99 s10 overheating

Try bleeding air from system like this remove cap from overflow tank fill with antifreeze to full mark leave cap off and start vehicle and let idle you will notice level in tank start to rise this is normal once thermostat starts to open level in tank will drop add more antifreeze to full mark then put cap back on.

do the fans come on when it gets hot ? What does the exhaust look like ? look under the oil fill cap what do you see ?

The fans do come on, exhaust is good, may have a leak but nothing crazy

I had the same problem with a 2002 ZR5. When your engine has an overheating problem and you have checked the obvious possible answers, the two remaining problems usually are; 1. DA CAT! If your 1999 S10 has the factory catalytic converter then it is probably so stoped up by now it's killing your engine trying to push the exhaust out by now! This usually starts with overheating and ends with a junkyard. It will throw an error code to your dash, but if you get rid of your OLD cat it WILL help your engine! There is an aftermarket chip you can get to plug into your upstream exhaust harness to keep that dash light out. 2. This may be hard to believe but If your truck from the factory has been depending on a 50/50 mix of antifreeze and TAP WATER then by now you probably have enough minerals built up in the water jacket of the engine to overheat it by now! It would be best if we all used premix antifreeze but it may be too late for your engine by now. I suggest disconnecting your engine from both the radiator and heater core and as best as you can forward and backflush your engine to get rid of as much mineral buildup as possible! After all that get the lowest thermostat you can find and new hoses for the radiator. Good luck.