91 LeSabre heating issue

So I took my car for a nice long trip through the mountains last week and discovered that my heating system wasn’t up to snuff anymore:

At idle, the car produces decent heat, as it does when driving around town or climbing mountain passes. It generally seems to give good heat at highway speeds on the flats too, but that’s harder to say for sure since the only flat areas on the trip are coastal BC and therefore warmer to start with.

Soon after starting the descent into whatever valley town on the mountain highway, my heater starts giving only lukewarm air. It was only just enough to make things comfortable in the cabin while wearing a sweater, and probably wouldn’t have been enough had it been colder outside. This can be for 30-60 minutes at a time as I descend in third gear, using very little gas. As soon as I get to the valley and stop, idling, the heat comes back, same as if I put the engine under load again.

Curiously, the coolant level in the reservoir never really moves above the cold line, whether the engine is hot or cold. Every time I check the radiator, while cold, it’s full. Water pump was changed this summer, but the bearings are already going and it will have to be changed again - in the meantime, it’s still pumping and not leaking.

So, where to start looking: half-clogged heater core? air bubble? thermostat? I’m at a loss.

If the bearings are worn in the water pump the fins may be perforated as well and that may be causing a coolant circulation fault, but perhaps not enough to cause an overheating problem.

Changing the t-stat may help. Are both heater hoses hot to the touch?

You may have air in the coolant system now.

I will make on other suggestion. You may have a leak in the vacuum system somewhere and when you start downhill and let up on the accelerator your vacuum goes down and it may not be able to hold the blend door open.

Had similar problem a few years ago after a water pump was replaced. A piece of debris was stuck in the thermostat which didn’t let it close completely. The coolant was continually circulating and never warmed up completely. Temperature gage also showed low temperature. When it got down to about 15 outside I replaced the thermostat and had heat again.

Besides the other good tips, look at the engine cooling fan(s) after the engine has warmed up. The fan(s) could still be running, and over-cooling the engine.

Don’t say that - I’ll never find the vacuum leak!

To try to rule this out - first, the heater blend door is cable actuated on this car, and second, if there was a vacuum issue somewhere in the heater box, wouldn’t it be present when idling at park as well?

Single fan, and I typically haven’t seen it running while the car is parked and hot.