Coolant gauge and Heater up and down
 

I bought a 2009 3.8 liter JK. Immediately had to put a new radiator in it. I bled and topped off the coolant. It warms up normal to 50%, drops to 25% or to 0% and the heat fluctuates cold and warm. If i pull over, it warms up within a couple minutes to 50%. I’ve added coolant, if I can. Parked it cap high and filled. Pumped the upper hose. Tried a 20 inch elevated funnel on the filler. Added the Prestone Flush-Tee to the upper bend of the heater hose. I cannot get the coolant or heater to stop bouncing from 0% to 25% while driving. I’m out of ideas.

Welcome to JKF. Initial thought is Thermostat is sticking open; therefore when sitting still no air flow through as when moving while driving. When you had the front elevated (did you have a burping kit) and the rad cap off did you get temp to over 200 degrees which would open the thermostat and flow coolant thru radiator.

First you need to determine a couple of things. Do this starting with a cold engine.
1. Bleeding the air. Also called burping. Simple procedure is raise the front right side of the jeep. Open the radiator cap, fill the radiator, start the engine and you should see bubbles start to come out, after a couple of minutes the fluid level will start to rise, put the cap back on the radiator.
2. With your engine cold in the morning, is the upper radiator hose, big and round or slightly collapsed or is the hose very very collapsed almost a flat hose.
3. Next day, observe the reservoir coolant LOW level and mark it. Start the engine. Do you hear noises like a grind or swoosh (AIR). After running the engine for a while the reservoir coolant level should rise about 1/4 inch. Mark it. This is HIGH level.
Do you have a new radiator cap on your radiator.

Feel like an idiot.
 

I have all the things I tried, the Most basic was the problem. The thermostat was stuck wide-open. I put a new thermostat in the jeep and so far it’s working great. Thank you for all the help.

At 51 years old. I should know this. How many times when I was younger did I chase problems just to find out the fuse was blown. I guess I over thought the entire process

Keep an eye on the reservoir coolant levels when the engine is cold and after it gets hot. It should vary 1/4". Apparently radiator caps go bad without warning and can cause engine damage.