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1985 500 SEL
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175 Posts
Discussion Starter #1
I was looking at some parts for my vehicle when I came across a spec for the thermostat.

What is the difference between using the 84C thermostat and the 75C thermostat?

Is there any benefit to using one over the other?

Thanks.
 

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As I recall my 1980 used a 80 degree C thermostat, which is about the same temp as older American cars. I would not go lower than that unless you lived in Saudi Arabia. When manufacturers went higher is was generally to clean up emissions a bit. In the winter it makes great sense since the increased temps help drive moisture out of the oil etc. In summer it is obviously harder on the oil.
 

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1991 420sel
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267 Posts
Always use the exact degree thermostat specified for your car. Don't try to keep it cooler or warmer with a different thermostat. All the engine electrics and emission stuff needs to have the engine at a certain tempurature. That being said, there is a slight difference between thermostat brands, and between ones made now and ones made in the 1980's. The current ones seem to run a few degrees hotter than the old ones, so don't be surprised if your replacement runs a bit hotter. Also, the brands are a bit different. I like the Behr thermostats. they are built better than the Wahler t-stats. But they are all better than one that opens too early or not at all. Good luck!!
 

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1985 500 SEL
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175 Posts
Discussion Starter #4
http://www.allimportedparts.com/epcksoap.epc?action=Search&cookieID=1FD0QRTO3&drillid=7&[email protected]@Thermostat&clientid=allimportedparts

I should have provided more information about the part specifications. Both parts are manufactured by BEHR, and are for a 1985 500 SEL, and both show up as the thermostat replacement part for my vehicle.

I personally purchased the 84C, but this could be more than confusing for any DIYer. This is more about the difference in operating temperatures than it is about part quality or manufacturer.
 

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Depends on the day!
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13,337 Posts
A lot of times the hotter T-stat are something the factory implemented to clean up the emissions.. I run a cooler stat in my super charged buick w/ no issue 65,000 miles later.
Jonathan
 

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1991 420 SEL
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85 Posts
Yes I'm digging up an old thread ;)

I'm probably going to use the 75 C t-stat. I have used cooler tstats for years, in any car I think is too hot, and it just seems this engine runs on the hot side. Heat can be brutal on materials.

Is there any other reason than emissions to use the 84c?

Thanks

Pete
 

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1999 500SL, 1988 SEC
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1,069 Posts
All you out there that change your stats, please post what the degree of stat you put in and what the gauge says after warm up. I have a new 74 stat and it uns at 90 with all new parts
 

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1985 500sel and 500sec 2012 E63 1989 Porsche 911
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5,092 Posts
hah...bringing up nightmares of my temp issues from last yr

what did i learn from there..
gen 1 cars, there is only an 84 stat no matter whats its labeled as

my SEL sits at 80 most of the time and i havent changed coolant in 10 yrs

my sec ( problem child) sat at 100 when i first got it, put in new radiator, clutch and tried new and current new stat and it runs mostly at 90 ish

so be it

nex move eventually will be the aux fan resistors under the fuse boxes
 

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1991 420 SEL
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85 Posts
With a new radiator and new standard temp Rein t-stat I was running 87-88 or so (best guess). Working under the hood on a hot engine it just felt too hot.

I'll report back when I get her back on the road with the lower temp t-stat.

Pete
 

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1985 500sel and 500sec 2012 E63 1989 Porsche 911
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5,092 Posts

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1991 560SEL
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563 Posts
MB's every other year was based on old coolant formulations. Most current era coolants provide same protective properties for at least 5 years/150k miles now. Even the cheapest..

 

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2004 C 230 Kompressor Sport Coupe
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9 Posts
Yes I'm digging up an old thread ;)

I'm probably going to use the 75 C t-stat. I have used cooler tstats for years, in any car I think is too hot, and it just seems this engine runs on the hot side. Heat can be brutal on materials.

Is there any other reason than emissions to use the 84c?

Thanks

Pete
Yes there is. It is common to use a lower temp stat to make the engine ECU think it is colder. It will put more fuel in to run richer and give slightly more power. New age Chevy engine builders like to use that trick. The down side is that running at colder temps for prolonged periods will cause premature ring wear. I prefer to use the correct temp stat for the car. If your car runs hot after swapping out the stat I would recommend getting the radiator flushed.
 

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2004 C 230 Kompressor Sport Coupe
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Ring wear? Diggity-dagnabit!

The radiator is new, the coolant fresh... maybe my cheapo tstat is a little off. I'll put in a better stock temp one. Thank you!
You are welcome. the 75 C is very close to the 160 F that chevy guys use to enhance their mods. I think the 84 C is closer to 185 ish...although some US spec cars use 195F (stock chevy).
 

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1991 420 SEL
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85 Posts
I knew about the race guys, I just want my car to last longer. I've been running a cooler tstat on my last couple 2.5l S-10s for years with no issues, a 185 vs the stock 195 you mentioned, but I'm just a little more worried about my big fancy Sonderklasse ;)

All you out there that change your stats, please post what the degree of stat you put in and what the gauge says after warm up. I have a new 74 stat and it uns at 90 with all new parts
I pulled the Rein 80 c out and installed a Walher 80 c and - no real change, it still runs right near the middle of the 80 and next mark up on the guage. Under the hood still seems on the hot side to me, but what you gonna do.

You guage might be off. Just thought. Do you have a temperature gun?

Pete
 
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