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#1
03-05-2002, 08:38 AM
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no heat on the highway
'85 300 TD
For some reason I have no heat on the highway. I get warm air once the engine is warm, but when I begin to drive at highway speeds it just blows cold air. What could cause this? Could the thermostat be stuck open? -Erik 
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#2
03-05-2002, 08:45 AM
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Good guess, it sounds like your t-stat is stuck open to me.
__________________
2001 Dodge Ram 2500 Cummins Turbo Diesel - 4x4, auto, 3.54 gears, long bed ------------------------------------- '92 300D 2.5 Turbodiesel - sold '83 300D Turbodiesel - 4 speed manual/2.88 diff - sold '87 300D Turbodiesel - sold '82 300D Turbodiesel - sold
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#3
03-05-2002, 09:06 AM
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Well... what is the coolant temp gauge reading? Does it go drop down close to 80 or lower once on the highway with a previously warmed engine?
__________________
2006 E350 w/ 155k miles (Daily Driver) Previous: 1993 300E 3.2L Sedan w/ close to about 300k miles 2003 E500 Brilliant Silver (Had 217k miles when totalled!) 1989 300E with 289,000 miles (had for <1 yr while in HI) 03 CLK 500 cabrio (Mom's) 2006 C230k (Dad's) 1999 S420 (Mom's/Dad's) 2000 C230k Sport sedans 2001 CLK320 Cabrio (Mom's) 1995 C280 My First Mercedes-Benz... (155k miles. EXCEPTIONAL AUTOMOBILE. Was Very hard to let go of!)
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#4
03-05-2002, 09:42 AM
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Yeah, my engine temp looks to be around 70 on the highway. Is the thermostat easy to change? Do I need to drain the system first?
-Erik
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#5
03-05-2002, 11:07 AM
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Sounds like a bad mono valve.
__________________
Mike Tangas '73 280SEL 4.5 (9/72)- RIP  Only 8,173 units built from 5/71 thru 11/72 '02 CLK320 Cabriolet - wifey's mid-life crisis 2012 VW Jetta Sportwagon TDI...at least its a diesel Non illegitemae carborundum.
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#6
03-05-2002, 11:17 AM
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How does a bad mono valve behave differently than a thermostat that is stuck open?
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#7
03-05-2002, 12:07 PM
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Besides inspecting the monovlave, the easiest way to tell is to drive the car for 15 minutes around the neighberhood, shut the car off for a few mins, then start it again. If it blows hot for a minute or so (heat built up in the engine comp't), then starts to blow cool air, it's most likely the monovalve.
It usually fails because the rubber seal tears on the valve or the rubber diaphragm.
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