Need some help to an old problem on more than one Blazer
 

It all started with my moms 94 Blazer with a 4.3 and over 240,000 miles (not sure the digital dash stopped working). She used it on her mail route for 4 years and then the problems started. It started using coolant but had no leaks. Eventually it got worse to the point where it misfired and used a quarter tank of gas to travel 5 miles. Now my mom has a 99 with a 4.3 4x4 around 230,000 miles and counting, and she uses it for the same mail route. Her truck overheated last week and the bottom hose blew off the radiator. Its back together and her mechanic said its using coolant but theres no leaks. So the question is, whats causing it?

Any input would be helpful, thanks guys
:icon_stooge:

My first thought would be to check oil for cross contamination with coolant.
check plugs.
Did they pressure check the coolant system?
Search for lower intake manifold gasket....known issue on these rides.
If coolant is missing and its not showing up externally around the engine then it has to be going into the engine somewhere.

I have checked the oil (nothing but oil in it), Ill suggest the pressure check and Im not sure how to check the gaskets, as sad as that statement is,lol. I will have to check the plugs when I get around to it for her. Thanks for the suggestions, Ill keep you posted but itll take a while, just a heads up...