Outside air temp related engine problem: VTEC-like power delivery 3-5k rpm

[goldNSX]

My engine shows the following symptoms: Now with the colder air temps of about 10 degrees Celsius outside the engine shows a somewhat VTEC-like power-delivery at part of the throttle between 3-5k rpm.

Scenario
2nd gear, 3k rpm, starting with constant speed, then stepping on the throttle slowly within about 1-2 seconds and redline. As long as the throttle is not fully pressed the engine seems to accelerate normally and suddenly delivers more power (VTEC-like). It has nothing to do with VTEC as this happens at around 3-5k rpm. The engine doesn't sputter, vibrate or something unusual, just pulling less during part-throttle maybe for half a second. If you don't know an NSX you could believe this is normal but it certainly isn't.

This could not be observed at air temps higher than 20 degrees Celsius. I had this last winter too. It's very obviously outside air temp related.

I suspected the fuel pump but 40 psi at idle are ok and I could not imagine it being bad as it should have showed the same symptom at > 20 degrees C. outside too.
I just measured the thermo unit which sends the intake air temps to the ECU. The values at different temps (-15 to 80 degr. C.) are a little bit higher than specified in the SM but not off by far. I'll change it as it doesn't cost a fortune.

My last thought will be the o2-sensors being the first ones. I have a Taitec header, could this be a problem? Exhaust gases cooling down too fast? I know that at part of the throttle the o2-sensors are regulating while at full throttle they're 'neglected'.

Any suggestions highly appreciated.

EDIT: fuel consumption is normal and no CEL or anything just the observation.

 

[pingu81]

I know exactly what you are talking about. I had kind of the same problem except that on top of the hesitation, sometimes my CEL would come on and would throw a code 1 (primary 02 sensor). My O2 sensors were pretty much new. I have aftermarker headers, exhaust, and test pipes.

I installed 2 mechanical O2 simulators (basically just extend the O2 sensor further out). After that, the problem happened only a couple times. I then reset the ECU and haven't seen the problem since. It pulls nice and hard now.

Mechanical O2 Simulator:
http://www.ichibausa.com/o2siadcelfix.html

The ones I used were home-made for $12 from parts available at Napa. The tuturial was online but I can't remember where.

 

[GreyHoundSteve]

Following thread. After i installed my headers and test pipes i am also getting the occasional CEL and the car is not as smooth as it should be when accelerating.

 

[goldNSX]

Thanks very much for your input!

The interesting thing about my engines behaviour is that it's only at low temps outside (< 10 degrees Celsius/<50 F).