Block drain plug
 

Hi Guys -
Dumb question of the day - where is the drain plug for the block located on an M103? Car is a 1990 300SEL. I'm having higher than normal temps and want to do a system flush.
The temp at 60-65 mph runs 90-95 degrees ( used to be 80-85). When I slow to 30-35 mph temp goes to 100-105. When I set at a stop light the temp goes to 110-115. It got to 120 once. I can hear the aux fans come on around 105 but the temp still climbs.
I've had the car for a year and a half and don't have any idea when a flush was last done. I want to flush the system, replace the thermostat, and put in new antifreeze (G-05) to make sure all that is correct.
Any other suggestions and comments would be greatly appreciated. This is my baby and I worry when it has a fever.
Thanks in advance for your help - Ken

You might think about a new radiator. Twelve years is a long time for scale to build up.
Anziani
'93 300CE 200K
'95 E420 122K

Drain bolt on m103 w124 is under the 5th or 6th spark plug. You may be able to get to it from above with regular socket and extension between/around exhaust, maybe swivel too. Easy from below with wrench.

Also there is a bleeder bolt on top of engine that will help during refill. New cap will not hurt and check for cracks in expansion tank or just replace it.

I got a couple of pounds of citric acid off of ebay for about 10 bucks delivered; mixed it with about 2.4 gal of water for flush. Distilled water with the G-05.

http://www.w124-zone.com/downloads/MB%20CD/W124/w124CD1/Program/Engine/103/20-0100.pdf

http://www.w124-zone.com/downloads/MB%20CD/W124/w124CD1/Program/Engine/103/20-0160.pdf

Would like to suggest changing the thermostat while at it. It is money well spent & shy away from the cheapy ones! Also, get the correct temperature range for the car.

M 103 block drain
 

Block drain is opened and/or removed w/ 19 mm wrench. Hope the pic helps.

I think yours has a viscous fan clutch - due for failure fail at 100K+

Ken, before flushing the cooling system install a new thermostat. A new thermostat resolved a sudden increase in our 1992 300CE's engine operating temperature.

No doubt you realize the optimum coolant is a 50/50 mix of MB coolant (or Zerex) and water with a ph between 7 and 8. If the ph goes above 8 the coolant mix should be changed.

ph 8-9
 

does this signify depletion of coolant additives ?
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FIXED
 

Thanks for all the input guys!
Ferdman, you were right on. I changed the thermostat first and the temps went to 80-85 at 65 mph and 90-95 at sustained idle in heavy traffic.
Again, thank you all.
Ken

The bleeder bolt mounted on top of the M103 head is made of very soft metal.

Can you say strips-real-easy.

Forget that and simply refill thru the coolant overflow tank with t-stat out. Once you have fluid at the top of the t-stat housing - most of the air is out of the system - not all - but most.

Put the t-stat back in and fasten t-stat cover.

Leave cap off overflow tank - start car - allow to reach normal operating temp - heater on. Attach a sock to the part of the overflow tank where the radiator cap is fastened.

Blip the throttle several times with a short rest between blips. This allows the water pump to push coolant thru the system and residual air exits out the coolant over flow tank opening. The sock keeps the coolant being pushed out from getting all over everything.

Top off coolant if necessary and refit coolant overflow tank cap - otherwise known as radiator cap.

You may need to top up some more a day or two later after things settle down.