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Old 02-03-2000, 06:51 PM
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Join Date: Nov 1999
Location: Germany
Posts: 63
During the winter, my 420 SEL's coolant temperature gauge doesn't go higher than 60 degrees centigrade, while in the summer it runs at 80 degrees - and gets better gas mileage by two or three MPG. Shouldn't the thermostat keep the engine at the same optimum temperature year round for efficient combustion, give or take a few degrees for days when it is extremely hot or cold?

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Old 02-03-2000, 07:25 PM
AMGmercedes's Avatar
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Join Date: Jun 1999
Location: Baltimore, MD
Posts: 203
Indeed it should. You most likely need a new thermostat. It pretty cheap and easy to replace. I had the same problem with my 16v, after a new thermostat 80C all the time. Good luck

1987 300SDL-T
1985 190E 2.3-16
1984 280GE Cabrio
1978 240D
1957 220S(running one day, I hope!)

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Old 02-03-2000, 08:45 PM
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Thanks, that's what I suspected. Will I need anything more than the thermostat (80 degrees) and seal? Visually, it looks like a straight-forward job. By the way, how tight (ft-lbs) should I tighten the thermostat housing?

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Old 02-03-2000, 09:08 PM
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Location: Suwanee, GA, USA
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Those are small bolts. Use a short 1/4" drive to tighten and you will be fine. Do so in a uniform fashon.

1981 280GE SWB
1987 16V

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Old 02-04-2000, 08:27 AM
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Thanks, I noticed that on my 89 420 also. Guess I'll replace the thermo too.
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Old 02-04-2000, 02:19 PM
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It is even more important than you think to replace this thermostat.

Engine wear is accellerated in an engine which is at less than operating temperature. It doesn't seem that this would be true, but it is. This is one of several reasons why you shouldn't accellerate hard before your engine is warm. In that case not only do you have the accellerated wear due to cooler overall engine temperature, but you also have oil that is too viscous to flow properly because it is still cold. By the way the oil temperature doesn't come up as fast as the coolant temperature, so just because the coolant temp guage is telling you're at operating temperature, doesn't mean that the oil is.
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Old 02-05-2000, 06:54 PM
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Thanks to all for your help!
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Old 02-06-2000, 11:22 PM
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Join Date: May 1999
Location: Lehigh Valley PA
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My '84 SD only warms up to 60 deg C after an 8 mile drive. To save money, I'm tempted to attempt to change the thermostat myself. I'm not sure where the thermostat goes, but I would expect to have to drain the coolant from the radiator.

How much coolant do I need to / should I expect to catch to get below the level of the thermostat? Is there anything else I'd need to buy, such as a gasket, etc.? Thanks in advance.

Robert W. Roe
1984 300SD 167K mi

[This message has been edited by Robert W. Roe (edited 02-07-2000).]
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Old 02-14-2000, 07:58 PM
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I was also going to change mine myself, but I finally had it done by a MB mechanic, who told me that it really wasn't a DOY job on the 126 series. FYI, after the change in thermostats my 420 SEL runs right on 80 degress C.
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Old 02-23-2000, 08:46 AM
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Most engines have the thermo housing on the other end of a radiator hose.

Its the housing with three 10mm spanner size bolts...they're 6mm bolts and the nominal torque is about 11-15 Nm.

Only do this when the engine is cold and remove the radiator cap.

Drain the radiator and drain the block.

Remove the above 3 bolts and replace thermo and Oring.

Fill with water and start engine.
Do dynamic flush of cooling system.

Be Very carefull of airlocks as temp maybe stable until you drive car and overheating will occour.

So it is better to manually bleed cooling sys while engine is at operating temp and danger of burns has increased.

If not completely confident in above procedures ...better to let the shop do it.

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Old 02-24-2000, 11:01 PM
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Join Date: Jun 1999
Location: Los Angeles, Calif, USA
Posts: 521
Change the thermostat on a 300SD (w126) is a very easy job. Note: Install the new thermostat with the little arrow up.

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