Moisture under the oil cap... milkshake

[Waterloo]

That dealer is taking you for a ride...

Do everything you were told to do in the other posts.

Why was the water pump and heater core replaced? Those normally last for quite awhile, like into the 700,000 to 900,000 mile range.

Any other issues with coolant hoses or coolant leaking out of the cap on the coolant jug?

What gearing and transmission is in the truck? And, when you are under a load, cruising down the expressway, where are your RPM's? 1200, 1300, 1400, 1500, 1600, 1700?

Now, you have bubbles and steam rolling out of the coolant jug? Does the coolant level fluctuate? Pressure in the bottle? Bubbles are not normal in the coolant jug. Me and Rawze just in framed my motor in his driveway, well, two years ago, doesn't seem that long ago... Anyways, I was just under my hood, there are no bubbles in my coolant, the level stays consistent too as I cannot recall the last time I even added any coolant.

Get that block test kit from NAPA, it is a normally stocked item, trust me I know...

Warm up the motor, idle her up and get her HOT, as close to 200* as you can get her, may take up to an hour in this cold weather. Follow the instructions on the test kit, very easy. Stuff the test tube rubber stopper in the coolant bottle opening securely, twist it in and make sure it is seated. If the test fluid gets a hint of exhaust, it will change color, but it may take up to 15 or 20 minutes, it is not a test of seconds.

Follow my instructions, I have been through this one too many times.

If it passes that test, you are still not out of the woods. Do the block test kit first and then we will go from there.

You will need these, if not now, eventually... These are the same items you will get off the Snap-On truck.

Here is the coolant jug adapter to pressure test your motor.
https://www.amazon.com/gp/product/B000VS...UTF8&psc=1

Here is the pump I use.
https://www.amazon.com/gp/product/B0002S...UTF8&psc=1

To fill your coolant, you need this vacuum kit or engine damage may occour. A 90 psi Home Depot style compressor will be needed to operate this kit. I have a 10 gallon compressor that works fine. Something like this will get the job done if you do not have a compressor.
https://www.homedepot.com/p/Husky-20-Gal.../206189626

Coolant fill kit.
https://www.amazon.com/gp/product/B0002S...UTF8&psc=1

This kit has everything you need to fill your coolant system, but I did swap out the lines for longer ones, any type of tubing will work, the clear stuff from Home Depot or any fuel line or coolant hose will work too as long as it has the correct I.D.

Good luck!