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  #1  
Old 08-25-2000, 06:12 PM
RunningTooHot's Avatar
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This is a follow up post about an overheating problem with my US spec 1980 450 SL (~240,000 miles with METICULOUS maintenance history). Everything, and I mean EVERYTHING (YES, >*EVERYTHING*< ) is working correctly Ė you name it & Iíve checked it. Belts, hoses, thermostat, thermal clutch, thermo switch, auxiliary fan, temperature sender, temperature gauge (calibration checked with a digital infrared thermometer), water pump flow capacity, the radiator header tanks removed to check the core, fuel mixture & ignition timing, refilled the cooling system twice to assure the removal of any air bubbles, - EVERYTHING IS FUNCTIONING CORRECTLY Ė yet the car still runs way too hot in all conditions: highway, city, traffic, etc.

Once it is warmed up & the heat has a chance to build up, it is running between 90 at the lowest, and usually just above 100, sometimes reaching as high as 115. YES, the thermostat is good - it is not only new, I pulled it back out and checked it in a pot of boiling water with a thermometer. It is NOT installed backward (not even possible on this engine).

This is on a car that consistently ran between 82-85 degrees, and would sometimes reach 90+ in traffic on a hot day. But now, now what? WHAT AM I MISSING HERE? Could this be a symptom of a head gasket starting to let go? Thatís the only other thing I can think of, but there are no symptoms of that Ė the compression is good & even across all cylinders, there is no external leakage, no internal leakage, the spark plugs are burning clean, WHAT GIVES ? I am going NUTS!

Please someone help me before they haul me off to the rubber-padded room!

THANKS FOR ANY SUGGESTIONS (other than euthanasia for my baby).
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  #2  
Old 08-25-2000, 06:27 PM
Steve Wengel
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If it's any consolation, my '93 400E with 83,000 miles and also meticulously maintained does EXACTLY the same thing in the summer. Even at highway speeds, if the ambient temp. is over 90 and the AC is on, it runs at least 100-110 or so. I have heard from long-time MB owners that this is usually not something to worry about. One factor here is that MB temp. gauges are LINEAR, which means they read accurately, whereas American and Japanese auto manufacturers deliberately use NONLINEAR gauges, i.e. gauges which do not move much when in the mid-range despite a temp difference of 10 or 20 degrees. They do this deliberately to avoid raising our anxiety by having us see a gauge which reads "higher" than we think it should . . .

------------------
'93 400E
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  #3  
Old 08-25-2000, 06:46 PM
WmHarlow
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This same thing happened to me after I had the cooling system flushed and filled in my 300D. I beat myself to death to find the problem. I did what you are doing and nothing made sense.

I left it alone for the rest of the year and flushed the cooling system again the next year. Now it is still hotter than original, but less than it was after the first flush. The only thing different was the type of anti-freeze that was used. Now I am using Havoline, but I'm not sure what was used before (probably Prestone for Aluminum).

I doubt this made much of a difference, but you may wish to try a different anti-freeze.
What can it hurt? You've tried everything else

------------------
William
76 240D (W115) - 550K miles
78 300D (W123) - 200K+ miles
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  #4  
Old 08-25-2000, 06:59 PM
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What a pisser!

I went back and looked at the first post, and all I can ask is the obvious based on what I would try if I were you:

Is the engine really getting hot, as in open the hood and you can tell? Does the oil pressure drop?

Have you tried backflushing with a good flush the system to be sure there's no blockage?

Is the timing correct? Not only at idle but at 3000 RPM.

Have you tried running it with no thermostat, or with a 75 degree to see if the problem still exists? My mechanic swears that a 75 degree thermostat will prevent overheating in some cars.

Since this problem developed suddenly, my bet is a blockage somewhere.

Good luck,



------------------
Chuck Taylor
Falls Church VA
'86 300E 5 speed
'95 C220 (wife's car)
'98 Porsche Boxster
Past: '79 280E, '82 300D (18 yrs), '77 240D,
4 250C's
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  #5  
Old 08-25-2000, 07:05 PM
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Is the dilution of your coolant correct? At concentrations greater than 50% heat transfer can be inhibited. Use just enough to give you protection from seasonal temperature variations in your region. I have had customers top up with straight anti freeze and over a period of time end up with a coolant mixture that is too concentrated.

Bob D.
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  #6  
Old 08-25-2000, 07:13 PM
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RunningTooHot,

What was done to the car prior to your overheating problem? Did you just have it flushed? Anything replaced?

Suggestions:

1- Did you re-fill your heater core also when the coolant was replaced? To avoid air bubbles in the core, you should disconnect one of the heater hoses and fill the core using a funnel attached to the hose, reconnect the hose afterwards.

2 - What kind of anti-freexe are you using? Use only MBZ or Prestone for aluminum.

3 - I know you probably don't want to hear this but head gasket might be a possibility eventhough compression is good. It might be letting very little pressure escape once the engine is hot which may not be detected by a compression check. If that is the case it would be better to just bite the bullet and have the heads pulled. Don't forget to have the timing chain, rollers, etc replaced as good insurance.

Good luck and let us know how it goes.
Regards,
Joe Brasileiro
1980 450SEL
1987 Jaguar XJ6


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  #7  
Old 08-25-2000, 10:13 PM
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Hey everyone, thanks for the input! I will address each item that has been brought up:

1. the overheating problem was pretty much spontaneous, there was nothing done to the car to cause this problem. Although the summer temperatures have probably exacerbated the problem or brought it to light.

2. The heater core is filled with coolant, verified by heat output.

3. I am running a 75 degree thermostat, made by Wahler (OEM)

4. The ignition timing IS correct at idle as well as at high engine speeds, both with & without vacuum.

5. Itís very, very unlikely to be a blockage. The coolant has been changed *every* year for the past twenty years, and the original radiator didn't even have much scale on its internal passages. There has never, ever been even a slightly rusty appearance to the coolant whenever it was changed, therefore it is not a rust or corrosion related problem.

6. Coolant dilution is correct, in fact it still behaves this way when running a lower ratio of coolant/water, which has more effective heat transfer capabilities.

I donít really know what else it could possibly be! It seems unlikely to be a head gasket problem, at least from my experience. Maybe Iím wrong Ė but being a technically oriented person, could someone please explain to me how a (possibly slightly(but no evidence of it)) leaking head gasket could cause an overheating condition like this?

Iíve thought of everything I can, I even checked the camshaft timing marks to see if maybe the chain jumped one tooth Ė this is how desperate I am; I mean the likelihood of that happening is practically Zero!

Thanks Again, and keep those ideas coming!

Sign Me:
Still Going Crazy, (although somewhat comforted by the camaraderie).
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  #8  
Old 08-25-2000, 10:56 PM
bigfish
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Are you sure the engine is actually getting hot and the sending unit and/or gauge are not bad?
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  #9  
Old 08-25-2000, 11:16 PM
mattsuzie
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What ever happened when you felt the two hose when they were heating up? Did you sense blockage? You rad may have blockage>

What about running without a thermostate as previously asked? If you overheat w/o a thermostat, you gotta have a blocked rad (if you're not losing water volume).

------------------
'89 420 SEL
'90 300 SEL
'68 Olds 88 Convertible
'84 300 SD (sold it)
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  #10  
Old 08-25-2000, 11:25 PM
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More of my $.02

1. I echo Bigfish's question - is it really hot? Does the oil pressure drop?

2. What if it is a scale build-up that has blocked a part of the radiator or the cooling system? This could be a electrolytic or chemical reaction that would not show up in coolant flushes. A slow build-up could have finally caused it to go critical.

3. Have you tried Water Wetter to see if that helps?

4. I still think you should try it without the thermostat.

Good luck again!

Chuck
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  #11  
Old 08-26-2000, 12:12 AM
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RunningTooHot,
I hated to bring this up initially because your original post said that there was good water pump flow capacity. I don't know how you measured that, so here is my $.02 worth.

Many years ago, I had a car that was heating up and after checking everything, I found no clue as what the problem was. Finally, I made a desperate guess and installed a new water pump. Sure enough, the impeller was slipping on the pump shaft and was only moving a fraction of the coolant it was supposed to be pumping.

Hope you are able to find the source of your problem soon.



------------------
Ted
1979 240D
160,000 miles
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  #12  
Old 08-26-2000, 01:50 AM
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Location: Cremona, Alta, Canada
Posts: 263
Here is my 2bits. Have you tried a DIFFERENT thermostat? I have had many a thermostat that has checked out fine in the pot of water but ran hot or cold in the engine. If it happened suddenly TRY A NEW ONE OR ANOTHER ONE. My car ran hotter than it should, I pulled the stat and it checked out fine, put in a new one and presto it ran 10' cooler.

------------------
82 300SD 100K
91 Caprice SS
92 Jetta TD
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  #13  
Old 08-26-2000, 03:10 AM
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I just wonder; how did you check the water pump flow capacity and fuel mixture?

David
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  #14  
Old 08-26-2000, 06:10 AM
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Ok, here is the theory behind how the compression test may not show a slight head gasket leak at running temp:
Your cooling system is pressurized to provide a higher boiling point to the water and avoid overheating. If at operating temperature with the engine running the head gasket has a bad spot somewhere, you may be loosing some of the pressure needed to keep the water temperature from reaching the boiling point and therefore the car will overheat. The leak might be so small that no actual water enters the cylinders but just steam so you probably would not see any water in the oil or have white smoke coming from the exhaust.

Have you checked your expansion tank with the engine running to see if it shows any bubbles? If it does this may be an indication of a bad head gasket. Check it when the engine is not super hot or it will boil over and spoil the experiment.

The thermostat theory also seems to hold. Try removing the thermostat and running without it. If it still runs hot and you want to avoid jumping into the headgasket replacement, perhaps have the readiator checked for deposits or have it replaced.

From what you have told us so far, you have checked all the components, perhaps it is time to start replacing some cooling system parts systematically until all is absolutely ruled out. At that point all that will be left is the head gasket theory.

Good luck,

Joe Brasileiro

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  #15  
Old 08-28-2000, 07:52 PM
gselzler
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I'm not familiar with the details of this engine design, but I had a similar problem on an old car (Toyota Celica). In a nutshell, there was a pinhole leak in the head gasket running from a cylinder to the water jacket. Coolant never got into the cylinder, but exhaust gases blew directly into the water jacket. It caused coolant temps to go up very fast and high. Can you check for minute amounts of oil or exhaust gas in the coolant?
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