. Less than full gas and it runs to red line with no hazzle.
Anybody having any ideas for the reason for this, please let me know! Can't really participate in any track days with my "additional limiter".
 
Thank's for the idea. Unfortunately it has been done and did not solve the problem. Also, because the problem appears with all the gears at c. 5000, I have assumed that it is not mixture related. If it was, should not the symptom appear at different rpm, or am I lost here?
 
If the engine runs to the red line if the throttle isn't fully depressed I can only guess that there is something amiss with the ECU interpretation of the WOT switch input. My limited understanding of fuelling would lead me to believe that the ECU would adjust the fuelling when the throttle is wide open (WOT switch activated) and a failure to do so would result in a too lean mixture. If you've had the fuel/air ratio checked it would suggest that is not the case.

If the problem is only apparent when the throttle is wide open I can't imagine any other root problem than the WOT switch or it's influence.

I'm confused by your assumption that "should not the symptom appear at different rpm". The engine management has little concept of road speed - only engine speed. So any fuelling or ignition problem is likely to occur at the same engine speed, regardless of what gear or road speed.

Regards

Dave
 
Thank's for the good comments, Gents! The fuel filter has been changed and the injectors checked (I really have had a lot of time and money put on this culprit). WOT switch I think has not been checked, how do one do it? And the - maybe fuzzy - logic behind my comment on the gears and the rpm's was because I assumed that the EM would take into account the different strain for the engine accelerated flat out on different gear.
 
There is a description of the test measurements on page 390 of Adrian's book. I'm short of time at the moment else I'd repeat it here.

The significance of the WOT switch is that when it's in the Wide Open Throttle position it switches the DME into Open Loop Mode - i.e. it ignores any input from the oxygen sensor. If this doesn't happen the DME will still be trying to adjust the mixture for an optimum lambda (i.e. 14.7:1) and the engine may be starved of fuel. It's just a thought - I've got no evidence that I'm right!!!

Regards

Dave
 
Dave has a point here, I'd be getting a helper to check the throttle and WOT contact, then checking circuit continuity on the WOT switch.

Kevin
 
Thank you for the advice, guys! I'll get the patient to the Porsche service here. Only one in here - Finland - which does not do much good for the expense for the work. I'll let you know if the WOT -check will cure the problem.

Jyrki
 
who did the chip adjustment - I would start there - call whoever did this conversion. Seems something is reading throttle full on and restricting the fuel - so pretty certain it is not the usual fuel pump, injectors etc. Does the supercharger have a full throttle input signal I wonder? Regards Alexander
 
Hi Alexandre! Because the car worked great for years after the chip upgrade and supercharger installation I can only assume that all the necessary input/output were once in place. Also, because the test with an other car's ECU did not help, the fault has to be outside it. I'll try to get the car to the OPC and ask them to start with checking the TWO -sensor's functionality. Hopefully they don't look at a converted Porsche too crosseyed..

Jyrki
 
That's an easy one, bones. There are none here that are really concentrated on Porsches. The one I have used thus far to sort this problem out is a specialist on race and specialist cars but they gave up after having tried everything within their scope.

Jyrki
 
Sorry didn't read the earlier post where you said you were in Finland , I have a friend who lives in Riistavessi. Good luck with the OPC.
 
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