My 83SD exhibits symptom of thermostat failure. The bottom hose and radiator were cold while temp gage was just a needle shy of 100 degree C after the car was well driven. I thought beeter to fix it before messing with the boost. Took it out and boiled with a new one (Behr, 80 degree C), they both opened at the same time. I drilled a small hole in the top arraw area of the new thermostat. After installing the new one, the temp gage stayed around 85-87 degree C and does not vary much.

Strange failure mode. The old thermostat is made in France, and it appears to be well made. Probably weak spring to block off the by-pass after years of thermo cycles.

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Shane83SD

Cold lower hose doesn't indicate a stuck thermostat. Cold upper hose does. If the car is driven hard, 100C is nothing to worry about. I don't think there was anything wrong with your old thermostat.

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2004 VW Jetta TDI (manual)

Past MB's: '96 E300D, '83 240D, '82 300D, '87 300D, '87 420SEL

I think DieselAddict is correct... the Mercedes is an unusual design in that area... if you are worried about it ( my impression of your hole drilling , which is a good thing on most designs ) you can check to see if you have the updated thermostat housing.. with its ' air bleeding' tube inside it....

Was wondering since you decided to change your T-Stat, why didnt you go for the one rated at 78c instead of the 80c, out here in India, MB is fitting a 72c rated T-Stat. Keeps the temps on my OM 616 turbo to a steady 81c and only when the outside temp is above 45c the needle hovers to the 85c mark.

Dear shane83SD:


I explained why the lower radiator hose and the bottom part of the radiator stay cold (or very slightly warm) in this thread:

http://www.peachparts.com/shopforum/printthread.php?threadid=77194

If you drive your car long enough (I think at least 30 minutes or more), you will see that the lower radiator hose/radiator bottom will be just as hot as the upper radiator hose.

I agree with DieselAddict and Leathermang that there might be no problem with your old thermostat at all. Also, Gurkha lives in a hot tropical country (India), so it may be better for him to use a thermostat rated at less than 80C. However, you should not use it in the US. Temperature range (for coolant) from 85C to 100C is ideal for a diesel engine.

Eric

The bottom radiator is now warm with the new thermostat. With the old thermostat, it was cold (same 12 miles local commute route). The old thermostat is also rated at 80 degree C.

The outside temperature at Bay area now is around 45-65 degree F. With heater on and no AC, running near 100 degree C is too hot.

Some reference from Mercedes maling list:

http://db.mercedes.cx/mb/200204/03/0006.html[/URL]

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Shane83SD

I agree. On an OM617 with a good cooling system, temp should be 85-90C and stick there like a rock, except in brutal summer heat. My turbo 84 does just that, but has a fairly recent radiator, fan clutch, t-stat, gauge temp sender, and proper MB coolant (no green death). Oh and the radiator fins are clean.

Per the MBZ.org diesel email list, the failure mode described here sounds a LOT like the corroded t-stat housing. It only takes a couple MM to make it run hot, as the bypass doesn't seal properly as the t-stat opens. Drilling the hole is a Band-Aid cure, the proper fix is a new (or good used) t-stat housing. It's true that 100C temps are not dangerous, they are also not normal in cool weather, and indicate a problem that should be addressed.

Just my $0.02...

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I did checked the inner by-pass round tube of the t-stat housing. It is in good shape, no corrosion. I think the old t-stat just a bit worn out, and cannot maintain perfect blocking of the by-pass.

The small hole is 7/64", mainly for positively getting rid of the air pocket at the expense of "slightly" longer warm up time. I think some older models have the small hole. As I have/had many Bimmers, I'm paranoid about the air pocket and potential head gasket problems.

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Shane83SD