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1974 450SL with 128,000 miles
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125 Posts
Discussion Starter · #1 ·
I got a new water pump to replace one that gave out recently, and a new thermostat, and my temperature gauge is reading hotter than it did before. I searched the archives, and most of the posts seem to have a gauge that is different than mine, or they are discussing things in that Celsius system that Americans don’t understand. So my question is just “Is the picture showing an acceptable temperature for driving around in 70 degrees Fahrenheit in a 1974 450SL?” I believe it is, but would appreciate some reassurance from those who know. Thanks.
Watch Gauge Speedometer Motor vehicle Vehicle
 

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Premium Member
1980 450 SL
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474 Posts
That's where my 80 450SL runs on a hot day 100 deg F with the air on and the aux fan working 75 to 80 MPH on the highway. My radiator was boiled out and tested. With the air off it runs at 180 where the thermostat opens town or highway.
 

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Premium Member
'72 350SL, '85 300D, '98 E320, '19 Subaru Outback (sold '14 GLK250)
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12,357 Posts
On my car, it runs just above the 175F mark, even on a hot day (we do have them in Canada :) )

One thing to look at, is the timing. More advanced will run cooler. I would set at about 5degBTDC at idle and check that it gets to about 27degBTDC at 3000rpm. Sometimes advance mechanism is stuck and engine does not advance as it should.
 

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1986 560SL
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289 Posts
...new thermostat, and my temperature gauge is reading hotter than it did before. I searched the archives, and most of the posts seem to have a gauge that is different than mine, or they are discussing things in that Celsius system that Americans don’t understand.
80C thermostats should regulate the coolant to roughly 176 degrees Fahrenheit. Maybe you've got a common 87C (190 degrees F) in there. Keep in mind that your engine is more efficient the hotter it is, up to a point of course, due to less heat lost to the coolant from the cylinders and heads. Also, Google is an easy way to convert C to F without a calculator.
 

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1974 450SL with 128,000 miles
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125 Posts
Discussion Starter · #7 ·
I concur with JHT. Certainly not dangerous but a few degrees high for 70 degrees. Have you done a radiator flush ?
Just checked my records, and the PO had the coolant flushed when he was prepping the car for me. This was in February 2021.
 

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Premium Member
1976 450SL, 1992 190e 2.3, 1984 300D turbo, 1966 VW bus
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1,060 Posts
I did a radiator flush on all 3 of my old Mercedes this summer. Never done one before. It lowered temps 15-20 degrees on all 3 vehicles. The coolant passages in the block get hard water deposits.

Put the flush in and drive car for a couple days. You will be shocked at what comes out, I promise. Whatever brand they sell at autozone will work fine, just follow directions.



**never mind, sounds like you already did it
 

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Registered
1974 450SL with 128,000 miles
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125 Posts
Discussion Starter · #9 ·
80C thermostats should regulate the coolant to roughly 176 degrees Fahrenheit. Maybe you've got a common 87C (190 degrees F) in there. Keep in mind that your engine is more efficient the hotter it is, up to a point of course, due to less heat lost to the coolant from the cylinders and heads. Also, Google is an easy way to convert C to F without a calculator.
Thanks for this info. As a new Mercedes owner, I had no idea there were 80C and 87C thermostats. Before the new water pump and thermostat, it was usually just a touch over the 175F mark, so I think I will just watch the temp carefully and not worry unless it gets drastically higher than what I calculate as the 190F degree mark.
 

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Premium Member
'72 350SL, '85 300D, '98 E320, '19 Subaru Outback (sold '14 GLK250)
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12,357 Posts
Thanks for this info. As a new Mercedes owner, I had no idea there were 80C and 87C thermostats. Before the new water pump and thermostat, it was usually just a touch over the 175F mark, so I think I will just watch the temp carefully and not worry unless it gets drastically higher than what I calculate as the 190F degree mark.
Ronsurf - The correct thermostat for your car and all 450SLs up to 1980, is actually a 75C unit. Not 80C or 87C. 380SLs and 560SLs may have used those?
Do you have a record of which one was recently installed?
 

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1974 450SL with 128,000 miles
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125 Posts
Discussion Starter · #11 ·
Ronsurf - The correct thermostat for your car and all 450SLs up to 1980, is actually a 75C unit. Not 80C or 87C. 380SLs and 560SLs may have used those?
Do you have a record of which one was recently installed?
I just have a part number, which looks like the generic water pump number. 1162000015.
 

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Premium Member
'72 350SL, '85 300D, '98 E320, '19 Subaru Outback (sold '14 GLK250)
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12,357 Posts
I just have a part number, which looks like the generic water pump number. 1162000015.
That is the correct part number for the 75C thermostat your car requires

 

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Outstanding Contributor , Bob's Your Uncle!
83 280 SL- 5 speed-The PIG
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32,325 Posts
Use Zerex G-05 coolant diluted 50/50 with distilled water.
Last batch I bought was already diluted 50/50.

Probably paid more to get less........but no muss no fuss.
 

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Premium Member
1980 450 SL
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474 Posts
On my car, it runs just above the 175F mark, even on a hot day (we do have them in Canada :) )

One thing to look at, is the timing. More advanced will run cooler. I would set at about 5degBTDC at idle and check that it gets to about 27degBTDC at 3000rpm. Sometimes advance mechanism is stuck and engine does not advance as it should.
Curious about your reply on running at 175 F on a hot day. Is this with the Air conditioner running ?. The air conditioner makes a difference with mine. If it's not running it never gets above 180. The condenser is clean and the Aux fan works. It doesn't make hardy make any noticeable difference in power with the AC on.
I just assume it's the amount of heat generated and common for these cars. The noisy large aux fan seems to be an extreme attempt by MB to deal with it.
 

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'72 350SL, '85 300D, '98 E320, '19 Subaru Outback (sold '14 GLK250)
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12,357 Posts
Curious about your reply on running at 175 F on a hot day. Is this with the Air conditioner running ?. The air conditioner makes a difference with mine. If it's not running it never gets above 180. The condenser is clean and the Aux fan works. It doesn't make hardy make any noticeable difference in power with the AC on.
I just assume it's the amount of heat generated and common for these cars. The noisy large aux fan seems to be an extreme attempt by MB to deal with it.
A hot day in our area is likely still below 90F. And my A/C is not working. I had it work one summer, but that was over 30 years ago :) I drive mostly with top down anyway.
I read somewhere that the A/C can require 4-10HP. May not be noticed when driving car with 200HP engine. But fuel consumption will be higher. Engine will have to work a little harder, so more waste heat that has to go somewhere. So not surprising engine may run a bit hotter.
My aux fan never comes on.
 

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1980 450 SL
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474 Posts
A hot day in our area is likely still below 90F. And my A/C is not working. I had it work one summer, but that was over 30 years ago :) I drive mostly with top down anyway.
I read somewhere that the A/C can require 4-10HP. May not be noticed when driving car with 200HP engine. But fuel consumption will be higher. Engine will have to work a little harder, so more waste heat that has to go somewhere. So not surprising engine may run a bit hotter.
My aux fan never comes on.
Yes it wouldn't come on without the A/C unless the water temperature is 212 Deg F. With the A/C it comes on at 126 Deg F. before it's actually needed. It doesn't seem like a 10HP drain should cause the amount of heat increase. Perhaps it is just the cooling capacity of the system on the 450SL is marginal and the Aux fan is not really pushing that much increased air in at highway speeds.
Just wondered what others were experiencing. 90 Deg is like a cool front hit in the summer in Texas.
 

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CH4S Admin , Outstanding Contributor
1985 500SEC, 1991 190E 2.6.
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45,381 Posts
A remote IR thermometer works well to check older radiators for possible cold spots, meaning blockage by sediment - calcium etc deposits. Radiator shops can do a professional flush to remove sediment / calcification, at fairly reasonable cost.
A de-oiling flush with a liquid dish detergent water solution, can be followed by a citric acid flush, to remove calcification / sediment from the cooling system / radiator etc. Citric acid flushes are quite powerful, and the solution should not stay in the system fore extended periods of time. At the very least, a new thermostat should be installed afterwards. Likewise, the installing of new cooling system parts should be done 'after' a citric acid flush. For example, if a water pump was already weak, the citric acid flush will likely kill it.

After a de oiling followed by citric acid flush, I replaced OEM pressure cap, pressure container (mine started seeping from around the seam), water pump, thermostat, radiator, and all hoses with good quality hose clamps.

Small stuff like the cap on the pressure container is often forgotten. An old and weaker spring will lower system pressure, and raise temperature,
Cheers, and stay safe.
 
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