'89 300SE weird HEAT, 2/11/13 update
 

Ok, now I'm frustrated. Here is the issue that started about 45 days ago. I mostly get heat while idling, but usually after few minutes it gets cold.. When I'm driving I usually loose heat. Its not the loss of air...the fan blows nice and steady, its the temperature of the air that is the problem...it gets cold!

I have replaced the monovalve TWICE. I have replaced the climate push button dash unit. I have replaced the vacuum unit with all the vacuum lines. I have replaced the radiator. I have replaced the thermostat....she runs just over 80*C..... It still acts the same.

What controls the temperature? Is it the monovalve? Is it closing for some reason and turning off the hot water? What would cause this?

If I put 12volts to monovalve leads I can hear/feel it clicking open/shut

I am at a loss and getting very frustrated and COLD......I hope someone can throw me something!!

Try it in defrost mode. That puts it in full hot and all air to windshield.

Paul

Tried evry mode, no heat at all when it goes cool....no matter what button is pushed, or where wheel is turned.

What controls the temp? Is it the monovalve? I need to see what is telling what to turn cold, or stop the water flow...

How's the littlevaux coolantnpump working?

Unplug the monovalve. No voltage is open. Did you replace the complete monovalve or the insert that goes in the top of the valve? The aux pump should not cause a problem. It only helps during idle. The aux pump in my 300D is disconnected but the heat works fine.

Paul

Plugged in or unplugged, still makes no difference. I have just replaced the pencil thing, not the whole valve. When I put 12 volts directly to leads on monovalve I hear it open/shut. I checked the fuse that goes to this operation and its good, with good contact. What tells monovalve to open/shut? Is it the push button/wheel unit? - I have replaced that also....

If you still have no heat with the monovalve unplugged (no power) then there is something mechanically wrong with the monovalve or something is plugged up. It doesn't matter in this case but the monovalve is controlled by the control panel. I'm not sure where the heater hoses hook on the 300SE (mine is a V8) but both should be hot. If one is cooler than the other, the system is plugged up, either the monovalve is stuck off, or the heater core or one of the heater hoses is plugged. The only thing I can think of now is to unhook both heater hoses from the engine, rig up an adapter so you can connect a water hose to one of the heater hoses, turn on the water and see if it comes out the other heater hose. In other words, flush out the heater core.

Paul

Thanks for the info Paul! Both lines to the heater core are hot. Its not that it never gets warm....it does, then after a few mins gets cold and stays cold until I turn off heat and restart the process, or turn car off.

Im wondering if I have air in the system, bubble maybe? If I squeeze on rad hoses, I get heat. If I stop squeezing, air turns cool again. How do I 'burp' the system?

Is the recirculation pump working?

If it does not work you loose heat in idle.

I lose heat idle or running down the road

This sounds so flow related I wonder if your main water pump works sufficiently. Does it overheat in the summer?
I think there is a block vent plug on your M103 passenger side, if you wanted to try.

I was leaning to flow problem also...thats why I replaced the thermostat and radiator. I just got the car in early November so dont know about hot weather yet. I know she runs at just over 80* running or sitting.

Block vent plug? What do I do??

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?

I only had your motor's brother, the M104, and the vent plug was up under the exhaust manifold. You just loosened it and filled till coolant came out.