Of course, now when the weather has decided to head towards 0 F, my '81 300D ('84 eng/tran) has decided to take a dump heat-wise...sort of.

I haven't had a problem with heat for the one+ years of driving (this the second winter currently), but today, what was happening was that, at slower speeds, but definitely at idle, the air was...lukewarm to cool at best coming out of the side "bullet" (window?) vents. However, if I stomped on the gas or was going at a good rate of speed (above 50 mph), the car's heat acted normally. Note this was in the "defrost" setting, which is supposed to give you all heat, all of the time, as I understand it.

Changing to other settings and playing with the temp wheel did nothing; the wheel behaved normally depending on its setting, but was still doing what I mentioned above.

So...where to start? I've looked at some posts on this forum and elsewhere (using my Google Fu), but am at a loss where to begin. My thoughts as to causes (from what I've read):

1. Probably not the monovalve. If monovalve was wonky, it would behave in reverse of what's described (hot at idle, cold at road speed). I bought an MTC valve and replaced last year---which did the hot idle/cold road speed thing; put original back and normal "defrost setting" behavior returned. Only reason I replaced was because I thought monovalve was defective; instead...

2. Found last year that the stupid foam tube above the glove box...had become a crumbly mess. Replaced with a piece of PEX and...heat, monovalve and temp wheel worked like a charm afterwards (using original---how original I have no idea---monovalve as well).

3. With that in mind, got me to thinking---could it be the temperature sensor in the dash? Of note, today when I was driving, the sun was facing right at the dash, and the coat I was wearing was very warm to the touch because of the sun. I really don't think it's the sensor, because IF you are using the "defrost" setting, the heat is supposed to be MAX regardless of the temp wheel setting.

4. The aux water pump? Since I've driven this car, I've always noticed the heat was not as good at idle; from forum posts, I've read that likely meant the pump was bad. However, the heat was never quite as bad at idle as it was today. I'm real disappointed that, upon quick searching, I only found a $500+ aux pump on eBay, and that the Pelican only has an MTC pump for $89 besides the super-expensive original. Thought about going the route of a universal Bosch (see below), but it is a 90 degree flow angle, not a straight pass-through. Plus, the wiring harness would have to be adapted from the car to it, I believe. I'm sure I'd get that part (from Estonia) about the time I'm drinking in a pub to celebrate...St. Patrick's Day.

BOSCH universal auxiliary additional electric water pump for car/vehicle/heating | eBay

5. Heater core full o' garbage? Backflushing the core: of course, this is JUUUST the type of messy repair I'd like to tangle at 10 degrees F outside this time of year---not!

6. The frickin' CCU head unit/control itself? Just had the cruise control box electronically refreshed by JamesDean here on the forums (thank you, sir!). Perhaps it's time for it to be as well.

With my head full of too many potential solutions; what I thought I would try first troubleshooting are these things:

---Completely unplug electrical connector from monovalve. If I understand right, that will "default" the valve to a "wide open", yes? If the heat works perfectly then, well...time to use the MTC monovalve solution below:

MTC Monovalve No Heat Issue and Solution

---If that makes no difference, completely unplug the power from the aux water pump. Perhaps there's something with that goofy thing causing the issue.

Folks, I appreciate your responses. This car, while a neat old car, is starting to frustrate the living he77 out of me. Seemingly every month or two, something breaks on this damned thing! I realize it's 35+ years old, but I'm starting to lose my patience with it being such a damned diva.