Yes, and also rule out a problem with the cap at the overflow reservoir. The system is pressurized, and if the cap is old it will overheat at lower temperatures than normal. It may well be that there are a combination of things wrong, rather than just one. If the problems persist, don't forget about doing a citric acid flush to clean out the radiator. I would look at all of those suggestions before even considering the head.

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Tjohn

82 300 SD
77 450 SL (gone)

I am going to go ahead and order the water pump tody, as well as a pressure cap. While I did a thourough flush of the system, since I have to pull the radiator anyway, it will go the shop and be rodded.
How do I test the fan clutch? The fan is spinning, what else is there?

I'm not an expert on the clutch fan, but it can be bad and still be spinning the fan without being fully engaged (i.e., the fan is not spinning at full speed). If you look in this forum you will find other discussions on testing the clutch fan. My understanding is that if you shut of the engine when hot (100C) the fan should not spin freely, and should only rotate a portion of a revolution if you spin it by hand. I tried this on my car and I can only get the fan to spin one or two blades before stops (at 80C). My mechanic tests them by trying to "grab" the fan with a shop rag with the engine running and feeling the resistance. I have never tried that because I'm too attached to my fingers (and want to stay that way). I'm sure there are others here that can give detailed advice on testing the clutch.

Bruce,

You could have a number of things wrong, all at the same time OR just one thing. Rather than buying parts and guessing, you should rule out the head/head gasket.

You'll need to borrow/rent or go to your local euro mechanic and use one of the pressure guages that mount onto the overflow bottle. This gauge will measure whether your engine is leaking pressure into the cooling system. The pressure ought to be constant once the engine is warm but should NOT continue to rise. A good radiator shop will also have one of these. Once you determine the condition of the head/head gasket then proceed (I'm bit concerned from your description about the overheating in the driveway).

You should also determine, with a digital infrared thermometer, the condition of your radiator. The gun type device can accurately measure the temp drop across the radiator, look for cold spots and accurately measure the temperature AT the sending unit.

Your coolant bottle cap is suspect (the radiator shop can also easily check that) AND I'm wondering if the water pump is actually pumping (could the impeller have come loose and spin on the shaft). You can open the top right hose a bit and see if the pump is flowing fluid.

Your 1985 SD has a 212 switch built into the thermostat housing (at least according to my MB electrical schematic). The 212 switch turns on the aux fan (in addition to the ac switch on the receiver dryer), two switches controlling the same fan. If the fan is not coming on, you need to check that too.

Report back to us with your findings.

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84 300DT Puke Yellow. Totalled after 438,000
84 300DT Orient Red. 169,000 (actual mileage may vary)
2002 Explorer EB (wife's)

The aux. fan should come on based on the temp. of the a/c at the receiver/drier but I do not recall the temp. setting to activate. It will only come on if the a/c is on. You can check the fan operation or even set the fan to run full time by pulling the two fan leads from the switch on the receiver/drier and connecting the wires. However, that aux. fan has little or no effect on the engine temp.

You might need to check your fan clutch on the main radiator fan.

How did you "wash" the exterior of the radiator. It can be really gunked up in the fins. Can you see daylight through the radiator when you have it out of the car? I messed around with all of the same stuff you have mentioned last summer before I really cleaned the exterior radiator fins.

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1984 300SD 326,997 miles and counting . . . No wait, my odometer is still dead

Thanks for all your help. The radiator is at the shop being rodded, should be done monday. I pulled the water pump, as the bearings were making a slight squeeking noise when I spun it by hand with the belts off. Also there was a bit of play in the shaft. I'll put the new pump in this evening, the radiator on monday, and go from there.I'm still not convinced I don't have a bad head gasket, but this other stuff needed to be done anyway. I will post follow up, and again thanks for all your help. I washed the radiator with a power hose, can see light through it, looks clean.

Check your viscous clutched fan...if its not activating the fan properly, you will be overheating like that.

Got my radiator back this am. They said it was horribly plugged up, took quite a bit of work to get cleaned out. Maybe I got lucky and this was the bulk of the problem. Lesson learned however- even if it looks real clean and has been well maintianed a twenty year old radiator may still be full of s***.
Hopefully I can get the time to button things up and see what happens.

Follow up. Still running hot. The last thing I need to check is the fan clutch. I watched the fan when shutting off the engine when hot, > 110 degress, and it did not spin after the motor stopped. When I grab it by hand ( not running), I am able to move it around, buit it does have some resistance. I see no evidence of oil leaked out of it. The fact that iot doesn't spin when shutting the motor off would indicate to me that it is ok, but I don't know how tight it should be. It seems that at over 110 it should be tight as a drum.
Running out of ideas here........

By "still running hot" do you mean it is still getting hot enough to boil over or that the guage is showing 110 degrees? If it is getting hot enough to boil over, then you still have a problem. Did you check the thermostat opening temp by boiling on the stove and checking with a "candy" thermometer when it open (partially and fully)? Did you check actual engine temp with an IR temp guage?

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1984 300SD 326,997 miles and counting . . . No wait, my odometer is still dead

What I mean by running hot is that at idle it will hit 110 degrees,Test driving the car by driving 14 miles the gauge went to nearly 120 before I cooled it down by running the heater. The thermostat is new, from the dealer. I have no way to measure the engine temp except by the guage. The thermoatat has been replaced, the radiator has been to the shop and rodded out, the system thouroughly acid flushed and filled with the proper antifreeze and the water pump replaced. Unless the fan clutch is bad, I can only assume a problem with the head or head gasket.

Ok, so after quite sometime things are back in place. Turns out I had a bad headgasket. So, I replaced the theromstat, the water pump, had the radiator rodded, and put new viscous oil in the fan clutch. Now can run down the road with ac on, nearly 100 degrees outside, temp stays shy of 100 on the guage. These are all good things.
Now the bad, and lessons learned. First rule of benzdom- if you don't do the work yourself, make damn sure the person working on it really does know theses cars. I did not have the time or inclination to pull the head myself. I took it to a shop that I have been using for the better part of twenty years. They swore they would have no problem working on the car, that they do them all the time.Well, they were fos. First, they had trouble with some kind of shroud/shield thing on the fuel injectors (? don't know what the hell that is?), and had to borrow a tool from a mercedes shop. Then katrina hit, and they needed to close up to go help family. They finished the car up late at night and headed out of town. I picked the car up the next morning from the parts guy, and it would hardly even run. Hammering to beat the band, shaking like a cement mixer and smoking like a bug fogger. So it sat for week till they got back. My guess was they didn't have the cam timing right and I told them that.They said it was right, that was not the problem, that they needed to change the timing on the injection pump. I got them about half pissed when I told them not to touch the pump until the cam was right. They did it anyway, and it still ran like crap. So they ended up having to talk to mercedes shop in town to find out how to get the cam timing right and then had to again adjust the injector pump. Fun and games I am sure.
So I finally pick the car up and it runs ok, but when I get it home and look under the hood all the little return lines on the fuel injectors were leaking like hell. Turns out they did not realize that it is not the same size hose that they have a huge roll of in the shop. So, they ordered the lines, my wife picked them up, i put them in, and all is well.
DO NOT HAVE ANYONE WORK ON YOUR CAR WHO DOES NOT KNOW IT.
Ironically the thing that kept me from pulling the head myself was fear of messing up the cam timing and fooling with the timing chain.

Have unfortunatly found in my location that if you want to keep your wallet semi-intact and the repair done without too many complications doing it yourself is manditory. At least if the rad shop was not lying the rad needed a good cleanout plus the waterpump should not have been squeeking with no belt load. Have suspected for years that some of todays antifreezes contribute to the build up in the rads. My really old cars rads are jet clean internally and suspect are the original rads that came on the cars in some cases 65 years ago. Anyways hindsight is a great teacher in my experience. Some of the puzzles are quite perplexing on this site from time to time but a great learning experience for me when solved. Must be hard to be the originator of the thread at times though.