Help with draining and flushing coolant w115

[mpofpa]

Ok. My new to me 1975 300D w115 has been running hot, around the 200 mark and slightly higher when stopped at a light. Yesterday After a 10 mile ride, upon getting back to the driveway and turning the engine off it boiled over. Ironically I was driving it back from an Indy shop. I know the PO put in a new water pump 2 years ago. After searching around on here I realize there can be multiple problems. Being that I am a total novice with diy car care, I thought the first thing I would try is to drain flush and refill the coolant. Ive never done this on any car. Looking for some advice or detailed steps for a total novice. I have some ramps that I planned on using. Thanks for any help, really want to learn how to work on this car.

[Left Coast]

First thing I'd try is check the thermostat. It has to come out to do a good flush anyway. Unfortunately, that procedure is a bit involved on a W115.

[mpofpa]

Well I wound up just draining and flushing the radiator a few times from the bottom hose as the drain plug is rusted shut. Filled it with water, ran the enginge, let it cool and drained again. I did this a few times with water then added 50/50 zerex G-05. I replaced the thermostat and found that one of the previous owners had placed a gutted thermostat in there, no valve, just a shell. I guess they were having overheating issues as well. He failed to mention it though. Thanks

[joshhol]

If you are having overheating issues, I would suspect the following:
Radiator is in bad shape - i.e. fins are smashed, pipes are clogged/rusted/leaking
Auxiliary fan is not operating as it should, usually only kicks on when ambient temps are high, if mild outside, it's not the fan.
Water pump is bad, yours isn't that old but if the belts are loose, it may not be turning properly
Thermostat stuck, you've already dealt with that.
Cooling system full of gunk. If you are just draining and refilling, it may not be enough to dislodge the crud in the system. Had a friend with a 300E that ran hot, they tried everything. Ultimately they had to pull the head and found water passages were gunked up. The car sat for 5 years so the normal flushing didn't free it up.
There's also a block drain point for coolant to make sure you get all of it out.

Forgot to mention to test the cap and system pressure. You can rent a little pressure tester kit from a zone of auto parts. If you don't have adequate pressure in the system, it runs hot.

Do you have the Haynes manual for Mercedes Diesel? It starts with the '77 diesels but much of it applies to the 115. It was a good starting point for me for DIY.

[280EZRider]

Originally Posted by mpofpa

I replaced the thermostat and found that one of the previous owners had placed a gutted thermostat in there, no valve, just a shell. I guess they were having overheating issues as well. Thanks

Gutting the thermostat certainly worsened the problem, if not causing it in the first place. With no thermostat the water pump can't create any pressure, which is necessary for total and balanced cooling. This is a good way to warp a head, especially if it's aluminum. It's good you replaced the thermostat, but I would keep an eye on the cylinder head gasket for possible leaks.

[Mike D]

Just to clear up a common misconception.

Automotive water pumps do NOT create "pressure'. They CIRCULATE the cooling medium, not compress it. Sealed systems create pressure through the transfer of heat from the engine block causing expansion of the liquid.

Proper circulation is affected by not having the correct thermostat as stated by 280EZRider.

[mpofpa]

The Aux fan did not kick on at all which I suspect has something to do with the A/C not working? I would have liked to get the drain plug off the block to flush the whole cooling system but I didn't have the tools to get in there so I am sure I did not get a very thorough flush. It was over 90 degrees when it boiled over so that certainly played a part. I have driven the car since changing the fluid and thermostat and it seemed to hold a steady temperature between the 175 and 200 mark even when stopped at a light. I only went a short distance so not sure how it will hold up on a longer drive. The service records I got with the car show a new water pump was put in 2 years ago. I got a CD/DVD of a service manual and owners manual which has been a big help but I would love to get a hard copy as well. Thanks for all the help.

[Mike D]

The auxiliary fan engages at 100C. To test the relay, ground the wire leading to the top of the upper radiator hose mounting with the ignition in the "run" position. The car does not need to be running. The fan should engage so be careful of anything which might be in the path of the blades.