I just replaced the plenum on my 5.2 magnum, and while in the process, the coolant temp sensor broke and I replaced it, I got it all back together and everything is fine except it's running hot, it's not in the red but it's around 240 I'd say. I put coolant in the resovoir but Im not sure if theres a leak or what because it's not there now. I replaced the thermostat as well

 

Could be an air pocket in the system.
Did you bleed all the air out of it?

 

Most likely your truck isn't getting along with the new sensor. My parts store sensor reads 30 degrees hotter than the actual temperature.

 

I figured it out, I think the previous owner put some kind of stop leak in the coolant hose and now it's as hard as a brick

 

Maybe.....which hose exactly?

It'd be nearly impossible for stop leak to clog upper or lower rad hoses and those are the only two you really need for proper cooling. Is the coolant brownish red and sludgy anywhere? It's possible stop leak might clog passages in the motor or clog the radiator. It does seem particularly adept at clogging heater cores

 

It's the lower hose I guess, it's not really like a brick but something is definitely in there, whatever it is is solid, I'm about to go get it out and see what it is

 

How do I go about finding the leak?

 

Quite possibly an expansion spring. Quality, OEM lower hoses often had them. The aftermarket skips 'em because it's more costly and time consuming to build them properly.

If you have a hose with an expansion spring and the rubber is in good shape, I'd run it as long as possible

 

Well, seems to me we're not positive you have a leak. After you replaced your sensor did you first top off the radiator at the radiator cap (not just the plastic reservoir)?

Ideally you want to "burp" the system and you do this by idling the truck with the rad cap removed. If you can park it nose-high all the better. The BEST way is to also utilize a funnel meant just for this purpose because as air in the system expands you'll lose coolant along with air bubbles, however you can do it without the special funnel if you're ok with the mess. Run the truck until the thermostat opens -- you can tell because the upper rad hose will get very hot. I typically squeeze the upper rad hose periodically (watch your fingers!!) in an attempt to help burp the system....no idea if it actually helps but it's habit at this point.

Then you can shut down the truck, make sure rad is full, reinstall cap and put a little in the plastic reservoir if you want.

Here's an inexpensive funnel kit that works great

 

Alright I burped the system and it's still running around 210-215, before it ran right at 200. I'm not sure if it's on the verge of overheating again or if it's because the engine got a lot of it's power back and it's running a little hotter. Last night it overheated and there was puddle of coolant under the truck so I thought maybe there was a leak. I've ran it today and it hasn't leaked or got in that 240 range again so I'm not sure what the issue is or was.