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Weird heater problem--2002 Chevy S10

I have a problem that I can’t quite figure out. My truck heater started blowing out cool-to luke-warm air at best. I first checked the antifreeze/coolant level first and it was a bit low. I topped off the radiator. I know the thermostat is operating properly. My temp gauge shows about 190 degrees when the engine is fully warmed up. The inlet heater gets hot, the outlet hose only barely warm. So I figure I have a clogged core. The wierd thing is, when I go up a steep hill, I get hot air coming out of the vent. It is as though when the heater core is at the lowest point in the system, the gravitational assist provided by that orientation allows for the core to fill with the hot fluid, providing the hot air flow. What is going on? Thanks!

It sounds like there’s air in the cooling system. Here’s how to purge the cooling system of air.

Get the engine up to operating temperature. With the engine idling, slightly loosen the upper radiator hose clamp. Take a small flat bladed screwdriver and slip this between the upper radiator hose and the radiator hose neck. Let it idle for a minute like this and remove the screwdriver and then tighten the hose clamp.


I have come to believe if Tester says it it must be so!

When I change the coolant on my 2000 S10 Blazer I do the following to get the air out of the system. I drive the truck up on ramps to get the radiator as high as possible. I leave the radiator cap (not the reservoir) off with the engine running and the heater turned on high. As the engine warms up the air should work its way of the system. I top off the radiator as needed, once the coolant starts to overflow the radiator cap, I replace the cap.

Ed B.

Thanks guys! I really appreciate the info. I suspected that it might be an air bubble, but then I had people tell me that air should work its way out of the system eventually. I didn’t know who to believe. Since it is under pressure, it seems logical that an air bubble or two stategically placed in the system COULD remain and cause problems. Could you not also purge the system by parking on a steep hill and simply running the engine with the radiator cap off for a few minutes? Will that not accomplish the same things as loosening the top radiator hose?

your diagnosis that this is intermittant confirms the system works, so it is not clogged. you have either a failing water pump, or an air pocket.

try to eliminate the air as the fault, when the engine is cold, open up the radiator then look in the radiator cap for circulation.

I have yet to be able to try some of these suggestions. I am waiting until the weekend when I will be able to work on the vehicle during daylight hours (It is dark when I leave for work in the morning and getting dark when I get back home in the evening). I do know that their is circulation; I can see it when the truck is running and the radiator cap is off. I had very warm air (not hot, but definitely warm–about 100 degrees F according to a thermometer I placed in a vent) for a few days earlier this week. That occurred after letting the vehicle run with the radiator cap off on level ground, but I ran it only for a couple of minutes. This morning, I was back to having only cool to luke-warm air.