The day before yesterday, I replaced the thermostat on my 300SD.
Every time I work on this vehicle I have it shoved in my face that it's meant to be serviced on a lift, and only on a lift. I actually went and bought ramps for this job, which only works because my driveway slants down, and I put the ramps there, somewhat leveling the vehicle (it was still down slightly in front.)
Why do you have to jack the vehicle just to change the thermostat? Well, you don't. But, in order to relatively cleanly change the coolant, you have to drain the vehicle from the block or you will succeed only in draining the radiator.
Let's take a quick look at why this is. Coolant enters from the bottom of the engine block (more or less) and flows upwards to the heads. The thermostat is closed until 80 deg C, and until about 100 C it is half-open depending on load. Well, supposedly dependent on load, but since water pump speed is a function of RPM, it's really based on engine speed. Anyway, at this point, some of the water passes through the bypass and back into the block, and the rest goes through the radiator for cooling. This is intended to keep engine temp in the operating range, which means it runs just a tad over 86 C. Over 100 C, the thermostat opens fully, and all the water passes through the radiator. Thus, you can't remove the thermostat for better cooling (a bad idea on any vehicle - see thermostat.)
Anyway, the thermostat itself is at the top of the water pump, and more or less at the top of the block. The bottom hose to the radiator goes down from this point, while the top hose goes more or less straight across. There are also two hoses from the radiator going to the expansion tank, which oddly enough is also where the radiator cap is located. There's a small hose off the top of the radiator for overflow to go to this reservoir, and a larger one (about 3/4" ID) going to the bottom of the radiator, for the water to flow back into the active portion of the cooling system. The relief valve line from the radiator cap vents onto the inside of the inner fender (actually part of the unibody) and thus to the atmosphere. There is no drain plug on the radiator.
In order to completely drain the cooling system, therefore, it is necessary to use the engine drain plug. In theory, you remove the plug and bring the vehicle up to operating temperature, opening the thermostat and allowing the system to drain completely. In practice, you remove the drain plug, and then remove the bottom hoses from the radiator, allowing it to drain. This drains the entire cooling system. To refill it, you fill from the reservoir, and you DO have to run the vehicle to open the thermostat and draw coolant throughout the system. In addition, you have to set the heater to hot-normal or defrost, or use a "self-made tool" to connect 12VDC power to the heater solenoid valve in order to open it.
None of this sounds that bad until you go looking for the drain plug. It's located way up on the block, or so it looks, because it has to be above the crankcase since there's no water passages in there. The plug itself is not accessible from above on turbo models, because it's under the turbocharger. This means you must either dismount the turbine (big PITA) or raise the vehicle in order to drain the coolant.
Anyway, suffice to say that I got all this done and my vehicle is no longer overheating. The car actually runs hotter than it used to when I got it, leading me to believe that not only did it eventually stop opening (although at high temps and high RPMs it would seemingly open - VERY high RPMs) but it was originally not closing all the way.
The next critical projects are to replace the fuel primer pump and replace several front end bushings (and maybe rear end ones, but I haven't seen them yet) as some of the front bushings are all but gone. Following this I need to possibly rebuild or replace one engine damper (there's dampers/shocks next to the engine mounts) and probably adjust the engine stop. Another possibility for reducing engine vibration would be to add another damper to the top - this is a fairly common modification for small japanese imports with heavily modified engines. They typically mount to a strut tower and cross over to bolt to the block or head.
The next non-critical project is to replace the factory intake system with a cone filter. This requires either constructing or purchasing a high-quality oil separator, or cutting up the factory air filter in order to utilize the original unit, which is integrated into the housing. The separator has two outputs, one of which takes separated oil back into the system, and the other of which is an exhaust gas output which goes into the intake just before the turbine. This is of course a bad idea, because some quantity of dirty oil is consequently released directly into the compressor. This is not good for it, to say the least. The plan is therefore to run the exhaust line into an oil catch can, and empty that can back into the oil supply periodically. This eliminates the recirculation of dirty, hot exhaust gases, which means that the intake charge is both cooler and has more oxygen.
Another non-critical project is to rebuild the window switches. Unfortunately I only have a replacement for the passenger side, so I can only replace one module completely at my leisure. The switches are full of interesting little parts, including ball bearings that make contact. The contacts they roll on become crusted up over time, and replacing them (and greasing the whole with a nice dielectric compound) often restores them to function.
The fun never stops...