Heats at idle but not while driving
 

I have a 1984 300SD, the car heats really well while idleing but when I drive I recieve very little heat either at defrost or max heat settings. Could this be the monoblock diaphram? If this is the case does much coolant leak out after pulling the 4 screws out of the monoblock and removing the diaphram? In other words should I clamp off the hoses going to monoblock to prevent the coolant loss? Thank you for all the excellent information. I don't want to tackle this whimsically as I just had a complete Knee replacement 1 month ago and it still hurts like he## when I strain it.

Torn monovalve diaphragm. Five minute fix (or less). No coolant (or very little) should run out. It's a no-knee-strainer job.

Seconded on the monovalve. Just did mine last winter, and now my heat works perfectly. Just make sure when you remove the monovalve, you re-insert everything back the way it was before.

I mis-aligned mine after I installed it, and the heat was stuck on full blast until I did it properly.

Thank you guys for the conformation on the monovalve. I will attack it tomorrow.:D

yes....another vote for monovalve through experience1 It is an easy fix and well worth it!

my car does the same thing.... do i need to replace the mono valve or is there some way to repair it?

alright cool thanks.

is there a way to check to see if its bad? or is there a way maybe the screen could be clogged... i have heat at low rpms and speeds up to like 35 then it starts to cool off

what happens if u just take out the valve and dont use it? or if u have to rig somthing simple to not use it? what would happen would i just have heat every time i put the blower on?

A visual inspection will most likely reveal a torn diaphragm. Remove the four screws at the top of the monovalve to remove the insert.

That's a perfect fit for a torn diaphragm.

You will have heat whether the blower is on or not.

i dont mind if i will have heat all the time, if the blower is off and the vents are shut then nothing will come out anyway... if i can temp rig it without the monovalve just to get some heat in the winter then im all for that... would i have to put a pipe in place of the monovalve or somthing? i just want to be able to have heat, and if it gets too hot then ill turn off the blower or crack open the moonroof. im saving up some money to fix this car up but for now the little things i just want to make work it doesnt have to be fancy i just need heat for the next 3 months

Replace the monovalve and its two connecting hoses with a single hose and you will be warm.

sweet im gonna do that for now then... so the mono valve is inbetween 2 coolant hoses by the pass side fender or bulkhead?

you could add this and be able to shut off heat in the spring-
3/4" PEX ball valve, about $6 at menards.

http://t3.gstatic.com/images?q=tbn:v...4543105(s).jpg