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Old 08-23-2004, 06:28 AM
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Join Date: Jul 2004
Location: Illinois
Posts: 222
thermostat question


I have a question which has probably been answered previously, but with the search down, I am desperate. I just got done replacing the water pump, radiator hoses, belts and thermostat on my 82 300D. The last thing I did was put the new thermostat in and figured that it was a no brainer. I started the car and let it idle and the car promptly overheated to 100 degrees C. I pulled the top radiator hose off and poured antifreeze down the hose (supposedly to get rid of an air lock that was in there), and ran the heater full blast, started it up and again it went to 100 degrees before I shut it down. The thermostat is an 80 degree model by the way. I know the thermostat is facing in the correct direction, or at least the same direction as the old one. Before I make a mess and pull out the thermostat, I was wondering if I am missing something or is there a bad thermostat in my car? Has anyone here gotten any bad thermostats in the past? I am ready to put the old one back in, it was working just fine.

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Old 08-23-2004, 09:00 AM
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Join Date: Sep 2001
Location: central Texas
Posts: 17,277
Ok.... with regards to " making a mess" ..... does your car not have the arrangement where the access to changing the thermostat is a compressible rubber biscuit which does not use and sealer ? Thus it should not be any mess or trouble to open it up and look or replace it....
Did you check your new thermostat in hot water to see if it was opening up at the prescribed temperature ? What brand did you install ? Does your system have the seperate ' burp pipe' built in ? Does your thermostat have a small hole in the flat part ? If yes, is it at the top when installed ?
Why did you replace the thermostat ? IE, was your car overheating prior to this current thermostat being installed ? Was anything else worked on or changed at the same time this thermostat was installed ?
And , YES, some people have bought Brand new thermostats which were Bad out of the BOX..... a truely disgusting situation when it happens....
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Old 08-23-2004, 09:21 AM
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Join Date: Jul 2004
Location: Illinois
Posts: 222

The mess of which I wrote would be due to opening the thermostat housing and releasing the antifreeze. There is no drain plug on the bottom of the radiator, so it is either a mess generated by removal of the thermostat housing or the lower radiator hose. My car does have the compressible rubber sealer and I did not use any other liquid sealer on the assembly.

I did not test the thermostat in hot water before I installed it. I have never purchased a bad thermostat and it didn't occur to me to test it before putting it in the car. My bad, I guess. There is not a small hole in the flat part, and I do not have the seperate burp pipe.

I replaced the thermostat because I changed the coolant, replaced the water pump, fan clutch, belts and hoses and consequently had easy access to everything. There were no overheating symptoms or any other issues, just my miserable attempt at good maintenance.

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Old 08-23-2004, 09:34 AM
Join Date: May 2002
Location: Blue Point, NY
Posts: 25,396
If you are quite sure that the thermostat is installed properly, then it is possible that the new thermostat is not opening at the correct temperature.
You can avoid the "mess" just my putting a pan under the radiator and pulling the lower hose. Then, you can take the thermostat out and test it in a pan of water to see when it opens fully.

I do have a question/suggestion:

Prior to doing the above:

When the temperature climbed to 100 degrees, was the radiator cap on or off the vehicle? If it was on the vehicle, you may wish to leave it off the vehicle and try the process again. Any air in the system is now permitted to exit via the radiator cap at the tank. You may see the coolant level in the tank drop significantly and you can add more coolant while this is happening.
It might be worth a shot prior to removing the thermostat. Remember, bad thermostats are not all that common.
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Old 08-23-2004, 11:23 AM
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Join Date: Jul 2004
Location: North Texas
Posts: 22
Silly Question

Did you fill the block and hoses up with coolant before connecting them? I did this same thing with my 240d, and had the same problem. After looking at it for a minute I said, Da what a dumb butt I am. The water pump is not self priming. You have to fill up the block, the hoses and the radiator or it simply will not circulate the coolant.
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Old 08-23-2004, 11:31 AM
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Join Date: Jul 2004
Location: Illinois
Posts: 222

I will try the radiator cap trick, it makes sense that this would relieve the air lock, if one exists.


Thanks for the suggestion, but I did fill the block, radiator, etc.

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Old 08-23-2004, 06:24 PM
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Join Date: May 2004
Location: Savannah, GA
Posts: 794
BTW, there is a drain in the radiator. It looks more like a plastic screw head in the bottom of the bottom tank than drains of other radiators.
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Old 08-23-2004, 07:13 PM
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Join Date: Dec 2003
Location: Tarzana California
Posts: 111
I have the opposite problem. My '82 300SD runs at a constant 60C, at least according to the gauge. That works out to about 140F, which seems way too cool. I guess my gauge might be inaccurate, though.

Oh, the thermostat has been replaced, but the gauge still reads the same.
1982 300SD
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Old 08-23-2004, 09:41 PM
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Satyameva Jayate Ad vitam
Join Date: Jun 2003
Location: Boondocks
Posts: 1,026
Hi All,

Before you go through the harrasment and hard work of changing thermostat, dismantling radiators as well as doing radiator flush due to high temperature readings, please check the ground and also try and replace the temperature sender. I have suffered first hand with insanely high temperature readings only to find out that I had a faulty ground and a defective sender, these two problems are quite common with MB, specially the older OM 616/617 and 603 series.
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Old 08-24-2004, 06:40 AM
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Join Date: Jul 2004
Location: Illinois
Posts: 222
Thanks to all for their input. Last night I changed the thermostat back to the old one. Guess what? It worked perfectly. I will now figure that the new thermostat was bad, unless anyone has any other ideas. Go figure.

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