Carpokes Porsche Forum

I have a 1988 951 race car with a gremlin I can't find. When the car comes to temp I get an intermittent choking or bucking that feels like fuel starvation (even with a full tank). I've replaced the DME relay, the wastage (found holes in the diaphragm) ands still nothing. The engine is stock except for 3" Lindsey exhaust and a chip. IT has about 25 hours on a rebuild and ran fine until recently. If I cycle the car on/off it will reset but eventually come back. Could this be a throttle position censor issue? That's my next replacement part. Open to any suggestions...track time is getting expensive and its the only testing I can do since the car is not street legal. Thanks!

Welcome to Carpokes! Super hard to guess without testing and more info. Over the years, people have reported stumbling when hot and the causes have been all over the map: Coil/ignition weak/going bad, DME Temp sensor, O2 sensor, Speed/Ref sensor connectors, Fuel pump going out, cracked solder joints in the DME or KLR, AFM traces wearing out, wiring issues, etc. A bad TPS wouldn't be my first guess, but if the idle switch or potentiometer are sending erratic signals to the DME and KLR (triggering over-run erratically for example), it's possible. I'd be inclined to do free/easy testing first -- bench test the AFM, TPS, check that FPR hold vacuum and has no gas in the vacuum line, inspect ignition (how old are the wires -- bad wires are a common cause of stumbling on full boost), etc. You might also check for KLR blink codes, and add some data logging capability to see if you can better focus in on what's going on. Wish I could be more helpful --any other clues as to what might be going on?

Thanks Tom, I know it’s a long shot to “diagnose” on a website but was more hoping for a “oh yeah that’s common and is a result of X”. We’ll look into those items you mentioned. I don’t have any way to check the codes at the track and unfortunately once the car is off, everything resets.

A little more info is needed - does this happen randomly or at a certain point range or boost range? Once warm dies it happen consistently?

Happens inconsistently once it reaches temp. Doesn’t appear to happen at a specific boost reading (I run a stock 12 psi) and there is no odd reading to the air/fuel mixture. Once it happens though the gauges go crazy and sometimes it will over boost, other times it will not.

Sort of standard answer….check all your grounds! For something to affect the gauges and performance says power supply/ground issues

When you say the car is stock with the exception of a chip, does it mean you are still using the cycling Valve to control WG actuation?

If still using the CV, then for testing, bypass CV with a MBC. If problem goes away, I would suspect a faulty CV (or possibly a bad KLR).