Absurd T-stat problem 300SD - PeachParts Mercedes-Benz Forum

RobTheMod · #1 12-21-2005, 01:27 AM

My beloved 126 got very hot under the collar last Fri night... pegged the temp gauge, but as the coolant tank was full of cold coolant, and the bottom rad hose was cool to touch, I guessed thermostat. Removed it at roadside and limped home leaking coolant from the gasket area (need a T-stat to hold the gasket)Ran at 60 C degrees, though, so this confirmed my T-stat diagnosis. Purchased new T-stat (Motorrad, German made) on Sunday afternoon, installed after testing on kitchen stove (opens in boiling water, closes when heat is shut off). Overheated and acted as though air was trapped in system. Raised front of car and revved to a steady 2500- this helps Vanagons lose their air pockets... this got a noticeable amount of air out of the system but still running hot with a cold radiator. Checked orientation of thermostat- same as previously installed item. Bleeder "wobbly" at top, pellet towards engine. Parked car until tonight and drove Audi.

Tonight: Removed guts from defective original tstat, installed (this gives the gasket something to hold onto). 60-80 C temps, runs fine, I'd like to see a little more heat (duh, there's no thermostat!) Water is circulating.

How is a t-stat "bad" if it opens and closes on the stove? How can it *not* open when faced with 120c coolant in the engine? I want to get a stat in there to get heat and keep the motor at the right temp, but so far my methods aren't working. What's goin' on?

#2 12-21-2005, 01:45 AM

Strange, my only guess is that you still had some air trapped below the thermostat so it did not "see" the high coolant temperature and open. Wasn't there a posting (some time ago) involving drilling a small vent hole in the edge portion of the stat to ensure no air was trapped? That's all I can think of.

t walgamuth · #3 12-28-2005, 11:18 PM

up the engine and let it come up to normal op temp and then check level of coolant. do this a couple times to assure that the air is out of the system. and it isnt a good idea to run with a gutted stat.

tom w

Brian Carlton · #4 12-28-2005, 11:25 PM

I've had the exact same issue at times.

The solution is as follows:

Remove the upper radiator hose at the radiator and pour coolant directly down this hose until it overflows. Then, quicky reattach it to the radiator. The additonal fluid makes the difference.

Report back if this works for you.

73MB280SEL · #5 12-29-2005, 05:57 AM

Howdy,

Here's the problem as I see it: The reason an otherwise good thermostat won't open even though the car is running at 120 deg C is that it's not getting any hot water down where the thermostat is located.

I went through this on my 190D 2.2. The cooling system is not designed properly--the thermostat would work much better if it were located in the head (WHERE THE HEAT IS!!!!) and not way down low on the engine.

This boils down to a water circulation problem. My solution was to install an OEM dealer water pump to replace the crappy aftermarket rebuild. I also did several flushes until enough heat gets down to the thermostat for the coolant temps to run normally.

p.s. If you want to run without a thermostat without leaking, take your old thermostat, put it in a vice, and file off the two brass straps that hold the body of the thermostat on to the top. Then you can reinstall the top of the thermostat, compress the gasket, experience no leaks, and your engine will run very cool.

One of these days I'd like to redesign the cooling system by fabricating my own water neck with a thermostat installed. Maybe this summer.

Sholin

engatwork · #6 12-29-2005, 07:58 AM

Quote:

Remove the upper radiator hose at the radiator and pour coolant directly down this hose until it overflows. Then, quicky reattach it to the radiator. The additonal fluid makes the difference.

A little more insite to this issue. To make the above operation more effective hold the neck of the hose down about even with the neck of the radiator while filling then raise the hose up as high as it will go. The coolant will disappear

into the hose. Once it won't go down anymore you have the cylinder head full. On the '95 diesel you can just disconnect the return hose (at the radiator end) from the radiator neck to the reservoir. Fill the reservoir as high as it will go while squeezing the upper radiator hose. You can see the air bubbles coming out. When all you have is a steady stream of water you can button everything back up.

Diesel Giant · #7 12-29-2005, 08:04 AM

Ok this the the classic Mercedes cooling problem. I have battled with this for YEARS, tried everything under the sun and cannot figure out why it happens.

But I do have a WORKAROUND.

http://dieselgiant.com/thermostatreplacementandmodif.htm

Now this set up will run a little cool in the winter months, so what I did was use some cardboard to block off a portion of the radiator to the incoming airflow. (between the rad and condensor) Now the car will run a perfect 175-190 degrees even in the coldest temps. I spent about 5 minutes with a "fed Ex" box and a knife and got the perfect balance of restricted air flow and engine temp.

Brian Carlton · #8 12-29-2005, 08:11 AM

Quote:

Originally Posted by Diesel Giant

Ok this the the classic Mercedes cooling problem. I have battled with this for YEARS, tried everything under the sun and cannot figure out why it happens.

Careful on this one, Russell. He does not have an ongoing overheat problem. The onset was sudden and I'm going to assume that the cooling system is in proper operating condition.

He has the problem after replacing the thermostat. Many times the 617 won't give up all its air after opening the system. The air must be slowly expunged and the system evaluated before any thermostat "modifications" should be made.

In any case, the vehicle should have no issues with temperature now that ambients are below 40°F.

I do not support the modification in this case.

Diesel Giant · #9 12-29-2005, 08:21 AM

I am fully aware of this. I do not advocate doing this modification unless everything else fails. But he did say with the new t-stat it still ran hot, so he gutted it.

In my opinion "gutting" the t-stat is way overkill and is not an acceptable fix except in a dire emergency, at least when the thing is drilled the function of the t-stat is still operable.

I am doing some heavy thinking and mulling over the root cause of this Mercedes anomoly. I am trying to figure out how to correct this problem that so many people have, without the t-stat modification.

Brian Carlton · #10 12-29-2005, 08:25 AM

Quote:

Originally Posted by Diesel Giant

I am fully aware of this. I do not advocate doing this modification unless everything else fails. But he did say with the new t-stat it still ran hot, so he gutted it.

When he mentioned that it ran hot after the new thermostat, I took it to mean that it immediately ran up over 110°C. in his driveway, without any load on the engine. This condition would be markedly different than the "typical" M/B problem of excessive temperature under heavy load in high ambient conditions.

The system cannot "run hot" in the traditional sense when the ambient air is 40°F. He must have trapped air or a bad water pump. There are no more options.

Diesel Giant · #11 12-29-2005, 08:31 AM

I dont know, but I have worked on countless diesel MB's and seen all kinds of weird cooling problems. I really think its a combination of factors and will continue to try and figure it out.

I think that the basic design leaves much to be desired and lends itself to "air pocket" problems. Besides the fact that there are numerous little hoses and pathways that just compound the problem.

Without redesigning the cooling system, which I think is not the answer either, I do have some ideas that I will test and try.

Brian Carlton · #12 12-29-2005, 08:51 AM

Quote:

Originally Posted by Diesel Giant

Without redesigning the cooling system, which I think is not the answer either, I do have some ideas that I will test and try.

Just don't put Freeze-12 in there.

Diesel Giant · #13 12-29-2005, 08:58 AM

Hey Brian,

I did another A/C conversion on the 300cd I am selling and did convert it to Freeze 12.

I have a friend that has 30lbs of r-12 that he is going to give me so I will use that also in the future on other conversions. (when my Freeze 12 runs out of course)

Since I am getting my refrigeration degree I have learned there is SO much misinformation, even from the "certified" A/C guys.

Since I really dont want to open a new "can of worms" with certain members on this forum, I can just leave it at that.

I was also thinking about installing duel a/c compressors and using the 2nd one to cool the engine