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diplonasty 06-15-2019 06:58 PM

Yet another 450SL running HOT thread
Hi. I've read so many threads here on the subject of 450SLs running hot.....
I have a 77 450SL with 57,000 miles. I am the third owner.

In the cooler months, this car has had some "good days" running at about 185-195 degrees F. But it did have a tendency to get up to the 212 line, especially while driving on the freeway. At times, I feel this car is bi-polar.

I foolishly went "ALL IN" and replaced the radiator, water pump, and thermostat and saw no change whatsoever. I had a system flush don3 on the car as well.... no change.

I read several threads here on ignition timing and had Goody's Automotive look into it. I am told that the timing is set properly.

A typical drive starts just fine. Once I get to speeds of 55 MPH or higher, the temps slowly creep up from halfway between the 175 and 212 line up to the 212 line. Once the 212 is reached, the needle hardly ever goes back down, even on the freeway. This behavior is the same if the AC is on or off.

So.... New Radiator, Fan Clutch (well.... a new one only made the car sound like a Mack truck and improved NOTHING so I put the old one back on)... New water pump, new thermostat, chemical flush.....

What else should I look into?? Summer has finally arrived and the needle is going past the 212 line. I pointed an IR laser thermometer at the top of the radiator and got a reading of 220F.... I know there is a solution but I cannot seem to find it. The aux fan kicks in but does not really seem to have any effect on the temperature.

I am not loosing any coolant and there is no white smoke coming out of the exhaust so I don't think I have a blown head gasket.

Any thoughts? I am at a loss!

Thank you

diplonasty 06-15-2019 07:17 PM

1 Attachment(s)
Forgot to mention.... expansion tank and cap are in good shape - no pressure leaks.

Djenka018 06-15-2019 09:19 PM

The fan that makes the noise as large lorries is of a simple visco-flex and it is for the cars that do not sport the electric AUX fan at the front (AC related), all for warmer climates.

I changed the fan on my car from visco-flex (South African CKD) to thermo-visco coupled from a later 380 and the noise/cooling improved. The engine will seldom go above 90C whilst idling and as soon as the thermo clutch engages, it drops to just below 90C. Mostly improved the temperature swings and at idle it needs good 30min in hot weather to near to 100C.
This was a true upgrade that required only drilling 2 holes in the new clutch hub to fit the original alu fan (new plastic one was not right for purpose and 2 of the 4 holes were mismatched but hub had provision for difference - by design).

Now back to your problem, have you ever thought to replace the thermo sensor?
I mean, the right procedure would be to use IR thermometre and check the temp but a second hand thermometer is the easier less expensive route. Perhaps also check the resistance at 100C indicated to confirm that the indicator needle is in check.
Apparently, temp under load and in hot days is normal to come to 100C (water boiling) but a moderate driving should bring it between 80-90C

There's always a chance that some galleys are blocked or that the mixture runs excessively lean but IMO these all come at the bottom of the check list.

...and after I read your post in entirety, above suggestion is meaningless (I made it smaller)

Perhaps a good spot is to investigate correct thermostat fitting and if it does open as required. This would cause insufficient coolant bypass to the radiator. Bear in mind that this thermostat is of a change-over type, it allows coolant to circulate the engine if temp is good or low and as soon as the temp goes above the required it diverts the coolant to the radiator. Some thermostats have tiny hole to assist hot coolant transfer to the back side of the thermostat thus assisting timely and full opening of the thermostat. When I replaced this on mine, it had a metal pin that acted as a poppet valve.

Jyuma 06-16-2019 07:03 AM

At highway speeds there should not even be requirement for a fan. 60 mph unlimited air ramming through the radiator due to the forward motion of the car should completely overpower any amount of air your fan could push. In fact, at highway speeds your fan may even be an impediment to air flow.

Did you check the A/C radiator fins for blockage? Something may be restricting airflow through the radiator.
How about oil? Are you using the correct grade of oil? Is there an oil cooler?
That new fan clutch you installed that made you car sound like a truck (fan roar) happened to me as well, I too put the original back and the fan roar was gone.
If you turn on the heat does the temperature come down?
Are you using the recommended type of coolant?
Does you car roll free or is there excessive drag?
Is the fuel mixture too lean? To be honest I don't know how to check it.
Any blockage in the exhaust?

Just thinking out loud.

cushjbc 06-16-2019 07:37 AM

I know you researched this by reading threads but I fought this problem for 4 years. My car needed a rebuilt radiator with more capacity for heat dissipation. I had a custom core installed into my OE tanks. When I installed it, I noticed for the first time my cooling system had the ability to recover from a hot run in surface traffic. As soon as I got air flowing through it again, the temp would drop noticeably to a comfortable level.

The viscous fan is what cured the "stop in traffic" heat rise. These fans are annoyingly loud but they really move air well when low or no speed provides low or no airflow.

The only thing I will ever do reference a system flush is a de-oiling process followed by a citric acid flush. I've seen first hand what that removes from a system I thought was pretty clean. I am also a permanent advocate of MB coolant (blue at the moment) mixed with distilled water (arguments continue on this ingredient but my mind is made up due to results). The problem with this is, it is a DIY project because nobody in a shop in ABQ will spend the time to do this correctly.

All of it - including mistakes along the way are outlined here:

Best of luck to you on this hot topic

diplonasty 06-16-2019 07:58 AM

Thank you for the responses.
Goody's Automotive in Scottsdale set the ignition timing as well as the air/fuel mixture. They appear to be the GO TO shop for the 450SL in Arizona, so I have to assume those factors are in check.

The condenser fins are straight and so are the radiator fins.

Motor oil is 0W40. I'll check if it coasts well to determine if there is excessive drag.

I am not an engine mechanic but I think that the issue may not be a lack of cooling but one of generating more heat that what the cooling system is meant to handle (if that makes sense). Not sure if this relevant of not, but if I open the hood when the engine is hot and running, the exhaust manifolds read mid 400's F (cylinder 4 reads mid 500's).

Goody's mentioned that 212 F was "PERFECT" and "Normal" but now that we have triple digit days in AZ I find the temp readings of 220/225 ALARMINGLY high.

I just don't want to make the mistake I made earlier of replacing good parts blindly.

okyoureabeast 06-16-2019 08:50 AM

In the summer time, especially Arizona I would run a heavier weight oil like 15w40. 0w40 is great for cooler running climates.

You might see lower temperatures.

cushjbc 06-16-2019 09:30 AM


I think that the issue may not be a lack of cooling but one of generating more heat that what the cooling system is meant to handle (if that makes sense).
It does make sense and I fully agree. These early systems (that have aged) do not have enough overhead to handle Phoenix with the A/C on. Later cars had better capacity.

diplonasty 06-16-2019 09:34 AM

Thank you. How much of a difference would you say it would make?
Would going synthetic help as well?

One other thing that may be worth mentioning: I mentioned earlier that the temps start out fine and start to creep up after the car reaches 55MPH..... In terms or RPM, 55MPH equates to a little less than 2500RPM. Could the issue be related to excessive heat generation above 2500 RPM? If so, what could cause the excessive heat generation?

I just took a 1 hour drive. Ambient Temp is approximately 90 F. I never exceeded 2500 RPM and the temps stayed below the 212 line (half way between 175 and 212 MOST of the time). Needle crept up at a stop and went back down once I got going.

rumb 06-16-2019 09:44 AM


Originally Posted by okyoureabeast (Post 17816056)
In the summer time, especially Arizona I would run a heavier weight oil like 15w40. 0w40 is great for cooler running climates.

You might see lower temperatures.

I dont know if that is correct.

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