engine heat goes up, no cabin heat, then relatively sudden reversal - PeachParts Mercedes-Benz Forum

brilliantblue · #1 12-04-2004, 03:54 PM

hello. i posted this in diesel discussion but thought i'd fly it here as well.

the other day, just after it turned cold here, i started my '84 300D and let it idle for 15 mins to warm up. i turned the heat on as well, jumping it with the defrost button. when i came back to the car the engine temp gauge read over 100c and cold air was still coming from the vents. i had a short drive to get home (3 miles) and a little space on the gauge so i figured i'd start driving and see if the cold air rushing through didn't bring the temperature down. after maybe a 1/4 mile the temperature gauge dropped fast (too fast for the cold air theory - i was in a 35mph zone) and heat started pouring through the vents. i was relieved and drove homeward. one more time on the way the heat rose onthe gauge and the cabin got cold.

in the Diesel Discussion forum i received suggestions of Monovalve problem or fan clutch and thermostat. the thermostat seemed to make the most sense so i bought one and replaced it.

but no change!

where do i go next. i was told the Monovalve didn't have anything to do with engine temperature (as in overheating). and my big fan runs fine. when should the little one kick on?

Benz300 · #2 12-04-2004, 04:02 PM

a bad thermostat usually makes the engine run cold if it fails.
Happened on a toyota once where the water pump failed, choking the internal circulation of the coolant and made the temp rise almost to the red zone, then dropped back to normal after a while. From what you've said it seems like something gets stuck that chokes the internal system and blocks the heat in the cabin as well, and when the normal process resumed the engine heat got back to normal as well as the circulation in the cabin was back to ok. check the schematics as to how the war air is pushed into the cabin, perhaps the inlets are somehow blocked into the cabin and creates additional heat in the engine area. just a theory !
good luck.

psfred · #3 12-04-2004, 06:35 PM

Check your coolant, I'll bet it's low. This will cause circulation failure in the heater core at idle and will also cause poor circulation through the radiator. The head will get hot due to lack of coolant circulating, but the thermostat won't open becuase the coolant isn't moving past it. When it does heat up and open, a slug of cold coolant from the rad will blow through, etc.

A bad thermostat will have no effect on the heater circulation.

Peter

Roncallo · #4 12-04-2004, 07:57 PM

Im not sure about the 300D but my 560SL has a small electric circulation pump right before the monovalve. Its purpose is only to boost the heater circulation at idel and is not required during normal driving. It may be possible that yours is out and when you started driving the engines water pump took over. Also there is a temperature switch on my intake manifold which shuts off the blower motor in the heating mode but not the defrost mode. Im not sure if this switch also controls the mono valve. But you can check the switch operation by seeing if the blower motor turns off in the heating mode when the engine is cold and then it should turn on when the temp reaches I belive 40C. Also low water level will effect the operation of these switches. If the level is low the switch may be dry especially at idel.

John Roncallo

brilliantblue · #5 12-06-2004, 06:31 PM

coolant was low when this happened. i refilled it but with no change in performance. i've not had much chance to move on, but i did find coolant dripping from somewhere beneath the thermostat. i couldn't see the exact spot but will investigate further on wednesday and no doubt have more questions/comments.

thanks

psfred · #6 12-06-2004, 10:32 PM

Just replaced the monovalve insert on my brother's SDL to cure this problem. Coolant was down 3 qts as well, hmmmm.

Seems to be working OK now.

Peter