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Old 08-26-1999, 08:18 PM
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I am in central Mexico getting ready to return to the U.S. and my 300D is overheating with coolant overflow. When I am driving on the highway the temperature stays normal but when I get into slow city traffic the car overheats and overflows. The thermostat has been removed. The radiator has been removed and sent away for flushing. The fan clutch assembly seems to be in good working order. All belts are tight. The problem persists. I am thinking it might be the water pump getting ready to go.
When I squeeze off the top radiator hose and release it I think I can feel flow. But nothing that I would describe as a pressure surge. Is there supposed to be a pronounced surge or just some flow? It's a pretty big hose so I think i should be feeling something more than I am. The water pump weep hole is clear. There is no squealing or any other unusual noise coming from the engine. If it's not the water pump. Are there any other places I can look for blockage. Any help will be greatly appreciated appreciated.
Old 08-26-1999, 09:59 PM
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Join Date: Mar 1999
Location: Suwanee, GA, USA
Posts: 4,712
Reinstall a new thermostat with the arrow pointed up!! Then if the problem is still there, istall a new radiator and the problem will be solved if the head gasket is intact. If the head gasket is blown it will over pressurize quickly!! Make sure the aux fan for the A/C is working.
Old 08-26-1999, 10:03 PM
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Join Date: May 1999
Location: Queens, New York, USA
Posts: 38
My brother had a similar problem as your and it turned out that the water pump was not working properly. It was hard to find the problem since the water pump appeared to work. However, after some test, the mechanic realized that the pump was not pumping water with the right preasure. I'm not an expert in this field. I'm just relating the experience to you.
Good luck!

Old 08-26-1999, 11:41 PM
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In so. calif, both our 300SD temperatures routinely can run 100-110 especially with AC on in city traffic, with coolant fan on, new radiator, and new water pump, thermostat. The dealers here say that there's nothing much more that can be done, although it makes me uncomfortable to park a car with the temp at 110C

I understand that removing the thermostat does NOT help improve the circulation, even though there are some indep mechanics here that'll drill holes in the side of the thermostat to improve flow.

Has anyone tried installing an aftermarket second aux fan on a 300SD or any W126 and has it helped? Sounds like this water wetter stuff helps...where can I buy some?

Seems like the engine runs a lot hotter with the new R134 retrofitted AC? anyone else experienced it?
Old 08-28-1999, 04:38 PM
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Join Date: Jul 1999
Posts: 239
Temperature controlled Viscofan coupling for the main engine fan should also be looked at. This temperature sensitive arrangement causes the engine fan to engage and increase its RPM proportionate to the engine RPM, but not to exceed 3500 rpm.
When not engaged, the fan RPM isn't supposed to drop below 2100 RPM.
Anyway, this fan clutch deal is oftentimes overlooked, causing folks to suspect perfectly good radiators, thermostats, aux. fans, and many other expensive stuff and still scratching their heads over overheating problems...
This fan clutch problem applies to many other brands. I mention this because I've noticed whenever there is an overheating discussion the fan clutch is often left out as suspect and I learned the hard way with my Volvo.

According to the engine manual, testing involves revving the engine at 4,000-4500 RPM and wait for the coolant temp to rise to 90-95C, at which point the fan clutch will engage and increse the fan speed by about 1000rpm, which should be noticeable by the different pitch of the fan rotation. (I'm guessing they're not suggesting you rev up the engine from dead cold and maintain that until reaching 90-95C....)
They also mention this is not a rebuildable part and needs to be replaced.
Another interesting point is the coupling has to be transported and stored vertically and should never rest on its front end if it needs to be laid down during assembly.

[This message has been edited by VA300SD (edited 08-28-1999).]
Old 08-29-1999, 09:26 PM
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I've tried that too in the past! (another expensive part replaced, ?unneccessarily)

Its the EXTREME heat in the desert areas in So-cal that seem to be impossible to combat with. I guess these cars weren't built for such extreme temperatures
Old 08-31-1999, 04:29 PM
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Thank you TXBILL. You really nailed it. I went out and checked the coolant cap. Not only was there no washer but the brass disk that sits on the washer was cracked on both sides. I then bought a new cap and took the car for a spin. I went out on the highway for about 40 min. or so to make sure it was really heated up. Then into heavy city traffic with steep hills for another 30 min. or so. No problem! The temperature guage never went higher than 85 degrees.

Thank you again, TXBILL

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