I have a 99 GMC Jimmy SLT. The other night I was driving home and my temp gauge started rising super fast. I got it home before it pegged out luckily. I opened the hood and noticed that my coolant reservoir was fuller than normal and that it was bubbling like the radiator cap was backing up fluid and air pressure into the tank. So I started off by replacing the radiator cap and the thermostat for good measure. The next day, same problem occurs. I check the radiator and I notice that its pretty low on coolant but the reservoir was full again. I put more coolant in it and it was fine for a week. But tonight it happened again. This time curiosity got the best of me so I decided to let the temperature rise up to 260 degrees. I pulled over so I wouldn't have to kill the engine in traffic and right before the needle pegged at 260 it dropped like a rock back to running temperature. I got it home and checked the radiator. Again it was low and my reservoir was full. This time I decided to look at the tube that allows coolant to drain out of the reservoir if it over fills and it looks like coolant had passed through there. The overheating problem is intermittent because I drove it about 80 or so miles on a small trip and it never overheated once. I want to mention that around 3,000 rpm, I can hear fluid running through my heater core and once in a great blue moon I think I can smell coolant when I first turn on the heat. But this thing with the core has been going on for the better part of a year and I've never noticed coolant in my floor so I don't think its related. I also replaced the water pump this past summer and the engine is rebuilt with about 37k on it. Tips or suggestions would be great. Thank you in advance for answering.

 

You might have a blow head gasket is my guess. Or a head gasket on its way out

 

pressure test the cooling system and inspect everything for leaks. some vehicle are unable to feed from the overflow if there is no suction due to there being a leak. also please post if there are any MIL codes.

 

I would start with seeking the direction to the problem so you don’t waste time and money on guessing (I speak from experience ). You can rent a block tester from an auto parts store like this:

OEM Block Tester 27145 - Read 1 Reviews on OEM #27145

Haven’t tried that one, so I can’t attest to it’s accuracy, but it’s the same concept as the very popular one below. You have to buy the fluid but you get your money back for the tool itself.

Or if you want to buy one to add to your toy box, this is a popular one used by shop mechanics. It's like $60 on Amazon:

Combustion Leak Tester | Cooling | UView

Shop mechanics use these types of tools for initial checks for blown head gasket (or cracked block or any other reason combustion gasses are getting into the coolant). The fluid changes color based off of CO2 in the coolant.

This is a pretty cheap and easy way to give you direction to determine if you are looking at an engine issue (head gasket, etc) or a coolant system issue (radiator, etc). Then you can go from there with more tests to pinpoint.

 

I checked for codes. The only code I have is no activity detected on o2 sensor bank 2 sensor 3. There's no catalytic converter and no sensor but that's a different story lol