I have a late '03 6MT Type S coupe, originally a California car, currently 37K miles. It is in stock trim (no perf upgrades that i am aware of). I seem to have a periodic issue either with fuel supply or ignition that manifests itself as a slightly off-power and roughness or vibration that is the worst between 1500-2000 RPM's. When she's singing full song, it's smooth all the way form idle as I would expect, and runs like a raped ape, but when this issue is showing up, it just doesn't seem like it wants to rev. It doesn't matter whether the A/C is on or not, or what the electrcal loads on the car are: i've tested it under as many conditions as I can. It doesn't change during running: it will eithe be there or it won't starting from a full stop. I almost wonder if there is some economy mode in the engine management system that has gone wonky. Any clues would be appreciated - Stealth Mode

 

Is this during clutch engagement or is it an auto? Throwing any codes?

 

Your throttle cable and/or linage may need to be lubed. The TB plate may also be sticking. Try to clean out the throttle body with some cleaner & lube those areas.

 

Clutch is at full engagement, doesn't matter what gear I'm in. I'll feel it in 1st at pull-away or in 4th or 5th when the R's drop into that 1500-2000 range

 

That's been one of my suspects, I have some cleaner I'm going to try this weekend. It could definitely be a lack of airflow from a sticking TB, thus the computer is probably backing off on the fuel flow to manage the A/F ratio. I wouldn't think that would necessarily cause the roughness, but stranger things have happend. The throttle never feels sticky at tip-in or over it's travel.
Thanks guys!