Good thoughts. Fillup late last week was 14.2 mpg. MPG has been steadily declining since August or September. New tires in September, and AWD was reactivated right after Thanksgiving. New fuel pressure regulator in June. Average mpg mostly in town is now 13 on the dashboard (usually correct when I check it against odometer at fillup), and it was 17 before all this. Last trip was 22 or 23 mpg and before would be 25-26.

@jwalker, you may be right that the combination of tires and awd have decreased mpg. I didn't even consider that reactivating awd would affect mpg. Xemodex did a rebuild on the DEM. I was able to confirm that everything else is in working order and within specs. A new filter, drain and refill Haldex fluid, and a lot of aggravation later, voila the awd works.

The whine developed during this time. I've not been able to isolate the source, but best guess is between the radiator and engine. It starts 20-30 seconds after starting the engine and can also be heard on acceleration. I initially thought it was the turbo getting more noisy, but I was able to rule that out.

I expected mpg to drop with new tires and reactivating awd, but not 4 digits worth.

I'll check the trans fluid this afternoon. It seems I can rule out the oil trap for now. It's not raining today, perhaps I can do the glove test this evening.
--
94 944 na 319,000 and rolling; 05 XC90 2.5t AWD for the wife; 97 Chevy Suburban 3/4 ton 6.5 diesel daily driver; 92 745t beautiful condition (sold); 88 764t failed experiment (sold)