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93 Ford Ranger Heater only works at high rpm

I have a 93 Ford Ranger with a heater that produces hot air only when the rpms are running high. The fan works fine but blows cold air if the engine is running at normal rpm, even after the engine has warmed up.

Anyone have ideas where I should start looking for the problem?

When was the coolant last replaced?

Both heater hoses hot to the touch when the engine is up to normal operating temp?

Remove both heater hoses from the engine and using a garden hose reverse flush the heater core. If the flow through the core is restricted at all, it’s plugged up and will have to be replaced. Check the hose fittings you removed the hoses from. If THEY are plugged up, clean them out.

Roadrunner has a good question… are the heater lines both warm? If not, locate the heater valve. Feel if the hose is hot on both sides. If it is not, you may have to replace the valve. Or, you can do what I did if you are el-cheapo and remove the valve altogether and put a coupler in it. In the summer, I rerouted the hoses so the line coming out of the water pump went straight back to the pump and hooked up the return line to the inlet of the heater.

I’ve heard that running coolant straight and not following the directions to dilute it will cause a vehicle to run cool.

Another possibility is a thermostat that is stuck open. The truck will even out and run like it is hot but the coolant never stays in the engine long enough to get hot enough to give you heat. You may be able to take the hose off the water pump and shine a light in the housing. It should be shut when it is cold and you would be fool hearty to check it when it is hot. If it is open (it stays shut until the coolant reaches about 180 degrees normally), you can even take it out, set your oven to about 220 degrees. After a few moments, the spring on the thermostat should move the valve open.

Thanks for the advise. I’ll be working on it this weekend and now know what to look for and where to start. Cheers!