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Old 08-19-2000, 01:23 AM
RunningTooHot's Avatar
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First, I want to thank all of you guys (and gals ?) for your help! This site is GREAT. I hope that I may be able to contribute in the future, but for now I have a problem / question.

I have a 1980 450 SL (US spec, about 240,000 miles - never had the heads off) that runs way too hot. It has a new thermostat (First thing I checked, but I still have the same problem). The fan clutch is working properly (it disengages at about 3000 rpm). The coolant temperature sender is good (I checked it against the one in my other 450). The belts & hoses are good. I am running a 50/50 mix of Havoline extended life coolant (no aluminum radiator to worry about).

I just flushed the system to make sure that the internal surfaces are as clean as possible (for good heat transfer) and the flushing agent came out perfectly clean – there is no rust (the car has been meticulously, no, make that FANATICALLY maintained its entire life). What came to light in the process of flushing it was this: Even when it was being fed cool water directly into the radiator, the temp gauge still read just above 100 degrees (centigrade, obviously)!? I don’t get it – And yes, the new thermostat works (I checked it in a pot of boiling water, with a thermometer.

The gauge is reading accurately, as the auxiliary fan comes on at the proper temperature (as read on the gauge), and it runs at about 82-83 in the mornings, once it is initially warmed up, but before the heat really builds up – then it goes up, up, and away.

The water pump is putting out a good volume of water flow, and the top of the radiator does get very hot to the touch. It feels like the air coming off the fan is warm, so there is heat transfer taking place (maybe not enough?). I am completely out of ideas. Well, except one – is there a possibility that an air bubble in might be causing the temperature sender to read too high? And if so, (but it seems unlikely) how do I alleviate the problem? I have never had this problem before, and I have changed the coolant many times before. Plus, this problem just arose spontaneously, before I even flushed the system. (I flushed it Because of the problem).

I thought that I knew a fair amount about auto mechanicals, but now I’m feeling pretty dumb & helpless. I hope that someone might be able to clue me in.

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Old 08-19-2000, 02:22 AM
Mike Murrell's Avatar
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Join Date: Feb 2000
Posts: 2,141
You sound like a competent DIYER mechanic; at least one who's had the good sense to maintain his/her vehicle.

I too am a DIYER so here's my 2 cents.

I've encountered the problem you've described and my guess is it's one of 2 things:

- air in the system
- early stage of a leaking head gasket. Yeah,
I know you didn't want to hear that, but..

With the engine cold first thing in the morning, top off coolant, pull radiator cap and start. Watch for bubbles.

Consider pulling the thermostat and filling with coolant 'til it gets to the thermostat.
I'm not familiar with the 450, so this may be impractical.


Mike Murrell
'91 300-SEL
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Old 08-19-2000, 09:21 AM
shoe's Avatar
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Join Date: Jun 2000
Location: Baton Rouge, LA.
Posts: 583
A question, does it run hot all the time e.g. idling and highway running. If it just runs hot at highway speeds it could be a partially clogged radiator. If you can get to all the parts of the radiator see if you have any spots that are a geat deal hotter than the rest of the radiator. Flushing may not always get several clogged tubes. this may not be it given how well you have kept the car. Just an idea! You may not have an aluminum radiator but if I'm mistaken it has an all aluminum engine.

Keith Schuster
99 Harley Davidson Ultra Classic
97 Dodge Ram
83 300D
auto\hydraulic mechanic 20 yrs
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Old 08-19-2000, 10:40 AM
RunningTooHot's Avatar
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Dear Mike & Keith - Thank you for your input, it is greatly appreciated.

Regarding the postings: 1) I hope that I don't have a bad head gasket...but I'll try that type of diagnosis later this morning. 2) To avoid air entrapment, I add the coolant while the car is parked with the nose pointed slightly uphill, so that any air in the cylinder heads will be pushed forward and (hopefully) out.

Also, 1980 is the last year of the iron block V-8; it is a 450, in 1981 they went to the 380 with the aluminum block, just in case you wanted to know.

I am hoping that some of the other knowlegable people out there may have some additional ideas?
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Old 08-19-2000, 05:12 PM
Posts: n/a
Let me give you some food for thought altough I am no guru.

How high does the temp go? above 120?

Does it cool down after getting to speeds greater than 35 mph?

Can you try running it without the thermostat, then you can definately rule that.

How does the auxilary fan know to kick on? If it is run off the the same temperature element as the water temp guage then you may have a bad / uncalibrated temp gauge. But if they are different, then you can probably rule that out as you previously determined.

How about your fan shroud? Is it cracked or missing? That would be a problem.

Can you put a temperature element inside the radiator while it is running? Running it would external cold water should keep it at 80C so I would guess you either have a clogged radiator or uncalibrated / bad temp gauge. You said that you "flushed" the radiator. Could something have lodged inside the radiator and reduced flow?

'89 420 SEL
'90 300 SEL
'68 Olds 88 Convertible
'84 300 SD (sold it)
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Old 08-19-2000, 07:49 PM
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
Sounds like a radiator problem to me. The V8 motors almost never have cooling system head gasket failures. The ones that happen come as a result of severe overheating.

Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician
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Old 08-19-2000, 11:22 PM
Join Date: Mar 2000
Posts: 37
Ok i have had this problem with a 80 450sl before, it was the pre cats in the exhaust pipes just below the exhaust manafolds, you will have to take the pipes off the engine and see if you can blow air through them ,if they are bad like mine was they will look like someone has poked steel wool in the pipe the ones on my car were stoped up about 90% and the car would not go over 50mph, and was getting HOT!, there is another cat on furter down in the exhaust system ,that might be stoped up as well, good luck, hope this might help you....

85 500 SEL EURO (no cats on this one)
73 450 SE
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Old 08-20-2000, 10:19 AM
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Join Date: Sep 1999
Location: Cremona, Alta, Canada
Posts: 263
Here is a thought. When things are running hot have someone hold the revs up2-3000rpm and then feel the LOWER radiator hose. If it is hot adn your fan is engaged then you have a problem somewhere else, but if it is cold or just warm then your radiator is sealed off in side. Concidering the year of car and if it is an original radiator than that is likely the problem. Had this problem with a truck and took me a long time to track down.

Good luck

82 300SD 100K
91 Caprice SS
92 Jetta TD
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Old 08-22-2000, 03:01 PM
Posts: n/a
change the temp sender if that dont work make sure that the thermastat is fitted the correct way round if it is backwards you will get this problem. Dont run with it removed it will definately run hot.
Sounds like you bled it correctly.
Did you change the thermastat before flushing?
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Old 08-22-2000, 08:05 PM
Registered User
Join Date: May 2000
Location: Scotts Valley, CA.
Posts: 145
I've run into a similar situation with other types of cars. I found on more than one occaision that it was the water pump in the early stages of failure...

Despite your statement that the water pump is putting out a decent volume, please see many other postings to the list that all say that the MBZ factory water pumps are (relatively) cheap and provide much better flow and volume than anything else.

Jay Yambrovich
1989 300 E
1992 190-E 2.3
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Old 08-22-2000, 09:18 PM
Posts: n/a
The thermostat reduces flow. No thermostat will mean that the maximum amount of water will flow through the block, thus running cold not hot.

Please explain to me how running w/o a thermostat will cause an engine to run hot.

'89 420 SEL
'90 300 SEL
'68 Olds 88 Convertible
'84 300 SD (sold it)
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