[4RE KLR]

Mike
It is very possible the core is stopped up.

You can take the top hose running to it and start the car. If you have circulation coming out of the hose turn the car back off.

Take off the bottom hose and hook up a garden hose to the top inlet and turn it on. It may take a second or so but that will wash out the core if it it stopped up. The garden hose usually has more pressure than the water pump does.

My father in laws truck had the same problem and we did this very same thing this weekend. The heater works great now. There was just no circulation getting into the core itself.

It was stopped up with all kinds of crap from over the years. The little Ford Ranger truck has almost 500K miles on it and still going.