My buddy has an '05 auto with cummins. It is all stock accept for a K&N filter and a Van Aeken box. The truck has about 100k on it and suddenly his milage cut in half! He took off the Van Aeken box entirely but it made no difference! Truck doesn't run rough or smoke accessively! Put it on scanner with no codes thrown. Fuel pressure seems good (7000psi at idle- 22000+-) Wondering if injectors are on their way out??? Any thoughts? Thanks!