My Fuso truck has always had marginal AC. Number of guys on Expedition Portal complain about the AC problems. Last week, I took a non-contact thermometer on a road trip. Ambient temp was 90-95 degrees. At 62mph on the flat AC vent temp was 66 degrees. Later in the day in a headwind, uphill, temps rose to 72 degrees. When I pulled off the interstate and drove around town, vent temp would drop to 48 degrees within a minute or two and continued driving at low speeds around town temps got as low as 39 degrees. Back on the interstate, it jumped to 66 degrees again. 40 mph in 5th gear temp was 58 degrees.
Poster on Expedition Portal claimed problem was caused by the truck not having a valve to shut off coolant flow thru heater core. It just has a diverter flap in the ductwork. I installed a flow control valve in the heater hoses yesterday. If that does not solve the problem, does anyone know what might produce these odd symptoms of cold temps at low speeds and warm temps at high speed? Condensor is not in front of the radiator. It's off to the side and has it's own fan sucking air up thru it and that fan is working fine.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13