I want to replace the T-stat on my 84 300D turbo. The previous owner did a complete flush(approx 300 miles ago), but the engine still runs hot-100-120. After reading several threads I think the T-stat may be bad. My question is, do I need to drain the radiator before I replace the T-stat, and if so, what is the proper procedure for refilling the radiator?

Nah you can just pull the thermostat and refill what coolant was lost when you remove the housing. It shouldn't drain too much out in the process. Make sure you replace the lost coolant with at 50/50 mix of coolant and distilled water to keep the mixture correct.

The stat could be in backwards too.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

I'm having the same issue.....I didn't know it was possible to put the tstat in backwards. That would be awesome if flipping the darn thing would solve it. We shall see!

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77' 300D, "Cartman" SOLD @ 150K (didn't know what I had)
83' 300SD, "The Superdon" 325k+ @ 28mpg
95 E320 wagon, "Millennium Falcon" 231k+ @ 24 Mpg
95 E300D, "Sherley" 308k @ 33.69 Mpg, currently anticipating a head
99 Suzuki Intruder "Trudy" @ 45 mpg

About how much coolant will be lost? I had wondered the same thing.

FYI Diesel Giant's how-to looks helpful. http://dieselgiant.com/thermostatreplacementandmodif.htm

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 156k miles
06 E320 CDI "Rutherford", Black on Tan, 173k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 143k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 67k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

I did it recently and lost less than half of a gallon, but it can vary depending on how fast you do it and the angle your car is parked of course.

yeah, the next time I do it I'm going to put the thing up on the highest ramps I can find. That was me being lazy. I love dgiants recommendation of drilling a hole or two in the stat to allow coolant all the time. If my thermostat isn't upside down and backwards, I may give that a try.

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77' 300D, "Cartman" SOLD @ 150K (didn't know what I had)
83' 300SD, "The Superdon" 325k+ @ 28mpg
95 E320 wagon, "Millennium Falcon" 231k+ @ 24 Mpg
95 E300D, "Sherley" 308k @ 33.69 Mpg, currently anticipating a head
99 Suzuki Intruder "Trudy" @ 45 mpg

If the change was only done 300 miles ago, it's possible the system wasn't burped after refilling. Is your heater putting out high heat?

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

I was thinking of replacing my t-stat also because of no heat. How do you go about "burping" the system. One thing I haven't done yet. Maybe my t-stat isn't bad. Not trying to hijack the thread...

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1990 300D 2.5
1991 F350 7.3 Dually IDI

1997 F350 gasser dump
1980 H-D 93 inch Shovel
2000 SE Roadglide

On your 1990 300D, the system is self-purging, i.e. it should burp itself.

Does the coolant temp come up to above 80? If so, the problem is not your thermostat.

Bad monovalve is a common cause of poor heat. Has yours ever been replaced?

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 156k miles
06 E320 CDI "Rutherford", Black on Tan, 173k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 143k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 67k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

the temp went over to about 90C once. I haven't done the T-stat yet. Had the car for 3+ yrs.Just ordered one yesterday. I see you have the same--do I have to pull the one front fuel line to get at the housing, or can I get it out around all the vacuum lines, wires, and lines.

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1990 300D 2.5
1991 F350 7.3 Dually IDI

1997 F350 gasser dump
1980 H-D 93 inch Shovel
2000 SE Roadglide

No reason to replace t-stat unless it's malfunctioning....over 90, even 100, now and again is fine on these cars.....nothing to worry about.

I've never replaced my thermostat (but will next time it needs it) but it looks easy enough to access without removing anything else.

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 156k miles
06 E320 CDI "Rutherford", Black on Tan, 173k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 143k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 67k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

thing is I have barely any heat. Was out a few nights ago, and it was around 15F. I was pretty cold in the car. It was barely lukewarm air. So I'm thinking stuck open thermostat possibly?

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1990 300D 2.5
1991 F350 7.3 Dually IDI

1997 F350 gasser dump
1980 H-D 93 inch Shovel
2000 SE Roadglide

Burping is easy, just run the entire system on defrost with the radiator cap removed for about 15 minutes.


There is a very small arrow on the thermostat that should be facing towards you and pointing up. Make sure you find it prior to resealing the thermostat back in!

Oh and if you haven't yet, replace all of the screws on the housing with news one. Mine were looking rather pathetic.

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-Typos courtesy of my mobile phone.

But is the coolant getting up to operating temp (around 82)? If so, then AFAIK your thermostat is fine. Like I say, the monovalve is the most likely culprit, especially if it's never been replaced.

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 156k miles
06 E320 CDI "Rutherford", Black on Tan, 173k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 143k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 67k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife