I think it's doubtful that oxygenated fuel would cause these symptoms. By law, we have oxygenated fuel year round in CA and morning temps are now around 20F where I live with no detectable change in car performance or fuel economy.

I wonder if the engine temp. sensor for the ECU has gone bad? If it is signalling the ECU that the engine is always fully warmed up, even on a cold start, that may lead to stumbling and lack of power until the engine actually does warm somewhat and the O2 sensor starts to "run" the computer. Would not explain your poor fuel mileage though, but short trips in cold weather would. (Are you using lighter oil: 5W30)?

Don't trust those airbox thermostats. I replaced these in both 240's with units from FCP Groton about 18 mon. ago and one has failed already - keeps the preheated air from the exhaust manifold coming. In cold (30F) weather it won't get much above 100F (my actual measurement) in the intake plumbing, which is well within the temperature capabilities of the AMM, etc. This wouldn't cause the problems you're having, but watch out when warm weather returns, or you could fry your new AMM.


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Bob (81-244GL B21F, 83-244DL B23F, 94-944 B230FD plus grocery-getter Dodge minivan, MGB, and numerous old motorcycles)