New cooling system has started over heating

1985 MERCEDES BENZ 300SD
125,000 MILES • V8 • AUTOMATIC
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CCAMALI
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  • 1 POST
Six months ago I began to over heat a lot. I took my car to my mechanics shop where he determined I had a clogged hose and as a result I cracked my radiator. He replaced my radiator and clogged hose and restored the coolant for $547.00. For five and half months it ran great. Two weeks ago it began to overheat and i put water in the reservoir. It ran fine for ten days.
Two days ago, out of nowhere my car shot to high 100 mark indicating I was overheating again. I had only drove to the store and it happened on my way home. I have not driven the car since that happened two days ago. The car has more than cooled down, and i have a galleon of water I am about to pour into the reservoir. My question is;
What do you think the problem can be? Please note I owned my car five years and I drove the car four years without ever putting in any water on my part, that does not mean when I had oil changes that the mechanic never added water. Now I have a new cooling system that is six months old and i am having issues.
Also, should I consider adding radiator fluid again or what?
My mechanic is on vacation and unreachable, what do you think?
Oct 5, 2017 at 5:50 PM
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STEVE W.
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My first suspect would be a bad thermostat. Next would be a radiator cap or head gasket. Both can cause overheating and both could be from the earlier overheat issue that caused a failed radiator.
Two tests would show which it likely is. One is a cooling system pressure test. It pressurizes the cooling system to the pressure indicated on the radiator cap. Then you watch to see if it holds at that pressure or not. Being it is a sealed system, it should hold pressure basically forever. If it does not and there are no external leaks like the radiator, water pump, heater core or hoses, the only other option is internal leakage in the engine.

You can easily do this test yourself if you wish. The testing kits are available at many parts stores in the US as a "loaner tool" (basically they make you buy the tool, then when you return it in the same shape you get the money back)

https://www.2carpros.com/articles/radiator-pressure-test

Run the test on a cold engine and you can add a simple step that may point out a bad head gasket as well if it does not hold pressure. That would be to run the pressure test. If it holds great, it is likely to be the thermostat.

If it does not hold pressure. Leave the tester connected and have someone start the car while you watch the pressure gauge. If the gauge bounces while the engine is turning over and then starts rising rapidly when it starts, shut it off. Remove the tester and plan on a head gasket replacement. The reason for that is the cooling system pressures should rise slowly from the coolant expanding. If the pressure bounces or rises real fast it is combustion gas pressure which should not be in the cooling system.
Oct 5, 2017 at 9:56 PM