Strange Winter Thermostat Behavior - PeachParts Mercedes-Benz Forum

Strife · #1 12-07-2008, 07:54 PM

My 85 380SL's thermostat behaved typically for this car in the summer; it would bounce to 100, drop to 80, and then stay between 80 and 90, AC on or off.

This winter, it has had a hard time getting above 80 C. So, I assumed the thermostat was tired, and I replaced it. The old thermostat had the "arm" broken off; I left the new one as-is from the factory. I'm glad I replaced it because the hose behind the housing was getting pretty ratty looking, and I replaced that also.

So, the behavior was exactly the same, not going above 75 or so. I tried an experiment. I turned the heat in the car off, and the temp shot up to 100, then dropped to 80. From that point, I could maintain 80 (Ambient temps were in the upper 20's) even with the heat on.

Should I do the (incredibly ugly) cardboard-in-front-of-radiator trick, or what?

Deltacom · #2 12-08-2008, 03:20 AM

There are 1.0, 1.2 and 1.4 bars expanxion tank caps. Your SL probably has 1.0 or 1.2 bars that for summer are fine but for cold winters wont do. Use a 1.4 bars in winter and keep the 1.0 or 1.2 for summer. Mercedes has superseeded the summer caps and are difficult to find.

Werner W · #3 12-08-2008, 08:36 AM

Hello

I was about to start a thread when I read this one and thought perhaps I have the same problem. It was 10 degrees this morning and I had to take the SL to work while my wife's car is in the shop. The heat came on nice and hot even before the engine warmed up. It got so warm I was about to turn the heat down when suddenly no more heat. It appears once the thermostat first opened at 100 degrees, from that point it seems the heater core didn't get anymore hot coolant. The temp gauge repeatedly swung back and forth from 80 to 100 degrees as it should. Froze my but off for 45 miles. When I got to work, leaving the engine running, I pulled the glove box out to see if the cowel vents were closed and they were. I touched the heater core tube and it was cool. Am I having the same problem with the expansion tank cap? 1985 500SL Euro

Werner.

Strife · #4 12-08-2008, 12:40 PM

Quote:

Originally Posted by Werner W

Hello

I was about to start a thread when I read this one and thought perhaps I have the same problem. It was 10 degrees this morning and I had to take the SL to work while my wife's car is in the shop. The heat came on nice and hot even before the engine warmed up. It got so warm I was about to turn the heat down when suddenly no more heat. It appears once the thermostat first opened at 100 degrees, from that point it seems the heater core didn't get anymore hot coolant. The temp gauge repeatedly swung back and forth from 80 to 100 degrees as it should. Froze my but off for 45 miles. When I got to work, leaving the engine running, I pulled the glove box out to see if the cowel vents were closed and they were. I touched the heater core tube and it was cool. Am I having the same problem with the expansion tank cap? 1985 500SL Euro

Werner.

This problem sounds more like the thermovalve, which can be repaired with a kit and it's not too difficult. This is the "gate" for water to go to the heater core and it is electromagnetically operated.

Werner W · #5 12-08-2008, 12:52 PM

Thanks Strife

I'll give that a try. Should be inexpensive enough. My expansion tank cap is new and it has a 120 stamped on it (MB original).

Strife · #6 12-08-2008, 07:41 PM

Quote:

Originally Posted by Deltacom

There are 1.0, 1.2 and 1.4 bars expanxion tank caps. Your SL probably has 1.0 or 1.2 bars that for summer are fine but for cold winters wont do. Use a 1.4 bars in winter and keep the 1.0 or 1.2 for summer. Mercedes has superseeded the summer caps and are difficult to find.

I have a newish 1.4 on this 380SL it now, although I can get a 1.2 and in fact I have one on my (hot-running) nice-weather only 560SL. So, this isn't likely to be the problem.

Walrus · #7 12-08-2008, 09:53 PM

I may be way off base here, but I suspect a fan clutch that is locked up or otherwise, not "free-Wheeling" in cooler temperatures...

Strife · #8 12-08-2008, 10:08 PM

Quote:

Originally Posted by Walrus

I may be way off base here, but I suspect a fan clutch that is locked up or otherwise, not "free-Wheeling" in cooler temperatures...

Naah, I checked with an RPM gauge, and it's about 1/2 engine RPM with the engine cold.

Walrus · #9 12-09-2008, 08:07 PM

Just curious... What is the drive pulley ratio?

Strife · #10 12-09-2008, 11:41 PM

I don't know, I suppose I could take a tape measure and figure it out - but the manual says under 105 degrees C, 400-600 RPM fan speed, at which the fan will go up to about 3000 RPM, higher RPM's at that temp will decrease it back to 400-600.

I got 400 fan speed at 725 RPM engine speed, and the fan RPM seemed to go up somewhat when I pulled the throttle (I did this one-man, and it was tricky to hold the gauge and do this at the same time).

Earlier clutches (and their possibly different-shaped fans) don't seem to have the temp feature.

Fan clutches usually fail the other way, OTOH, a MB technician at a dealer told me that in his 20+ years of experience, he had seen exactly two (2) fan clutches fail, both where the fluid leaked out and were too "slow". I have a spare fan/clutch loose and my 560SL (which has the same clutch), and they all feel the same.

Strife · #11 12-11-2008, 01:05 AM

I know it is very bad to run an engine "too cool", because the oil never gets to truly flow freely and the piston/cylinder tolerances stay too loose. Well...

In a now apparently related thread, M116 - Source of Gas in Oil? ,I described the premature stretching of a timing chain and breakage of a guide, and the smell of gas in the oil even after doing the replacement of the parts.

In addition to the smell, I noticed that my oil was unusually dirty in the last (3k) change. It seemed to be getting that way again. After changing the thermostat, and allowing it to "pop" before turning on the heat, the color of the oil has remained stable (yes, it could get dirty that fast) and the gasoline smell has disappeared!

I like my car to be "toasty" in the winter. So, as soon as there is a hint of heat on the needle, I turn the blower on all the way and the temp wheel to the max. Doing this would not let the old thermostat "pop" at all, and would cause the newer thermostat to take a lot of time (several miles on the Interstate) to get that warm.

The new thermostat, combined with a little behavior modification, seems to have solved this problem, and possibly can explain the other. Strange, though. Am I the only one here who drives a 107 in the winter?

Werner W · #12 12-11-2008, 06:00 AM

Interesting.

I have a similar habit. I blast the heat even before the temp needle goes up. The morning I froze (mentioned earlier) that day my trip home was a different story. I had heat blasting all the way home. If I recall correctly, I turned the heat on later. Hmmmm. I'll have to take it out on a cold day and try it again.

Werner

RadioTek · #13 12-11-2008, 05:39 PM

When I did a heater core bypass, I was told that there needed to be some resistance to flow from the heater core in order to prevent 'dead areas' (no flow of coolant) in parts of the cooling system. I ended up blocking off the hoses to/from the core, instead of 'looping' past it, and gained some improvement in the system performance.

Possibly you could have an issue where, before the thermostat opens, you are getting cooled water through the core instead of fully heated.

On mine, the thermostat is 'double acting', closing one coolant path while opening another. If yours is the same, cranking up the heat (diverting full flow to the heater core) too early may have the effect of creating a flow path that has cooled water to the core before sending it to the engine.

Then again, the engineers may have fixed that problem by the time they got to yours, and I'm just blowing smoke.

There are more than a few similarities between electrical circuits and water circuits, and I'm just delving into some poorly remembered stuff from long ago and trying to apply it here. But it does offer some idea of an explanation.

Mistress · #14 12-11-2008, 05:58 PM

Quote:

Originally Posted by Werner W

Hello

I was about to start a thread when I read this one and thought perhaps I have the same problem. It was 10 degrees this morning and I had to take the SL to work while my wife's car is in the shop. The heat came on nice and hot even before the engine warmed up. It got so warm I was about to turn the heat down when suddenly no more heat. It appears once the thermostat first opened at 100 degrees, from that point it seems the heater core didn't get anymore hot coolant. The temp gauge repeatedly swung back and forth from 80 to 100 degrees as it should. Froze my but off for 45 miles. When I got to work, leaving the engine running, I pulled the glove box out to see if the cowel vents were closed and they were. I touched the heater core tube and it was cool. Am I having the same problem with the expansion tank cap? 1985 500SL Euro

Werner.

hey mine too! It gets to 80 and that's it. Slow moving as well.

Strife · #15 12-11-2008, 06:21 PM

Quote:

Originally Posted by RadioTek

Possibly you could have an issue where, before the thermostat opens, you are getting cooled water through the core instead of fully heated.

I did install a monovalve repair kit, and I'm fairly sure that it works.

My aux water pump seems to be noisier than it once was (and definitely noisier than the 560's), so I am going to replace that preventatively along with the 24 year old hoses. Mostly, because if this motor jams up, it can fry things.

This stuff is voodoo.