Driving the 300 TD today for about 125 miles. as we approached the destination, I noticed the heat was blowing cool air. made a stop at a food store, for about 15 minutes, and the heat came back. until about 15 minutes later inthe trip. finished the drive with little heat. Any Idea's ? I just replaced the thermostat, which helped a overheating problem. but now the car is running on the warm side as well or the gage was reading about 90 C. and the outside air temp was in the low teens. How do I test the water pump? It is not leaking, and the coolant is not low...???
Jason

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46 WDX Power Wagon

84 300TDT daily driver

heat/no heat cycle could mean failure of monovalve. I had similar symptoms several years ago. I would get no heat until I drove a couple miles, then shut engine off. Waited about 10 seconds and restarted. I would get heat until the cabin temperature reached the setpoint, then it would shutoff. Watch for similar symptoms. If this happens in your case, i would strongly suspect the monovalve. It's easy to replace, and a "repair kit" is available that is easy to do and saves a lot of money

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'82 300SD - 361K mi - "Blue"

"Good judgement comes from experience. Experience comes from bad judgement."

listen, look, .........and duck.

What Pete said.........

And, the engine is running right at the proper temperature if the new thermostat results in a gauge reading of 90C. The water pump is probably fine.

I have the same exact thing happen from time to time, the heat just "goes away" and only comes back if I come to a stop, turn the ACC on and off, wait a second, then go on my way. I will have to look into a monovalve rebuild kit soon, mine keeps giving me very sparing amounts of heat, which is no fun when its 5-10 deg's out around here so often, its warm right now though! 21!!

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-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)

Could be worse. My CD.. the monovalve is touchy, plus the blower motor is dying. At least I live in NorthWest Florida where it doesn't get too cold.

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'84 300CD Turbo 132k (Anthracite Grey) - WVO - My daily driver - Recently named coo-coo-coupe by my daughter.
'84 300D Turbo 240k (Anthracite Grey) - Garage Queen
'83 300D Turbo 220k (Orient Red) - WVO - Wifes daily driver

I'm not a certified mechanic, but I did stay at a HolidayInn Express last night.

So I will replace the monovalve with a spare off the parts car and see what the difference is. Thanks for the help.

jason

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46 WDX Power Wagon

84 300TDT daily driver

Hi folks;
Whenever you have the symptom heat at idle and low speed and then no heat at >1500rpm and hwy speed you are most likely suffering from having a mono valve that either has gone bad or you have a cheap replacement insert installed in it. The $15 replacement inserts WILL NOT work - their internal spring rating is incorrect. Aux pump and high engine speed will literally "suck" the valve seat into the closed position no matter the temp wheel setting and voltage on the mono valve. You HAVE to get the genuine part 000-835-06-44 (Peachparts shows it, but it appears to be out of inventory). The cheap repair inserts will drive you absolutely insane and you will only have heat traffic lights.

What about if your temperature wheel is in the cold/blue "min" setting and you get low amounts of heat, sometimes high heat? It never did that before, it just starting doing that.

My indy replaced the monovalve, ACC switch and aux water pump. Still no heat for about 20 min, if I turn the car off then on, I get heat for about 10 min. Do I have a vacuum leak? The floor vents are not working.

What year/model?

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Sam

84 300SD 350K+ miles ( Blue Belle )