mdj577

86' 951, 96K miles

A few months ago my car developed a rough idle, started back-firing and had very low power when accelerating. Mechanic replaced spark plugs, wires and distributor. This seemed to solve the problem as the car behaved perfectly after the work was done. Now the rough idle, backfire and anemic power has spontaneously returned.

Can anyone point me in the right direction to diagnose this once and for all?

ramius665

Sounds like it's running lean. I'd start by checking the fuel pressure.

mdj577

Thanks. I have a 3bar fpr and chip from Lindsey Racing. If it is indeed running lean how do I remedy this? I assumed that the chip was not interfering with anything as it has been running well for some time.

Tom M'Guinn

You might pull the plugs and see if they looked fouled by excess fuel. Other than coincidence, one could imagine you have some engine management issue that is causing excess fuel to foul the plugs, which might explain why it ran well for a while after changing the plugs but is now back to missing/backfiring. Do you have a wideband or any other data that might help? In addition to checking the regular engine management culprits (temp sensor, AFM signal, fuel pressure, stuck injectors, etc.) you might also confirm the timing belt is aligned since it is backfiring. Timing belt issues are not likely to come and go, so probably ok, but easy to check whenever you have unexplained backfiring. Water in the fuel also comes to mind...

mdj577

Thanks Tom. I don't have any other data. The fpr is new, the belt was done with the water pump, rollers etc 2yrs ago and checked and re-tightened shortly after. The car does have a Fabspeed exhaust with cat delete.

I will have it checked out for excess fuel. If it is engine management I feel it is going to be a pain to track down/fix.

asmperformance

There is nothing complicated about the 951 especially compared to today's cars. Any good tech should be able to do the basic diagnostic procedures necessary to solve your problem. The last place to look is with the chip you're using. Programs don't change so if your car was running fine then a problem occurs it's most likely not the chip.
These cars are old and I've seen many problems with fuel injectors manifest themselves in this way. The other basics you should check are fuel pressure and volume, throttle position switch signal, air and/or vacuum leaks. The turbo uses a different TPS than the other 944s and has a potentiometer that sends a linear voltage signal to the KLR. Make sure that the voltage signal is correct.
This is pretty basic diag for any professional tech. Hope you find someone local that can help.

black944 turbo

Have you done a boost leak test? This would be my first check. Then the distributor cap off and check to make sure the rotor is tight and has not moved. Then go onto some of the other tests suggested.

Willard Bridgham 3

I'm betting on a loose vacuum or pressure hose connection.

fejjj

2nd on the distributor rotor.

mdj577

Thanks for all the advice guys. I have a local Porsche mechanic who has been good so far. I will have him take a look.

When the car is running well, which it has been the majority of the time, it is a great car. The build quality seems fantastic despite its age, is plenty fast once it gets going, but more importantly it delivers a great driving experience at just about any speed. I really enjoy the mechanical. My friend has a C4S. Performance is impressive but the 951 feels just a bit more fun. Truly a wonderful experience.

mdj577

...mechanical feeling of the car