Thermostat Stuck Open? Need Confirmation
 

So I picked up my 300D this weekend and drove it home. I noticed that the temp gauge was riding a hair below 80*C. Not really concerned as the needle was still touching the 80*C line. I've driven it daily since and noticed the same.

This morning, we had one of our first mornings below 40*F. Car warmed up fine, but on the 30 min drive into work, I stopped getting heat. I cranked the ccu up to max and selected "auto". I got cold air that continued to get colder... that's when I noticed the temp Gauge was down below what I call the 60*C line (the mark between 80 and 40). I put the ccu on economy and the fan on low. Within 5 minutes I could feel heat coming back and the temp gauge went a mm or two above the 60*C mark, but no further.

I'm thinking that the thermostat is stuck open and needs to be replaced.

Your opinions?

we'll definitely need to know what year and model you're driving. 60c-80c sounds about right to me. no heat issues are well documented in the forum archives. most likely a problem with the monovalve i guess.

80c is about right. 60c isn't even close.

If you drove for 10+ minutes and the coolant never got past 60, I think you have your answer. New thermostat time. It likely isn't stuck fully open, but certainly enough for the engine not to warm up. Low heat is only one of the problems. A cold engine will kill your power and economy and fail to drive moisture out of the lubricating oil.

My 1984 300D was like that, usually running ~60 C on dash gage. I removed the T-stat and tested in a pot of hot water. It seemed to work until I made careful comparisons against a mercury thermometer, IR gun, and other T-stats. It opened a bit earlier and responded more sluggishly. Replaced it and the car has run 80-85 C ever since. I recall mileage went up. So, yes they can kind of work, but need replacing. My other opinion is that you should always tell us the year when asking a car question.

My bad... 1991. OM602

Zerex G-05 still the coolant of choice?

Yes. Either get the pre diluted version or mix the full strength version yourself 50:50 with distilled water. You'll need one gallon of G05 and one gallon of distilled water.