It almost sounds like we have similar problems. I never got over 60 deg C unless I drove at least 8 miles. I got under 15 mpg on one tankful of diesel. My mechanic changed the thermostat, and now I run about 85-95 deg C. My AC also quit late in the summer, but during the summer I'd seen 110-115 deg climbing a mountain with a full load with the AC running. 100-105 was a typical temperature with the AC on. I've been told I'm probably looking at a radiator, but now I'm not so sure that it might not be a thermostat. I still have 85-90 deg running temps, so my problem isn't fixed yet either.

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Bob Roe
Lehigh Valley PA USA
1973 Olds 88, 1972 MB 280SE, 1978 Datsun 280Z, 1971 Ford T-Bird, 1972 Olds 88, 1983 Nissan Sentra, 1985 Sentra, 1973 230.6, 1990 Acura Integra, 1991 Volvo 940GLE wagon, 1983 300SD, 1984 300SD, 1995 Subaru Legacy L wagon, 2002 Mountaineer, 1991 300TE wagon, 2008 Murano, 2007 R320CDI 4Matic 52K, some Hyundai, 2008 BMW 535xi wagon, all gone... currently
2007 Honda Odyssey Touring, 2014 E350 4matic