Brian935

My porker has flatspot (feels like a miss fire on a cylinder) when accelerating beyond 3800 rpm. Not sure if there's a mis-fire or non-fire of a cylinder. Possibly fuel injection related? Just had a valve job done and new sparkplugs installed......Is this common or should i try some fuel injector cleaners, i.e., STP, etc....Thanks!

Steve Weiner-Rennsport Systems

Brian:

Troubleshooting an engine miss requires a systematic approach and some backtracking, especially after a major engine service like this. Assuming that it didn't have that miss before it came all apart for the valve job, I would check out:

Plug wires
Plug connectors
Cap & rotor
Head temp sensor & its connectors
DME relay
Fuel injectors and their connectors
Crank position sensors

Then,...check for vacuum leaks at the intake manifolds and associated hoses and fittings.

madmmac

Pull your plugs and take a look at them. Add valve adjustment check to the list also.

jmech

Check the the large 3 pin plug behind the engine, on the left side of the engine bay, this can get loose and cause a similar problem. Might sound obvious but sure wish someone could have told me (would have saved hours of frustration and head scratching) my local garage spent a week on it, and i ended up finding it by pure chance! Anyway good luck.

LongLiveThe3.2

I had a 'high performance' chip in my 86 911 that did that to me once. I replaced it with the original chip and it cleared it right up, ran great afterwards. That's the last chip I'll ever buy I think.

srf506

My guess would be to start with the plug wires. If you put new plugs in but not the wires you might have one breaking down under heat and load. I'd also make sure all of your grounds are clean and tight.