I took my beloved 74 for a sunny Saturday sprint and encountered some problems I haven't been able to remedy. The only background I should provide is that this is a 74 911 with a 1981 3.0 engine. It is not rebuilt, drinks a quart of oil between changes, but otherwise is pretty reliable.

When the car hits operating temparture two things are happening. Under load, the engine is running poorly. I hear some backfires and the power is definitely reduced. It feels like its missing. Seperately, and this is strange, as I come to a stop, the car idles fine for about 45 seconds and then roughly starts to idle at 1200-1500 RPM. It's almost as though extra fuel is being supplied. This higher idle is very rough. Remember, this is happening only when the engine is at operating temp. or hot.

I've taken it to my shop, but of course this problem doesn't materilize. Does anyone have any thoughts?

 

It seems to me you might have developed a vaccuum leak or two which is quite common with K jet injection. They have so many joints in the intake manifold ,where they can leak. What you need to do first is tighten every hose clamp on the engine, then try to establish if the plastic injection box has split or cracked. Check the seals on the injectors, the fuel filter, and the gasket under the sensor plate housing.
Regards Mike

 

just like spyder says,vaccum leak may be the problem,also a good tracker would be to spay some starting fluid around the injection system and see it the idle drops,or dies(boots around injection box,intake gaskets,around back be egr tubing) If that isnt the case,best to start with trowing fuel gauges on there,just to be sure(fuel pup,check valve or warm up regulater) Ive had a few similiar problems with no vaccum leaks,and found out the cold start valve was bad. have you pulled the plugs to see how each cyl is working?

 

"When the car hits operating temparture two things are happening. Under load, the engine is running poorly. I hear some backfires and the power is definitely reduced."

It's unlikely you have a vacuum leak, since the problem occurs at open throttle under load,
unless the leak is great. More than likely, you have a fuel distributor control pressure
problem, i.e. the control pressure is too high when warm - the mixture is too lean.
Have the control pressure checked on the WUR, e.g. 3.4-3.8 bars approximately per
Porsche Tech Spec when warm.