Own a 1990 f150 with a 302 5 speed. I have been driving on the highway back and forth between Wyoming and Colorado. Every time I drive the anti freeze boils itself. I just replaced the thermostat and all the anti freeze. The water pump is about a year old. Please help me. Also whenever I put it into fifth it grinds really loud and doesn't allow it to stay in. Why is that.

 

Did you also replace the rad cap when you changed coolant?

And 5th gear grinding and not staying in could be a result of worn shift tab/fork or sleeve and only allowing the gear to stay in half way grinding the syncro, also could be the 5th and rev shift rail.

 

The cap was replaced also. And I'll look into that for fifth gear