At first it almost sounded like you have a headgasket problem, but on the other hand it Sounds like you have air in the system or a somewhat clogged heater core. I would try to bleed the cooling system first. I like to get the RF corner of the car jacked up to have gravity help me a little bit. Top the expansion tank off and bleed from the t-stat housing and the overflow hose going to the expansion tank. If that doesn't work, you mite want to try a CLR soak and flush of the heater core. I used to do that to Audi's/VW's that came into our shop. They have a issues with heater cores clogging because the coolant never gets flushed out on a regular basis.It worked about 80% of the time. Give that a try before you investigate headgasket issues

Headgasket, timing chain/set, valve train all new in Oct. I was thinking maybe headgasket failure also, but nothing else points to that...no loss of coolant, no water in oil etc. Yep, plan on heater core flush this weekend, but if it were clogged, wouldnt I never get heat?

This morning on my way into work, no heat and this: I heard a little 'click' behind the middle of the dash and WA-LA, HEAT!! Is there a vacuum pod or something back there that would not be opening/closing properly?

If I remember correctly, there is a small plastic filter basket within the mono valve housing. I believe you have to remove the mono valve to look down inside to see it. I'm thinking If you flushed your coolant system recently (recent radiator replacement) this filter may have enough sediment build up to cause your issue.

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BENZSTER

W123, '81 240D, Pastel Grey/Blue Tex
W126, '88 300 SEL, Artic White/Grey Leather

I didnt realize that. I have installed new thermostat and radiator and flushed the heater core. I have constant heat with the 'modified' monovalve I installed. When the new CCU arrives and I install the correct monovalve I will check for that screen in there...good tip, THANKS!!

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'89 300SE
'02 E320 Wagon

Slow down a little. You had said earlier that you still didn't have heat with the monovalve disconnected. The ccu can only turn the monovalve on or off. If it is unhooked, it is off. Off = heat. You now have heat with the modified valve, so that means the valve is bad. You can put in a new ccu if you like but I don't think you need it. Never replace parts until you prove they are bad. You have proved the valve is/was bad. The new ccu may not be as good as the one in there now. Fix/replace the valve and go from there.

Paul

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84 500 SEL (307,xxx miles)

Hey Paul,

I have replaced the monovalve twice. This last time there was nothing with the valve but at $15 for the new valve I figured it was worth a shot. It did not fix the problem. Now with the modified valve in I have full heat, I believe the CCU is bad, tellinig the monovalve to close when it shouldnt. New CCu should be here by weeks end so I will let yall know. In the meantime I am enjoying full heat!!

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'89 300SE
'02 E320 Wagon

Still full heat with the modified monovalve....drove nearly 200 miles yesterday comfortably!!

New CCU came in yesterday, hope to install that and new monovalve this afternoon, will report back....

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'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.

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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?

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'89 300SE
'02 E320 Wagon

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

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Prost!