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  #1  
Old 02-11-2014, 11:32 AM
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Location: Savannah, GA
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1984 300CD Heat question

I have a 84 300CD and I am struggling to repair the heat. The problem is that when traveling over 50 MPH there is no heat. At less than 50 MPH the heat is fine. I have rebuilt the mono valve and replaced the aux water pump. The valve seems to operate normally. I can hear it pulsing, it shuts off water flow when in A/c mode and controls the air temp very well at less that 50 MPH. The aux water pump seems normal too. I can hear it running when the engine is off and the system is calling for heat and I can hear the coolant recirculating and I can hear the mono valve pulsing. I just not sure what to check next, any ideas? Again, the air at the outlets gets cold very quickly right at 50 MPH.
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1984 300CD
1973 220DPK 4 spd (died)
1978 350SL 4 spd gray market
2003 SLK32 AMG
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Old 02-11-2014, 12:24 PM
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FYI

If you can hear the coolant recirculating, there is air in the system.

It should be virtually silent if the system is full.

.
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  #3  
Old 02-12-2014, 10:38 AM
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whunter, is that the reason for loss of heat? What is the best way to bleed the air out? I would think that it would happen as all of the coolant recirculated.
Thanks for your help, I do appreciate it!
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1984 300CD
1973 220DPK 4 spd (died)
1978 350SL 4 spd gray market
2003 SLK32 AMG
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  #4  
Old 02-12-2014, 10:49 AM
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Quote:
Originally Posted by jaybutter View Post
I have rebuilt the mono valve...
Loss of heat at highway speed is very likely to be a monovalve problem. There have been a number of reports on this forum of defective aftermarket monovalve inserts that cause that issue.

Quote:
Originally Posted by jaybutter View Post
Again, the air at the outlets gets cold very quickly right at 50 MPH.
Classic monovalve symptom.

Quote:
Originally Posted by jaybutter View Post
What is the best way to bleed the air out?
As designed, the 617.95 engine is self-bleeding. The thermostat housing features a passage that bypasses the thermostat; any air should wind up in the expansion tank.
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Old 02-12-2014, 12:38 PM
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FYI

Quote:
Originally Posted by tangofox007 View Post
Loss of heat at highway speed is very likely to be a mono valve problem. There have been a number of reports on this forum of defective after market mono valve inserts that cause that issue.

Classic mono valve symptom.

As designed, the 617.95 engine is self-bleeding. The thermostat housing features a passage that bypasses the thermostat; any air should wind up in the expansion tank.
In theory it is a self-bleeding system.

Real world experience proves that air generally becomes trapped in the heater core / cylinder head, causing issues.

If you can hear the fluid flow = there is air trapped in the system.

.
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Prototype R&D/testing:
Thermal & Aerodynamic System Engineering (TASE) Senior vehicle instrumentation technician.
Noise Vibration and Harshness (NVH).
Dynamometer.
Heat exchanger durability.
HV-A/C Climate Control.
Prototype Vehicle build.
Prototype Fleet Durability
Prototype vehicle instrumentation.
Technical Quality Auditor.
Automotive Technical Writer

1973 300D
1973 309D - stolen
1978 280SE
1980 240D
1983 300D
1984 190D
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