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Old 10-20-2002, 11:00 PM
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Join Date: Oct 2002
Posts: 45
how to remove the hidden thermostat bolt

I have been lurking and picking up tips for some time. Maybe I can contribute. I replaced the thermostat on my 420SEL today. I got to the hidden bolt with a 9 inch 1/4 drive extention (12 inch would have been better). Do this by going behind the alternator, between it and the timing chain tensioner, above the oil filter. I do not know if the stud with the small gauge wire was hot so I put a piece of heat shrink tubing on it; of course you could disconnect the battery. You can feel around with the socket on the extention, it's not too hard to find and remove the bolt. To install the bolt, I put the bolt in the socket and was just barely able to feel the hole with one hand while inserting the socket/extension with the other. With the bolt in the hole I removed the socket so I could start the bolt with my fingers out of fear of cross threading. It is very difficult to do this, you probably can't if you have large fingers.
The top bolt actually gave me more trouble because none of my wrenches would go on it and there is not room for a socket and ratchet. I took a 10mm cheap box end wrench and ground down the entire outer edge to make it thin enough to go on the bolt.
Hope this helps someone!

Last edited by Harold in NC; 10-20-2002 at 11:10 PM.
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Old 10-21-2002, 09:41 PM
Registered User
Join Date: Sep 2002
Location: Kingsport, TN
Posts: 375
Thanks Harold,
I assume you came from under the car, right?
Did you leave the lower hose on or did you take it off?
Reinhard Kreutzer
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Old 10-21-2002, 10:43 PM
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Join Date: Oct 2002
Posts: 45
No, this was from under the hood (1989 model year). There is really quite a bit of room to get to the bolt, you just can't see it. I removed the medium size hose from the bottom of the radiator, the one that runs to the expansion tank, and drained all the coolant I could. Looking back, I should have removed the thermostat to bottom-of-the-radiator hose, I could have saved more coolant as the coolant was almost new. I reused it by straining it before putting it back. I removed the top hose, easy to remove, makes much more working room. I did remove the bottom hose from the thermostat housing. The new thermostat APPEARS to have fixed the problem. The one I removed was a wahler, replaced it with one from the MB dealer, no brand name stamped on it.
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