My '98 SL 500 suddenly started overheating last week. I exited the expressway after a 20 mile trip and the oil pressure gauge, which is usually pinned right at the max started dropping and jumping around related somewhat to RPM, and the coolant
temp rose quickly into the red. Before that trip, I had noticed nothing wrong. Always ran consistently in the 85 degree C range. Outside ambient was average, in the seventies F. The electric fans are coming on. I thought the fan clutch might not be working properly so I temporarily pinned it to be always engaged - had no effect. I removed the dome-shaped manifold chamber above the thermostat where two rubber hoses, one coming from the top of the radiator and the other leading forward and out from the top of the engine between the air cleaners (return from heater core, maybe?) meet and combine their flow down through the opening in the base of that chamber, and then presumably through the thermostat located adjacent. I ran the engine with that domed cover removed and noted coolant pumping out of the hose leading from the top of the engine and conveniently arching and falling directly into the opening in the base of that chamber leading to the thermostat, which readily accepted all of the coolant and sucked it away. Again, I was able to observe this without the domed cover in place, the coolant cascading out of the hose and down into that opening through mid-air. Three things I noted: 1) The water pump was moving the coolant but I would have thought the force of the circulating coolant would have been much greater, enough to basically shoot the coolant straight out in a stream that would have hit the radiator instead of just kind of pouring out of the open end of the hose; 2) The thermostat must
obviously have been open for the water pump to have wisked the coolant away (but I must admit I am not absolutely certain that opening leads directly through the thermostat to the water pump. Does it?); and 3) There was NO coolant coming out from the rubber hose leading from the top of the radiator, none. So it seems like there is no coolant being pumped up through the radiator. I am assuming the direction of coolant flow through the radiator is into the radiator at the bottom, then up and out through the top hose, then combining flow with the other hose in the domed manifold chamber I spoke of, through the thermostat, then into the water pump, through the engine block, then back to the bottom of the radiator. Is this correct?
I can drive the car for about five or six miles before the oil pressure starts to drop, presumably because it's viscosity begins to thin as the engine starts to overheat. This happens when the water temp reaches about 85 degrees C, which is clearly not very hot but I guess the water temp sensor is not really reading true engine temp, especially if the coolant is not circulating properly. It makes no difference if I'm driving fast or slow, freeway or stop-and-go. I have about seven or eight minutes before heat buildup begins to negatively affect oil pressure. After 10 or 12 minutes, the water temp will be in the red and the oil pressure flirting with zero when at idle.
Because of the sudden onset of this problem, I'm not thinking the radiator is packed up solid. There is no evidence of oil in the coolant or coolant in the oil. I changed the oil just after this occurred. So, I think the head gasket is ok. The surge tank cap gasket seems to hold well against the rising pressure. Likewise, there are no leaks, even at boiling-over pressures, apparent anywhere in the system. What's going on? Bad water pump? Combination of weak water pump and partially blocked radiator. If the thermostat is malfunctioning (stuck open), could that unexpectedly cause these symptoms?