This is kinda hard to explain but my dads 3rd gen will run great one day and the next you start it and it feels like it goes into a low power mode and struggles to get over 55 under slight load (aproximatly 3500 lbs) and it has struggled even when it isn’t loaded just running to town, then you’ll start it the next day and it’ll pull a 35 foot stock trailer 80 mph like it’s not there, anyone know what could cause that?

 

Have you checked to see if you have any codes. Do you know how to do key on off trick ? Do you have a OBD reader ?

 

Fuel Filter change first

 

What are the boost levels? What are the low side and high side fuel pressure readings?

 

It’s my dads truck not mine, it’s not throwing any codes as far as I know, and it doesent have gauges but it sounds like it spools up pretty well, and the power is intermittent, one day it runs like a bat out of hell and the next it feels like it just doesent wanna go, the previous owner tuned the engine and the trans then pulled the runner out and flashed the ecu when he sold it but that shouldn’t affect anything right?