I tried everything myself before I was about to run the jeep off a cliff. The coolant flush gave me a 3 degree diff in heat. This could also have just been a warmer day. The garden hose did help, and gave me at least 5 degrees more of heat. My first understanding was yes water pump could be a problem and or the thermostat aswell. The jeep stayed in operating temps during hot days, so water pump was good and so was coolant level when checked. I checked the radiator hose on a cold day and that was pretty hot. What wasn't very hot was the return hose from the heater core. There was a significant temp difference between upper and lower heatr core hoses. Because I noticed improvement with the garden hose, I decided to turn up the pressure, but not with water but with air. I used a blow gun (Pocket Blow Gun with Industrial Coupler
) and I did one hose at a time, rinsing out with garden hose in same direction as air was blown each time, I worked my way up with increasing air pressure each time. I must have done this 8 times each hose and put it all back together. On a cold day my heater air went from 80-85 degrees tops to 110-120. I know too much air pressure can be dangerous, but it was risking that or replacing the heater core.