Why is my coolant getting over 250 degrees at a stop or slow driving?

DAN1970

MEMBER
1988 CHEVROLET CORVETTE
20,000 MILES

The level is not low yet the low coolant light comes on. The fan is working it is the first thing I checked. When I am on the freeway it cooed down rather fast to 190 degrees. The oil temp is 230 degrees. Is it a bad water pump? Is it a bad radiator? Could the fan be bad despite running what appears to be normal? How do I trouble shoot this problem before I start buying parts?

6 Replies

RIVERMIKERAT

MECHANIC
6,110 POSTS

What is the condition of the coolant? Does it look rusty? When was the last time the cooling system was power-flushed? Look in the radiator. Can you see coolant flowing? If not, remove the thermostat and check again. No need to put in a new gasket yet. If you don't see coolant flow, replace the water pump.

Check the thermostat. Boil a pan of water with the thermostat in the pan. You should see the thermostat open when the water reaches around 190 degrees. If not, replace it.

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Monday, December 19th, 2011 AT 3:28 AM

DAN1970

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7 POSTS

Flow seems fine. Why does it cool down when it is going down the freeway but when slow driving coming to stops does it get hot?

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Monday, December 19th, 2011 AT 3:34 AM

RIVERMIKERAT

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6,110 POSTS

Increased airflow across the radiator. It's cooler. What condition is the coolant in? When was it last power-flushed? Not just a coolant swap like a Jiffy Lube, but where a pressurized water source is hooked up to the cooling system. This flushes rust and contaminants from the system.

Does this have a direct-drive fan or a clutch-type fan? Is there a big aluminum disc behind the fan? If so, can you see moisture on it? Or "gunk"? Grasp 2 blades opposite each other. Wiggle them front and back. Is there any play? If so, replace the fan clutch. Is the fan in 1 piece? If not, replace it. Has the fan become bent? If so, replace it.

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Monday, December 19th, 2011 AT 3:45 AM

DAN1970

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7 POSTS

I will fush the system as you suggest. It appears to be rusty. I will also check the fan. I am thinking if the flow was a problem it would not cool down to 190 degrees when I am on the freeway. Thank you for your help.

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Monday, December 19th, 2011 AT 3:50 AM

RIVERMIKERAT

MECHANIC
6,110 POSTS

My pleasure. Even with lower flow, the fact that it is standing in the radiator longer and has increased airflow across the radiator while on the highway, allows it to cool down. Also, at higher freeway speeds, the water pump flows more coolant than at idle or in traffic.