OK, I don't think it is a vacuum problem. I have been doing extensive reading and have reached a tentative diagnosis - bad monovalve, or what is actually called the heater valve in this car.

The suction pipe is always cold, even when the vent temps are warm, and the heater hose that runs into the cabin at the driver's side firewall is hot. The problem is sporadic and seems to increase under load - if you sit and idle the A/C generally kicks in. It seems that hot coolant is counteracting the effects of the A/C system. A sticking valve, or some other electrical problem is the only thing that makes sense to me giving the sporadic nature of the problem. All of the vacuum stuff seems to be working right - if I push DEF then the air blows out the defrost vents, the Bi-Level setting works right and if I switch the temp wheel to 85 deg, then the floor vents kick in and no air out the center vents.

I suppose it could be the compressor cutting out - slipping belt or a temp sensor, but I haven't checked that yet??? It is harder to tell in this car when the compressor kicks on and off....

What do you guys think? I should also point out that this car has not been driven much in the last several years. I know for a fact that in the past 2 years, the PO put 400-500 miles on the odometer.

ryan

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RG Newell

1984 300D
1972 250
1986 560SL
1991 300CE