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Old 02-07-2005, 06:57 PM
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Join Date: Feb 2005
Posts: 2
Running Hot on freeway

1975 450 SL. Engine seems to run too hot. During street driving the temp is at 180 to 185. As soon as I get in freeway, after about 15 minutes of 70 mph...I switch on A/C and temperature will go to 200 and even 225....has not gone passed that. When I shut-off A/C, temp stays steady. When I turn on and blast on the heater...temp drops 30 deg. When I turn goes back to 200-225. Already changed and checked thermostat, drained coolant fluid and replaced, minor tuneup (plugs, points (and dwell check), cap, rotor) checked timing. Plugs look like they are too lean. Tried to do minor change on mixture... (inside passenger compartment)...but did not have results...went a little rich, then more...and nothing. Then I went to the other side (counterclock) and still no change. Please help!

Last edited by siankaan9; 02-07-2005 at 06:58 PM. Reason: Add
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Old 02-07-2005, 07:03 PM
1991300SEL's Avatar
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Join Date: May 2003
Posts: 545
Hot operation once underway is many times due to a plugged radiator, but...

You mention this occurs when the a/c is activated. The a/c's condensor sits in front of the radiator, so it gets first crack at the air passng through.

I wonder if the area between your condensor and radiator is full of debris? Pull the radiator and clean both sides and also clean both sides of the a/c condensor.

If that doesn't help consider having the radiator replaced.
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Old 02-07-2005, 07:11 PM
Ta ra ra boom de ay
Join Date: Jun 2004
Location: Pittsburgh
Posts: 1,915
also... Is your auxiliary fan running?

1986 300E 220,000 miles+ transmission impossible
(Now waiting under a bridge in order to become one)

Reading your M103 duty cycle:
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Old 02-07-2005, 11:38 PM
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Join Date: Feb 2005
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Thanks for your reply fellow forum friends. I have checked between radiator and condenser...and there was plenty of light coming through (with flashlight). I even went ahead and shot some compressed air through...almost no debris at all. By the way...the fan clutch does seem to be working ok. At slow speed the fan is engaged properly. I even brought up RPM to 3200 to see if shut-off ok...and it did. When I let the RPMs drop about 3000 rpm it kicked in again. So I think it is working just fine. Even so...if the fan clutch did not work...isn't it true that at freeway speed the cooling would not be affected...and the overheating would be happening at slow speed? Any help is greatly appreciated. Oh! and there anyway of checking the radiator for proper flow before being forced to pull it? and also...could a stuck EGR have anything to do with this? ( I'm reaching, I know...but I'm desperate!)
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Old 02-08-2005, 08:09 AM
ozzy's Avatar
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Join Date: Dec 2004
Posts: 221
One way of checking your radiator is to check for heat from top to bottom, first thing in the morning from cold, (If you can, remove fan cowling and fan), start engine and feeling top hose (being carefull of moving parts) as soon as thermostat opens (you will feel hot coolant through hose), let it run for a few more minutes and then shut eng off. If you can't remove fan, remove fan cowling and move it away from radiator, now being carefull not to burn yourself, feel the radiator from top to bottom,(It's a good Idea to remove the key first), If it doesn't feel uniform or cold in spots it's possibly blocked. The only other thing I can think of is maybe your water pump is cavitating due to worn pump vanes.
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Old 02-08-2005, 12:13 PM
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Join Date: Jun 2001
Location: Northern Calif. (Fairfield Area)
Posts: 2,225
I'd go with the above suggestion. Take the car for a run after the therm opens, shut the car down, and feel as much of the radiator fin suface as you can reach. Every spot you touch should be uncomfortably warm. There should be no cold spots. If you live in a state that checks for NOX, there is speculation that a restricted radiator will seriously ellevate NOX readings also. Unlike Hondas and some other brands deteriorated water pump vanes are not a problem with MBs.


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