Do you have 240k Since you rebuilt it ?

I get 28 out on the road...

I would start in this order.....

set the valves

check the chain stretch ( using the dial indicator method because this is what the FSM says when there is a performance complaint.)

check your compression

Run Diesel Purge through it using the correct procedure

change all your fuel filters , check your air filter

pull your precombustion chambers and clean them

take your injectors to a good shop and check for correct pressure and spray pattern.

If at some point your mileage goes up 4 miles per gallon.. stop messing with it and just drive it....