My 1985.5 NA 944 is having a misfire once the car warms up starting at around 4800 RPM and running till probably 5400 RPM then it straightens out and runs to redline. To my knowledge it wont do it in 1st gear and if it does its minor, but every gear after that it happens after the car has warmed up. It acts like it hits a brick wall and if I put the pedal to the floor it stutters and bucks incredibly hard and the Tac jumps around till it gets out of that RPM range and continues to accelerate to redline. It does not do it when not in neutral and sitting still. I replaced the spark plugs, wires, and the AFM ( admittedly this part was not new but i took it apart and cleaned both the original one and the replacement one) and had no change with it the issue. After driving I pulled off the vacuum hose on the fuel pressure regulator and no gas came out so I ruled that out.

 

Mine did this same exact thing early this week but it was at lower RPM (from take off) all the way up to 4,000 rpm. It idled fine, only did it when accelerating, not when driving at a constant speed. It was really bad when engine was cold, and got a little better when engine warmed but it was still doing, through all gears. This happened absolutely out of now where.... I drove it to work one morning and it was perfect, then I parked it, and when I went to drive it home at the end of the day it started doing this from the very beginning. It was severe, and really blew my mind that it was happening. Then a few days later I pulled the spark plug wires off each spark plug as I planned to pull each spark plug to check it. Everything I could see from the top side looking down in there with a flash light looked just fine, but it turns out I didn't have a socket to fit the plugs so I put the wire heads back on. So I go drive the car do some more studying and the darn thing worked absolutely perfectly.

I've had the car 3 months, and its a 1990 S2 NA, and it has run *perfectly* until this happened. No I kind of want it to happen again so I can get it fixed permanently but at this point, I will always fear that it could start doing it again any time.

 

Could be the speed sensor that determines the spark advance. The speed sensor is located on the bell housing bracket, there are 2 mounted, it is the one closet to the fire wall. Remove and see if there are any metal chips on the bottom, if clean, then the sensor could be in failure mode.

Maybe first make sure the reference and speed sensor connectors are ok and snapped tight.

Or it could be also the reference sensor is failing.

Both can be static tested by procedure in Clarks Garage Porsche website. To dynamic test, an oscilloscope is required.

Also, call Lindsey Racing and ask for Dave. he could offer advice

Another thought, there could be a leak in the vacuum system that controls the pressure of the fuel pressure valve as the rpm increases. Check if you have gas in the fuel damper valve