87 300sdl cooling system problems
 

Hi All;
Just changed the coolant in the car after suspecting a leak. I pressure tested it with the gagdet from autozone and found no leaks. the car seems to work ok, but on a long trip recently it drove at temperatures slightly higher than normal. Periodically I find some coolant under the car, almost as if the expansion tank is overflowing, but the fluid level is a little lower. After driving the fluid level is back to normal.
My questions are: what are the symptoms of a water pump about to go out? what about a thermostat or a pressure cap. I'm using the MB coolant and looking for signs of leaks (white crusty stuff) but not seeing any. It's making me uneasy to see the extra heat - I live in the Sierras and so I get to go up and down lots of mountains.
I'd appreciate anybody's comments that people have on this engine and car and any voices of experience.

Thanks Much,

Matt.

The most direct route to starting your investigation would seem to be a thorough cleaning of your engine compartment.... since you are finding drips at times... tracing where they are coming from may save you a lot of guessing and worrying....

Have you you checked the coolant system for pressure when cold? If the upper rad hose is solid when squeezed, and the car builds pressure rapidly after a cold start, before it warms up, you may have issues with the cylinder head. If the cooling system is pressurising itself through a crack in the cylinder head, the cooling system can get very irratic, temps can vary, seemingly for no reason, say climbing rapidly at idle, etc. The system is also unable to draw from the overflow tank hidden in the wheel well/front fender area (124). I sincerly hope I am wrong, and that it is something simple unrelated to a -14 cyl head issue, I'll keep my fingers crossed.

I would check your Pressure Cap before anything else.
If you are suspecting slight coolant loss from the expansion tank then that could be the cause. Also the fact that you are at higher altitudes will cause the cap to open a bit sooner since ambient air pressure is lower.

If you had abnormal pressurization of the cooling system (ie. head gasket or cracked head) I would expect a much greater coolant loss that would be noticable in the expansion tank.

When my cracked head was just starting to show signs, I would only occasionally lose coolant and had to add more maybe once every 5000 miles or so. If it is a cracked head you are seeing signs of, your coolant loss will get worse and you will see more temperature fluctuations. As Mack said, I will also keep my fingers crossed for you.