This is an urgent one, as my car ('90 944S2) is currently at my local Porsche Centre after they fitted a new TPS (they were fixing the A/C, so thought they could do this at the same time). Some of you may remember I am trying to trouble shoot a slight bunny-hop at 2000rpm in 4th and 5th... I have so far re-aligned the AFM and cleaned, new O2 sensor, new ICM, cleaned the throttle body and ICV, and also cleaned all the earths, injector lead connectors, etc.

Prior to today, the car always started fine, with the cold start working and then settling to approx 850rpm. Even last year when the A/C worked, I never experienced an issue. Sometimes, and only briefly when driving the engine warm and coming to an intersection, clutch in and the idle would fall to about 700rpm and hunt ever so slightly 750-850rpm, but usually settled down to around 800rpm.

But today when I was collecting the car, I started it cold and the throttle jumped to 1150rpm with A/C engaged, soon resting back to 1000rpm and slowly down to 900rpm. Then drove the car 500m and the engine throttle was bunny-hopping around at low rpm, and at the intersection the car almost stalled out. Took it straight back and resting warm at the dealer the idle didn't want to go above 600rpm, fluctuating up and down as low as 400rpm. Surely just a new TPS would not cause this???

Please help, as I am not sure Porsche know what is happening. Thanks!

 

Sounds odd, unplug the TPS if it idles normally then the TPS is bad or mounted/set improperly. Double check for a vacuum leak, especially if the IAC is messing up cause even a warm car can run bad or shut off due to that valve.

 

Thanks for your info... I went back to them and the tech just fitted the new TPS without adjusting for the idle... and they didn't test drive it afterwards, thinking it was just 'plug and play'. So the tech got a stern talking to by the PC manager, and the car is back and runs great.

One small strange side effect though, is now if I pump the accelerator while parked, pushing hard so it revs over 3000rpm, and immediately take the foot off, the tacho needle drops almost down to zero before settling again at around 850rpm???... the engine is definitely not dropping that low, as there is no sign of stall, vibration or low engine speed below 850rpm... but I have never noticed this before. If I just ease off the accelerator, it drops nicely back to approx 850rpm without the needle travelling any further. Weird.

 

The rpm dip to zero is indicative of a vacuum leak. When you Rev the engine, it's pulling alot of vacuum which would make the motor almost stall or goto zero rpms because of the vacuum leak. Hence the less dramatic effect when easing on the throttle.

 

Thanks for the feedback. Weirdly it only happens when I am tapping the throttle up to approx 2000rpm... if I tap it higher than that, the rpm and tacho needle drops rapidly down to approx 1300rpm for half a second before returning to a consistant 850rpm idle. Tried even higher rpm with immediate release of the accelerator, and the rpm/tacho never drops to zero. Also, when it does occasionally do the severe drop, the engine is no where near stalling or go to zero rpm... just the tacho needle for some reason. I would have thought a vacuum leak would give a consistent issue at all rpm fall? Maybe I should do a quick video and post it?

 

If any instruments are proving to be inconsistent clean all the electrical connections first before doing any other diagnosis, especially the connectors at the back of the instrument panel and the grounds in the drivers footwell. Many spurious electrical issues on our cars are due to dirty contacts and grounds.

 

Yes, understand that. I have already gone over all the grounds in the car, cleaning them thoroughly (the ones in the driver's footwell were in very good condition). I have also pulled out the dash, cleaning all contacts, polishing the football connectors, mew bulbs, etc. Maybe the actual tacho is faulty... as it seems weird that the anomaly I am experiencing only happens at a particular rpm.

 

Could be, if you’ve already tried all the common causes and you really can’t live with it, you could send it to PA Speedo for checking out. Thing is the instruments are 30 years old, stuff wears, things aren’t going to work like when they were new, so either getting it professionally repaired or buying a new one are the only reliable options.

Personally it wouldn’t bother me but if it did I’d buy a good used one, swap it out and see if the fault transfers. At least that would confirm a fault with either the instrument or the car.

 

Thanks again for the advice. Yeah, I can live with it too... just thought it may have been a byproduct of the new TPS. Unfortunately not really practical sending it to the place you suggested when I live in Sweden :P