What happened

[Dave Blaber]

Still waiting for advice why my z200 HPDI does not power up over 3700rpm?????

[boats.net]

On one of your posts you said exchanging the VST resolved problem so you need to clean VST and check fuel pump.

[99yam40]

Is there a reason you do not monitor fuel pressures like most recommend to see what is going on?
Probably need the software and cord to hook to laptop, but it is the best way to troubleshoot these HPDIs

[mitch]

you really need fuel pressure reading ....

[rodbolt17]

the high pressure pump can be monitored with a multi meter and a service manual. the medium pressure can be monitored with a standard fuel pressure test guage that can read 70 psi minimum.
BOTH pressures should be monitored simultaneously at the problem RPM and load.