rebuilt monovalve, still not heat at hiway speed....
 

monovalve, still not heat at hiway speed.... on 85 wagon, hmmm... what would next be. I'm thinking thermostat as car never gets much hotter than 80 on the gauge, never has. I replaced the thermostat several years ago, when I was running even cooler.

Did you use the MTC kit? many have said the spring is not strong enough, and coolant flowing opens the valve. The hot trick (pun intended) is to place an Oring down in the bore so the valve is tighter. It will still allow coolant to flow in the default position.

Think monovalve. 80C is plenty hot for the heater to work.

If the valve opened, the heat would work better, not worse.

When's the last time the heat worked? Is it possible the heater core is clogged up? Any idea if the aux pump is working?

I am betting it was the last time the car slowed to under 35 mph.

I am pretty sure your problem is the the valve, if you also replaced that. When I was getting at the exhaust manifold bolts to remove my trap oxidizer, I broke the plug and pins on the top of the monovalve solenoid. I had a new valve I had not installed, so when I replaced the solenoid, I put the new valve in. I had no heat at all over engine idle. I put the original, corroded valve back in and the heat worked, despite a plugged screen and slightly ripped membrane (or whatever it is called. It is supposed to fold neatly up into the bottom of the valve body when the valve is open.) I could not tell a difference between the spring strength of the MTC valve and the old or new Mercedes one with a travel scale, but in the open, retracted position, the MTC membrane/bellows did not neatly fold up into the valve body. It was rubber, and just kind of scrunched up unevenly. The Mercedes valve seems to have a creased rubberized fabric, and it completely folds up into the valve. My thought was that the bunched rubber caught the flow of water to the heater core, especially at higher engine speeds, and pulled the valve shut.

Anyway, I ordered a Mercedes valve from my closest dealer at less than three times the price of an MTC, and I have no problems.

1985 300 TD, 368,000, daily driver

The genuine part was less than three times the price of the MTC? Our sponsor has the MTC for $26.75, and the genuine for $39.

I worked at a Mercedes shop and was stumped when we installed a valve in a w126. Had me and my boss stumped for a while until I looked up price of a bosch and found the MTC super super cheap (like $12) and realized it was china crap. Confirmed, spring is too weak in the mtc to push against flow. Lame too, all they need to do is spec the spring stronger and it would work. Imagine how many they made.

Rollguy, That is a great price! It either was a lot more, or it was out of stock when I needed one pronto because of no heat about 10 or months ago. Was driving a couple hundred miles a day with no heat. I think they were quite a bit more even here not even that long ago. I know I was looking everywhere for one, and I certainly would have looked here first. A couple sites wanted over $100.

Bingo. I have tons of heat @ 40 or below, will bake you out of the car....

Ok, I'll buy the genuine part and see how that goes. good price

On another note, now I also have heat, even when I turn the temp all the way down below 40, it's like its stuck open. still I'll order the merc part tonight after I make a list of anything else I can add to the order to get the free shipping

Correct Mercedes made part solved the issue.

Excellent! Makes you wonder why the MTC part is even still being sold since it clearly doesn't work!