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 (sold)
1973 220DPK 4 spd (died)
1978 350SL 4 spd gray market
2003 SLK32 AMG
1988 300CE (sold)
1997 E420 (Totaled)
1998 E320 Wagon

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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__________________
ASE Master Mechanic
asemastermechanic@juno.com

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.
Vehicle build.
Fleet Durability
Technical Quality Auditor.
Automotive Technical Writer

1985 300SD
1983 300D
1984 190D
2003 Volvo V70
2002 Honda Civic

https://www.boldegoist.com/

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 (sold)
1973 220DPK 4 spd (died)
1978 350SL 4 spd gray market
2003 SLK32 AMG
1988 300CE (sold)
1997 E420 (Totaled)
1998 E320 Wagon

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.

Classic monovalve 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.

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When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.

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.

.

__________________
ASE Master Mechanic
asemastermechanic@juno.com

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.
Vehicle build.
Fleet Durability
Technical Quality Auditor.
Automotive Technical Writer

1985 300SD
1983 300D
1984 190D
2003 Volvo V70
2002 Honda Civic

https://www.boldegoist.com/