I recently purchased a 1999 Oldsmobile Intrigue 3800 and I changed the water pump thermostat to put new hoses also on a radiator cap and if I'm having heating issues where I can drive 10 miles or 15 miles easy with the gauge not getting to the middle but once I get to that mileage I get to the middle and if I shut the heat off I immediately have a problem where the gauge goes almost to the three-quarter Mark and I have to turn the heating back on to bring it back down is there a reason why my coolant might not be circulating or do I need to flush it because there is a rust build up when I open the cap that is new or going to the hose that is new . Both fans are running on the radiator and I don't believe the radiator is the problem or the fans but I can see the buildup of it is maybe the previous owner put some crap in there to try and seal a leak you never know some people are idiots. so I believe I need to flush it but I'm just curious to others opinions and also it seems like the coolant doesn't do anything from the Overflow container it'* like it just sits in there is that possible that it wouldn't either suck it up or run over into the container?

I'd like to see how much heat the radiator is dropping. Use an IR thermometer and check top in and bottom out. Should be losing at least 25 degrees.

I agree with carfixer007 ^^^^^^^

I also would like to get some clarification:

Did you change the water pump AND the thermostat?

Is there any chance that the thermostat is installed backwards?

Did you get all of the air out of the system?

Was it having any issues prior to your work?