Low power

hi this is my first time posting anything and wouldn't post anything if I didn't really need the help.. I have a Kenworth t800 year 2000 with a cat C12 with 400,000 miles on reman motor. A little over two weeks ago I noticed I was having a loss of power..most of the loads I haul are pretty light anywhere from 20000 lb to 30,000 lb my main hall is from Norfolk Virginia to Charlotte North Carolina which for the most part is flatlands with a few smallfoot hills if you would even call them that so most of the time I'm on cruise control every once in awhile I have to climb some hills like in Burlington I85 right before the DOT scales. For the most part on flatlan I'm ok but when I'm trying to climb these very small hills my power is limited .. I'm out for hours and RPM pretty much stays the same when pedal to the floor if not sometimes dropping a few miles.. as I said very small hills with load ranging between 20 to 30000.

At first we suspected turbo so inspect the turbo turbines everything was tight from front to back find ..turbine spin freely both directions.. the next day while truck is parked I rev up to 15 RPM load set cruise control sprayds soap water on air charger.. oh boy do we have bubbles everywhere !!
next day Friday I went to the dealership got an air-to-air put on myself that afternoon didn't run the truck all weekend came out Monday and to my heartbreaking surprise there was very little difference.
I talked to Caterpillar 2 days ago he suspected it was the wastegate. One of their head guys actually came out to me with laptop and fuel pressure gauge.

First thing he did was start truck it pressure gauge he said that it was slightly low but not that bad when he rev motor up it went to 80 psi and held so that was good. When he turned engine off gauge held at 20 psi he said that was concerning and thought there was air in the fuel system holding pressure and pointed at at the hand pump.

Then he hooked up the laptop to ECM restarted truck all the trouble codes we're good did a manual valve cut off on laptop everything was good no no motor Miss on laptop or through ear.

So then we took it down the road with the pressure gauge still attached to motor on dash went for a few miles down the road.
I watched the gauge hold at 80 psi the whole time he said it was making really good fuel pressure we can both hear turbo spooling up.. but as I told him I just didn't feel like it was pulling or grabbing when spooled up. He didn't say much other than didn't think it was a turbo wastegate and that wastegate would only work or not work...but I thought maybe wastegates sometimes get hung in between ??

He asked when was last time I did an overhead on it I told him it was last November.he said maybe they didn't do a correctly and I'm not saying they did or did not but it was through a shop and I heard they were the best around. But with that being said this is a cat motor and I know they have a lot more Cummins than cats they work but still this was last November and I've been feeling this loss of power for only a little over two weeks.

We got back to my shop and he suggested that he thinks,,,, but really doesn't like throwing parts at things.. and I don't either but he says he thinks maybe we should run a overhead on it and maybe change out the fuel transfer pump because it looked old and probably never been done before... Well I'm just a small owner operator I've only had this truck since last summer and I've already spent almost everything I have in it.. and finally start making a little bit of leeway .last thing I want to do is start throwing money at parts.. but if there is no other way , then I get it.. like I said most of my runs are only from Norfolk Virginia to Charlotte North Carolina we have other runs to do but with this loss of power it's really holding me back from other directions that I can run for better money and catch things financially back together... I know that was a mouthful lol.. but I desperately and seeking some advice because I don't know if the fuel transfer pump is the answer. Please help if anyone can

 

No . I wish I did

 

I could be wrong on the numbers..but I believe he said the computer said from a 1 through 4 it showed a 1 so it was making boost

 

Thank you for all the advice.
Air filter still pretty new just change little over month 15000 on it.
Fuel filters just changed two weeks ago
Even though we that cold weather I run the power service anti gel cond/Cboost.

Check lines and fittings both tight.
It does have an old-school muffler on bottom of the truck... I was hearing a lot of back pressure a while ago the last few weeks I haven't heard as much. We are taking muffler off in straight piping this weekend.

 

About two and a half months ago I had an issue going on where I had to keep changing my filters every week or two weeks..mainly the primary water separator when I change primary truck would go back to normal . When I would change the secondary she be so tight with pressure she would pop coming off. But I was using a lot of biodiesel at the time(Loves) and probably not filling up as much as I should had.. but I changed the place I was getting fuel at and topped off my tanks for every fill up... and haven't had that problem sense. But something was definitely goting on then... Could that have something to do with what is going on now ??