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Old 02-24-2009, 08:30 PM
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Join Date: Sep 2007
Location: Hillsborough, North Carolina
Posts: 110
A different HEAT question...maybe..

So I have been looking through all of the posts on heat and monovalves and the like, and can not find one that describes my here goes.

I have a 1981 TD that I have been driving regularly for three days(finally, long story), that is demonstrating a strange behavior. During normal mixed driving I usually have no heat. While I have no heat, my temp gauge crawls up to 100 degrees. The whole time I have my blower on hoping for I come to a stop light, as I pull away...HEAT! and the temp gauge goes down to around 80 degrees. All seems fine after that, but it does this every time I start driving it, I am thinkng thermostat, or monovalve, I dunno...any ideas? Seems like something is getting hung up, but what?

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Old 02-24-2009, 08:53 PM
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Join Date: May 2001
Location: Michigan
Posts: 1,373
Collect some other opinions, but I think its a monovalve. I remember mine fluctuating and sure enough, that was the issue. Plus, it's cheap and easy to repair, that is, if you can find a used one around.

I haven't priced new....

'84 300DT 298k (Aubrey's)
'99.5 Jetta TDI IV 251k (Julie's)
'97 Jetta TDI 127k (Amber's)
'97 Jetta TDI 186k (Matt's)
'96 Passat TDI 237k (Don's
'84 300D 211k Mint (Arne- Undergoing Greasecar Conversion)

'82 240D 229k (Matt's - Converted-300DT w/ 4 speed
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Old 02-24-2009, 09:03 PM
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Join Date: Jan 2007
Location: New Cumberland, PA
Posts: 833
Sounds like the monovalve isn't opening when it should. Chad300TDT just had trouble with his monovalve and he had just replaced it, and it was doing goofy things like heating until the engine got up to 2000 RPM, then no heat until he turned the heat off and back on (something like that).

If your temp gauge is getting up to temp after a reasonable amount of driving, your t-stat is probably okay, but if its climbing up to 100 with just mixed driving, maybe the t-stat isn't opening when it should, which could lead to other problems, but that could also be caused by rust/junk in the cooling system.

As for your heating issue, try unplugging your monovalve. When its not powered, its supposed to be open (someone will correct me if I'm wrong, but I'm pretty sure its supposed to work like that). Then turn on your heat and see if you have hot air. If not, maybe look into a monovalve kit. I've never taken one apart, so I don't know what to tell you to look for if its bad.
1985 300TD-euro 352,000 mi
1974 240D (1?)52,000 mi - has a new home now
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Old 02-25-2009, 08:35 AM
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Join Date: Sep 2007
Location: Hillsborough, North Carolina
Posts: 110
I figured it might be the monovalve, seems to be the common link it most heat related issues. I know were there is a good one in a junkyard, I will grab it and see what happens..t.hanks
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Old 02-25-2009, 01:53 PM
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Join Date: Jul 2006
Location: Stockton California
Posts: 637
Easy to verify. Simply remove the lid to the mono valve and ck the diaphram for being torn. I would also suspect the aux. water pump. If not working heat will be affected at idle, going cold. It will also restrict the coolant circulation until higher engine RPM. Just jumper wire the aux. water pump to the battery to verify it spins.

81 300CD (sold) 1972 280 SEL 4.5 (sold) 1966 250 S 4 spd (sold) 1974 450 SL (sold) 86 BMW 325ES (sold), 1973 280C (sold) 1988 300 SE.
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