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Hello out there.... I have a 1984 chev 4x4 1 ton p/up into which I installed a 1996 vortec engine from a Tahoe.

All seems good except that I get 10 mpg only. The plugs are correct and I have installed a new distributer as well. as it kept throwing the cam positioning sensor code. When the engine is cold it runs great but once it warms up there is a pft pft sound out of both exhaust pipes. It is adapted factory exhaust with factory kats as well.

The strangest thing is that it gets 11.4 mpg with the camper on and strangely, only 10.4 with the camper off. I took it to the dealer here in town and they scanned it and reported no misfire and are at a loss to tell me what it could be. Has anyone out there experienced anything like this? My next thought is to replace the mass airflow .
 

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What engine did you put in? How many miles were on that engine?
 

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What kind (ratio) og gears are you running? Makes a big difference in mileage.
 
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