Intermittent heat in '91 350SD
 

My heat is acting up and I would appreciate some help/advice - I noticed this morning that it wasn't warming up even though the coolant was coming up to temp. The fan runs, but the air coming from the vents is not warm. If I turn off the ACC for a moment then turn it back on, the heat works temporarily - but only for a few minutes, then the air from the vents goes back cold.

It seems like the heat is working, just not making it into the passenger compartment. Any thoughts?
Thanks!

Sounds like the infamous monovalve failure to me. Normally a failed diaphragm that won't let it stay open.

What is the remedy to this infamous monovalve failure? How can I determine if it is the monovalve or a failing diaphragm?

Sorry, I meant it was probably a failure of the diaphragm within the monovalve. It looks like this:

http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1CQ0J3JZ42911DM6VZ&year=1991&make=MB&model=350-SD-001&category=All&part=Mono+Valve+Repair+Kit

The usual solution is to buy this rebuild kit and install it (4 screws on the top of the monovalve to replace the internals). If you take out the internals, you should be able to see if the diaphragm is damaged.

It's likely you can get full heat by unplugging the monovalve until you get it fixed.

Is there a monovalve replacement/repair instructions thread? Is it an easy DIY?

Yes! I outlined it with pictures a little while ago....I'll find a link to the thread...

Edit: Here is the link, my pictorial begins at post #42

http://www.peachparts.com/shopforum/diesel-discussion/206723-replacement-monovalve-heat-control-3.html

Wow, thanks!

Okay, now that I know where the monovalve is and how to take it out, how do I go about determining whether it is the culprit of my heat problem?

From the car's symptoms and what I have read here, I don't think it is the core b/c I do have good, hot air from the vents - just not all the time. Anything else I should consider/check to get closer to isolating the problem?

Look at the pics I took. I showed a pic of a torn diaphram. If that little rubber piece that moves is torn (you should easily be able to tell with it out) then thats your problem. Only way to tell is to remove it.

I had the same symptoms as you did. Per guidance from this site, I installed a Mono Valve Repair Kit. The diphgram was not torn in the old valve, but the heat has never worked as well as it does now.