not to be an a$$ but have you checked the computer for codes? that be a good start. i just got a truck that a mechanic of 35 years told the customer the head gasket was junk and he wanted $2000 to fix it so they dumped the truck to me for free. i started it and the cat went to bright red on the pass side and it was boiling the heater hoses. i put the code reader on it and the ignition moduel failed it test. i replaced and all was good and i tried to give it back to the owner but they said it is yours and it is now going to the Youth group of our church for money for the group. sometime the computer will tell you wat is wrong (some times it does not but a good starting point)....mike