Hot-Weather Engine Lag

aero-head

I know there are several threads about heat-related performance issues, but I haven’t found anything that seems to explain what I've been experiencing in hot weather since I moved to Phoenix a couple years ago, so please bear with me. My Z is a 2010 with 80k miles, manual transmission, and no aftermarket mods or tuning. Whenever the outside temperature is above ~95 degrees F, the engine is very sluggish below 3,500-4,000 RPM, above which it suddenly surges to almost the normal performance level. In addition to the general lack of power in the low-to-mid RPMs, it sometimes feels very jerky - when I press the throttle (smoothly - I'm not mashing it), the car lunges forward for a split second before pulling back to reduced power. The engine is even slow to rev with the clutch disengaged, e.g. when starting from a stop or rev-matching for a down-shift. The RPMs also tend to take longer than normal to start dropping when I lift off the throttle, which makes it harder to up-shift smoothly.

There are no warning lights when any of this happens, the coolant temp is a couple dots below the middle of the range, and oil temp is between 200-220F. When it's cooler outside or the car has been parked for a few hours, it performs very well with NONE of these symptoms... there's a substantial difference in drivability between my morning and evening commutes. I also never noticed anything like this for the first three years I owned the car (in Philadelphia, which is obviously not as hot but does get into the high 90s and occasionally triple digits).

I have been to four different shops (one Nissan dealer, one independent shop, and two tuners) which checked/performed the following: cleaned the MAF sensors, replaced a faulty MAP sensor, did throttle body and fuel adaptation relearn procedures, checked a brake light switch/circuit (apparently there was a TSB related to this), checked the fuel pump pressure, and checked the O2 sensors to rule out plugged catalytic converters. I even test-drove a brand new Z with the same weather, oil temp, etc. and didn't experience any problems.

Does anyone have any other ideas of what could be causing this? I'd like to keep this car as long as possible, but I've been so frustrated with the way it drives in the heat that I'm just about ready to trade it in for something else if I can't come up with a diagnosis/fix.