Hey guys. I've been away from the forums for a while, I finished up replacing my windshield this winter, and drove the car for a little bit. I have been spending my time doing woodworking and woodturning lately.

One nagging problem I have had with my 85 300D is the monovalve. My air conditioning system has been rebuilt and is working properly, except sometimes my heater will overpower it. As near as I can tell, the issue is that the monovalve is getting forced open when the engine is at temperature. If i start the car cold, and run the AC for a while it works great. If I then stop, shut off and start the car while the engine is still at temp, then drive off I will frequently get hot air from the vents (not ambient temperature, hot as in the heater is running). This also happens after it heats up and I turn off the AC then turn it back on, or if I use anything other than max cool (which cycles the monovalve). I am 99% sure this is not an electrical issue, but is related to either water pressure or thermal expansion in the cooling system.

So, besides trying another monovalve (currently have a Bosch core - I think - with an MTC solenoid, which i have messed around with repeatedly, my current thought is to install a valve inline with the heater core that I can control manually- close the valve during the summer to ensure I have heat.

My questions are: who else has tried this, how well did it work, what size and type of valve did you use, and where did you get it?

Thanks!

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'85 300D - federal spec, built in late 84. 85 300D Complete AC System Rebuild