Hi all,

I'm looking for some help to diagnose an intermittent rough running on my 1998 3.4 996 (manual coupe, if that matters), which occurred twice over the last 600 miles or so.
What happens is the engine will start to run a little rough and feel like it's down on power and doesn't really want to rev, a little like an engine that's about to die due to an empty tank. The issue is transient and only lasts 30 seconds or so before the engine gets back to working normally, which in these cases didn't leave me the time to stop and check the idle. No codes, no lights.

Here's what both occurrences had in common as far as conditions:
-driving somewhat dynamically in twisty mountain roads
-dry weather, once at around 2000ft elevation, the second around 6 000
-engine fully warmed up, with the needle about 1/8th of an inch from the second temp bar
-tank almost full with fresh high octane fuel
-revs around 2-3000 rpm

I've ordered some MAF cleaner and cleaned the MAF (and the ICV while I was at it) but I doubt it'll solve the issue. Coils & plugs are almost new, lifters are new, variocam pads too.

Any ideas?

 

fuel pump failing? have you replaced recently the fuel filter?

 

I had that happen with my '05 997.1. Loss of power once in a while. Ended up as the brake peddle switch. Sounds crazy but a Rennlist member told me to replace ( cheap and easy) and never had the problem again. That switch gets worn out where when you brake the switch doesn't go back to normal. Other members may comment on this! Good luck!

 

I too am battling an intermittent rough idle in my 1999 C4 manual coupe, noticed only when the engine was hot and in stop & go traffic. Changed the injectors to the 996.2 style injectors 2 months ago, although I did notice an improvement for how "balanced" the engine seems to run at low RPMs, but the random rough idle still kept on until I recently changed out the MAF sensor with a new one. After ~1000 miles, so far so good, but havent had it really hot weather in stop and go traffic just yet. New coil packs, new crank position sensor, cleaned throttle body, and vacuum leak checks were all done prior to this issue as well. Will use the durametric to make sure my fuel trims are within spec.