Hello all. I posted a little about myself just before the crash last week so if you did not see it then I was just introducing myself and saying great forum. Anyway, my '84 300D is having a heater performance issue. The fan/speeds/auto work fine as well as the blend control. My issue is when I drive over 60 mph for over 10 minutes, the heat starts blowing cold, not ambient cold but definately not hot. The only thing I need to do to make it blow warm again is slow down like getting off the freeway. The thermostat is fine, the temp runs about a needle width over 80 degrees celsius. I know the symptoms sound like a head gasket but the engine is solid and shows no other signs of a bad gasket. The coolant is clean and stays full. I'm thinking there is a bunch of loose corrosion chunks blocking the heater core at times of high flow or something weird like that. The car had a new mono valve and aux water pump from the previous owner too. Just curious before I tear into the cooling system. Well I'm off to Sam's to get a couple cubes of oil. By the way, can I go as high as 80/20 oil/diesel in 30-40 degree weather? I am half done buying parts for an actual veggie conversion. When I have a few more minutes I'll re-post on how I repaired my B2 piston. You guys rock! Eric.

Do a search on mono valve. Sounds like your is inoperative.

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Jim