Truck Repair: general, gas filter, fuel pressure

Question
Hi there,

I have a general mechanical question.  My 2001 GMC box van mid size truck is losing compression or power when going uphill.  The speed will drop down to 30 to 40 mph and the engine seems really quiet....kinda like an electrical vehicle, as compared to a straining loud engine sound.

Then when I reach level ground again the speed returns to normal with normal acceleration, etc.

Do you have any idea what I should be looking to repair?  I'm guessing gas filter might need changing, but not sure if it might be a transmission or compression problem.

Looking forward to hearing from you,

Thanks!
Wayne

Answer
I always start with the easiest and basic things first. Yes the fuel filter will lower the fuel pressure if it is restricted and cause low power under demand. considering how much its used, it is my first place to look. after that i need more info on the specific engine we are working on. also change the air filter if its old. its a small investment to keep the performance up.   Rusty