Why is my 72 merc 280 sel 4.5 still overheating? - Mercedes-Benz Forum

 
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post #1 of 6 (permalink) Old 07-28-2008, 02:42 PM Thread Starter
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Why is my 72 merc 280 sel 4.5 still overheating?

My 72 280sel 4.5 has been overheating (up to about 225 degrees) when it usually stays around 190-200. It happens when I am sitting in traffic. I replaced the thermostat and waterpump and the coolant. What else could it be?
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post #2 of 6 (permalink) Old 07-28-2008, 05:12 PM
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Check the viscous coupling on the cooling fan. As I remember from my W108 (sold 20 years ago, sadly), if that coupling packs it in, the fan will slow to almost stop when it idles. It's the opposite of the coupling locking up so it spins at water pump speed all the time. It sounds idiotic to ask, but did you put the cooling fan back on when you reinstalled everything?

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post #3 of 6 (permalink) Old 07-29-2008, 11:13 AM Thread Starter
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I had a mechanic install the waterpump and thermostat. The fan seems to be working fine, but I will check into the coupling. The problem was going on before I had my mechanic replace the pump and thermostat. After he replaced the pump and thermostat the overheating problem is still going on. I suppose that it is the radiator? Maybe it is clogged?
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post #4 of 6 (permalink) Old 07-29-2008, 12:48 PM
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Did the mechanic back flush the cooling system when he changed the coolant? It could be the radiator is clogged as you suggest. It might also be that the water passages in the engine block are corroded or blocked. Check the radiator cap; if it isn't sealing or working properly that could be a cheap-fix cause. If the temp settles back to normal once you get moving it seems more likely it's the radiator than the engine. If the hoses are good and the coolant is holding its color (not discoloring from crud in the system), then there may be calcium deposits in the radiator. If all this stuff doesn't reveal a culprit, the next step would be to have the radiator checked for blockage I would think. Rodding out or replacing a radiator isn't a breathtaking expense compared to engine work, so let's hope it is that. If there hasn't been a consistent coolant mix or you have hard water in your area (or the area where the car spent most of its time) that also points to the radiator. Good luck.

"Don't pick a fight with an old man. If he is too old to fight, he'll just kill you."--John Steinbeck
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post #5 of 6 (permalink) Old 07-29-2008, 03:20 PM
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You may want to check the aux. fan in front of the radiator. The fan circuit is protected by a fuse located under the hood in a small rectangular box like fuse holder who's cover is retained by a thumb screw. Sometimes the fan can seize up. Quite often the contacts become oxidized at the point where the fuse makes contact. You can clean these contacts and replace the old fuse. You should also check the 220 switch which looks like a threaded brass fitting that has one contact wire that is ofset from the center of the switch.
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post #6 of 6 (permalink) Old 07-29-2008, 06:51 PM
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I didn't remember that there was an electric fan in the W108. That's an obvious place to check. A little WD-40 on the fan bearings might loosen it up if the fuse isn't blown.

"Don't pick a fight with an old man. If he is too old to fight, he'll just kill you."--John Steinbeck
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