what year is the bus? if its in the 90s it will have a TBI injection system... if the check-engine light comes on when you first start then goes off and ther engine is running perfectly then not much you can do.. and engione swap to a diesel or a more modern one is pretty heavy duty.. you'd be nbetter of selling the bus and getting one that has what you want.. 4-5 MPG. sounds a little low deoending on what transmission you have..


if you hgave TBI on your 366... coolant temp sensor and O2 sensors are a couple things that can go bad and screw up the fuel mixture and mess up MPGs..

Chris,
I sent a pm, this morning, but not sure it went through.
Short version is, what do you recommend (if it's doable) to increase mpg on a 366? 4-5 mpg won't cut it. Thought about swapping to a 6.0 (gas), cause I'm not sure if a 5.3 would get the job done.
This would be in a short 90s GMC bus, not a cut-a-way.

Thanks, and Happy New Year,

Harry