If you wired the valve to a switch, you would have full flow to the heater core no matter what when its turned on. You could turn the a/c on and mix the hot/cold air. You could even turn the pushbutton unit off and leave the valve on, and you would still have warm air come through the vents at highway speed. But this isn't really fixing the problem.

I was just thinking that maybe something is getting stuck when you reach the higher cruise rpm's, because there's higher water pressure there than at idle. But it still doesn't make sense. You would think something would get stuck and stay stuck, not go away at idle.

__________________
Streetside: 1987 Mercedes 300E Petrol Blue/Green
2014 Ford Mustang Grabber Blue
1999 Ford Ranger Green

Dockside: 1968 Eltro 17
1985 Silverton 34C

And I have no idea what the ball in the bottom of the valve does. I've never taken one apart. Maybe thats getting forced up into the valve when you reach highway speed, blocking flow? Did you replace this part when you rebuilt the valve?

__________________
Streetside: 1987 Mercedes 300E Petrol Blue/Green
2014 Ford Mustang Grabber Blue
1999 Ford Ranger Green

Dockside: 1968 Eltro 17
1985 Silverton 34C

Thats what I thought....


I'm going to replace the whole monovalve this time around, not just the plunger part. There might be something wrong with the housing, check ball thing, electromagnet piece that works the plunger etc

__________________
'89 300SE
'02 E320 Wagon

Alright, this weekend I hard wired the monovalve to a switch I installed in my cabin....basically detoured the temp wheel. With a flip of the switch I have full heat or full cold.

This is NOT a sloution, but part of my diagnostics

Same issue.....heat idle and in traffic, loss of heat when up to speed on interstate....this is with me flipping switch to the monovalve to give me full heat.

So, how is it that hot water is bypassing the heater core with monovlave open? (Operating temp never really moves past 80*)

Could it be a sign of not enough water pressure? like bad water pump, or a restriction? I am trying to rule evrything out before I drop $200+ on a monovalve!

__________________
'89 300SE
'02 E320 Wagon

Have you ruled out the blend (diverter) flap yet? I think you could rule this out by stuffing rags in the outside fresh air vents and then driving. If you get heat then while driving, sounds like it's the blend flap allowing too much cold air in while driving.

__________________
Prost!

I have ruled this out.

a couple weeks ago -- I have a modified monovalve plunger that I actually took the plunger, seal and screen out. There is nothing left to seal the hot water out of heater core. This gives me 100% heat always. The cabin even on a 45 minute interstate run would still give full heat....I had to crack the windows bc it was too hot!!

The problem is related to the working monovalve. Even when given 12v and it is supposed to give full heat, at speeds over 60 mph or so it does not. As soon as you slow, heat returns

Are the fresh air vents between engine and windshield?

__________________
'89 300SE
'02 E320 Wagon

I think on yours the intakes are to either side of the wiper.

__________________
Prost!