2003 kia sorrento

I have a car problem, and being a retired backyard mechanic I they to fix most things myself. My problem is a 2003 KIA Sorrento which only gives heat when moving above 40mph. I replaced thermostat, it has no visible leaks, blend door seems to work. I’ve thought about loose water pump belt, bad W/P impeller, water control valve and all the horror stories found on Internet about Kia’s. I can’t find a repair manual which doesn’t help. Hoping someone might have some words of wisdom or just an idea. I live in Mastic NY

My money would be on #2 “bad W/P impeller” Does this car have a timing belt? Is it an interference engine. If yes to the above, have you had a timing belt change? How many miles on it now.

If it were my car 2001 with 75,000 miles, I would get the timing belt and water pump changed. Generally a lot of the cost for either is the same so doing them both at the same time saves a lot of money.