Q: Why is my 1991 Chevrolet S-10 overheating.

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OK, first I've changed the lower hose and 3 different thermostats. I've flushed my system separately, radiator, heater core and engine one at a time. Good flow through all. At least 100 Gallons worth because I kept getting chunks.water will not flow through the radiator until I turn the truck off for a minute and start it back up. BAM all of a sudden radiator hot. Then BAM overheating, cold radiator. Shut truck off, radiator cold. Start truck up, bam radiator hot. So I started pumping the upper hose with the truck running and with a after market temp gauge right Infront of me the temp would keep dropping. I quit squeezing on the hose, it starts overheating. Rapidly squeeze on the hose again, temp drops back down. What the heck is going on? By the way when I start the truck back up temp drops 30 degrees quickly so I know at that moment the water flow is fast.
My car has a manual transmission.

Hello. It definitely sounds like you have a coolant flow problem. I would suspect that you either have a restriction in the radiator itself or a failure in the water pump. If the impeller in the water pump is failing then this will occur. I usually have the radiator tested first and if it is restricted then I replace it. If not then I change the water pump. Hopefully the engine never got hot enough to damage anything internally. If you need to have this checked out, consider YourMechanic, as a certified mechanic can come to you to [diagnose why it is overheating] https://www.yourmechanic.com/services/car-is-overheating-inspection

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