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Unusual Cooling System Problem

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I have a cooling system problem that I cannot seem to find a cure for. This is in a 2004 Chevy Silverado 5.3 Vortec model 1500.


Truck was overheating, I replaced the thermostat. It has a closed system, no radiator cap, so I went online to see how to properly “burp” the system. Once finished, it came up to normal temperature for this truck at idle and at normal driving speeds. I took it for a drive. It held the normal temperature until I stopped. Once I stopped the temperature dropped dramatically and instantly from 210F to 185F.


I thought perhaps I missed something when I burped it and so I burped it once again, but there was no more air to be bled from the system.


I have owned this truck since new in 2004. The temp has always run at 210F as a normal temperature. Once it comes up to operating temperature is never changes unless it is really hot outside and I am running the AC.


I checked the radiator, no bugs or deformed fins, no leaks. I checked the water pump, no leaks and no noise, no play in the shaft bearing. Also checked the fan clutch, it seems ok. Also there was no foreign matter in the coolant when I drained it. No collapsed hoses, they seem to be in good shape and no leaks. No water in the oil, or oil in the coolant.


The part than seems so strange is how the temperature changes so fast; it is almost instant once I come to a stop. There is no way that the actual coolant could change temperature this fast, it is instant. After continued driving the temperature will alternate between 210F and 185F intermittently as I continue driving.


I am at a loss to figure out what is wrong, even though it is not overheating, something has obviously changed. Is there anything else I can check that might cause something like this?

Edited by Jim Foley
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Thanks RJGTI. I had given the sending unit a thought, but also wondered why it would go out at the same time as I changed the thermostat. It had been working perfectly fine, with no fluctuations since it was new. It just seemed unlikely to me that the sending unit would go at the same time as the thermostat.


If it is the sending unit I thought it would be more likely to just stop working, rather than fluctuate like it does all of a sudden.

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It's possible that you are seeing the normal thermostat cycle. You may be watching the gage more closely now and didn't notice it before. As long as it's cycling below 210, I wouldn't stress about it.

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Thanks Podor, you may be right to not worry about it. Like I said, I have owned this truck since 2004 and the gauge always reads right at 210F once it is warmed up, It has never fluctuated before. I am just concerned that it may not be reading correctly and may overheat without my knowing it.

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