Low boost x15 was 30-35 now peaks at 29

[Yady]

Hey everyone looking for some ideas.

It used to give me about 30-35 boost when I first got the truck back in May this year. Now its giving me peak 28-29 boost and real slow to go up.

there are no air leaks and no coolant loss. We just replaced the exhaust gas pressure sensor and imap. We put a new turbocharger with vgt actuator because we thought maybe the power loss was due to one of those failing. The engine intake and exhaust valves have just recently been calibrated and the engine sounds and runs smooth.

Any ideas?

Here's a little backstory of this engine.

I purchased a 2020 Kenworth T880 manual transmission.
Original engine serial 80177801 ISX15 565 hp. In 10/2024 it got a 2017 X15 565 hp engine serial number 60851461.

After I purchased it in May I decided to demandate it.
I got the demandated ECM file flashed, hollowed out 100% the SCR, DPF, and DOC
Added blocking plates on the egr crossover pipe, exhaust manifold and egr cooler. Removed intake butterfly. Replaced crankcase filter with maintenance free filter.

.

[tree98]

It could be in a derate of some kind or the turbo positioning tables are trash. Copy the file out of the ECM and send it to Rawze for review.

Did this start before or after the turbo was replaced? Did you replace the turbo and actuator with genuine cummins parts that are the correct replacement parts for your exact engine?

[Yady]

(10-23-2026 )tree98 Wrote:  It could be in a derate of some kind or the turbo positioning tables are trash. Copy the file out of the ECM and send it to Rawze for review.

Did this start before or after the turbo was replaced? Did you replace the turbo and actuator with genuine cummins parts that are the correct replacement parts for your exact engine?

I'll get it copied and sent. All the parts I've replaced with oem parts.
Its always been low and it would ask for a regen but when I attempted it, it failed. It said it wasn't reaching the temp to do it. So I decided to demandate it and it better for a while and then it started dropping and now its hard to reachc 28-29 psi boost. This is why I replaced the turbocharger and sensors but its still doing the same thing.

I agree , it feels like a derate but not sure how to fix it.

[Rawze]

copy file out and e-mail it to me for a review of what someone did in there.

I sent you a PM with the email address to send it to.

[Yady]

(10-23-2026 )Rawze Wrote:  copy file out and e-mail it to me for a review of what someone did in there.

I sent you a PM with the email address to send it to.

Thanks. Just sent it.

[Rawze]

(10-23-2026 )Yady Wrote:  

(10-23-2026 )Rawze Wrote:  copy file out and e-mail it to me for a review of what someone did in there.

I sent you a PM with the email address to send it to.

Thanks. Just sent it.

The demandate file is a bit dated, but otherwise no issues there.

== Unhook the exhaust behind the turbo and take it on a test drive to see if boost levels are higher with it disconnected. - If so then exhaust may be getting clogged in one way or another.

== Also check for CAC other intake leaks.

[Yady]

(10-23-2026 )Rawze Wrote:  

(10-23-2026 )Yady Wrote:  

(10-23-2026 )Rawze Wrote:  copy file out and e-mail it to me for a review of what someone did in there.

I sent you a PM with the email address to send it to.

Thanks. Just sent it.

The demandate file is a bit dated, but otherwise no issues there.

== Unhook the exhaust behind the turbo and take it on a test drive to see if boost levels are higher with it disconnected. - If so then exhaust may be getting clogged in one way or another.

== Also check for CAC other intake leaks.

There aren't any CAC or other leaks that I've found. On the maintenance free crankcase filter am I supposed to drill holes in it? I just bought it and placed it as is.

Also, I did the demandate myself. I hollowed out the cans. But I've attached a picture of the fault codes I get thru the Motive app. I dont get these codes in Insite though.