VIS better or not?

[Mettler]

I own a 98 sport. The previous owner had to replace the engine and he did so with a motor from a 99. Ive discovered that the motor he installed originally came with the variable intake system, but now has the upper intake plenum from the 98 that doesnt have the vis. Also ive been getting 259 miles to a tank. And this truck runs at 75mph @ 2750rpm in 5th gear, everything is bone stock on this truck. Is this normal?
Should I return the motor to a fully functional vis? Will the engine management on the 98 support the operation of the 99 vis?

 

[Mettler]

Nobody?

 

[2000StreetRod]

VIS on 1997 & 1998

It was my understanding that VIS was incorporated on the 1997, 1998 and maybe early 1999 SOHC V6. It seems unlikely that the original 1998 engine did not have VIS and the 1999 engine did. Is it possible you're confusing the OHV V6 with the SOHC V6. Is the eight digit of your VIN an "E" or an "X"? Below is a photo of a VIS upper intake manifold.

VIS is only activated by the PCM when the engine speed exceeds 3,000 rpm. Not having VIS should not have an effect on your fuel economy with normal driving below 3,000 rpm.