Hello Gentlemen!

Heater works fine at idle but losing heat when engine revvs and when the car starts moving and driving.

Tried disconnecting the electrical plug on top of the heater mono-valve, same thing. Heat works fine at idle but losing heat when driving.

Even when the Defrost button is activated, same thing, hot air at idle and cold air when driving.

What could be the problem?

Thanks in advance and any help, ideas and inputs are greatly appreciated.

By the way, these conditions are all with water temp reads at 80 deg C at the gauge, after minutes of driving.

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'85 300D Turbo - CA Version

Classic symptoms of a monovalve issue. Check to see if the diaphragm is torn, you are getting proper connection at the electrical plug, you are getting power at the plug and that the monovalve operates when power is fed to it.

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1983 123.133 California
- GreaseCar Veg System

But isn't it that if the monovalve diaphragm is torn, I will have constant heat because power to the monovalve shuts the valve off that is why one of tha classic symptom of a broken or torn diaphragm is no cold air in the summer?

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'85 300D Turbo - CA Version

Pull the monovalve and look??? it only takes a minute. Don't drop the screws.

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.

Bad monovalve.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

The diaphragm has nothing to do with power to the monovalve. A torn diaphragm prevents the monovalve from remaining open under spring pressure when the solenoid is deenergized.

For sure, the most common condition that results from a torn diaphragm is good heat at low rpm and poor heat at high rpm.

Monovalve it is!

Just swapped the monovalve with a known good one and heat is restored at all engine speeds and all driving conditions. Surprisingly, the bad monovalve has NO torn diaphragm...there must be a micro-puncture that I cannot see..... But anyway, a monovalve from my stash of boneyard finds did fix the problem.

Thanks Gentlemen!

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'85 300D Turbo - CA Version

there is a fine copper gauze filter it can be plugged and or the rubber sealed off under pressure.
one of my car was plugged.

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