2016 DF 150 topping out at around 3600 rpms...

[supagruv]

Hey redlowrey thanks for your input. So yes, it starts out at 40 psi and then after 5-10 minutes of running at various higher RPMs it would suddenly just drop down to 20 PSI. I've tried pumping the primer bulb from the very first time this started happening with no results (or change in PSI with the added fuel pressure gauge) - though it does seem like the primer bulb goes flat quickly when the pressure drops. Once I bring the RPMs down to 1-2k for a minute or two I can then get back to 4k+ but it tends to bog down more readily after the first pressure drop. I did try and swap the low pressure pump from our parts motor last week but that didn't make any noticeable difference. With the injectors I just figured the parts motor had some dirty ones and might as well get them cleaned so we can try one more thing if the new fuel pump didn't fix things. No smoke or rough idles. The motor just purrs.

I changed the VST to one from our parts motor after cleaning it, inspecting the fuel pump and fuel pressure regulator (it was corroded/rusty). The VST on our parts motor looked like new. Again, these are 2016 motors that look like new... but were used for a shark dive tour boat so we know they were probably run fairly hard and not well maintained.

I ran the fuel pressure gauge where is says to do so in the manual (just before the injector rail). I suppose I could try putting it in different places to see if it changes... do you think changing that would give us a better idea of things? I've checked every fuel line for possible bad clamps or leaks at this point. The primer bulb stays hard up until the engine starts running... then it just needs a pump or two to get it hard again.

Anyway, I'm going to install the new fuel pump this morning and we will give it another try. Putting lots of positive energy out there for a successful go!

[redlowrey]

If the high pressure pump stopped pumping and the pressure dropped, the primer would only take a couple of pumps to go hard because the fuel in the vst would be full to the float height.
You would not think that the primer would go flat easily, plus you have already changed the vst and still have the same problem.
Have you changed the primer.

[supagruv]

Well I'm pretty frustrated at this point... went out fishing for an 8 hour trip the other day and it went fine up until we did 4 hours of slow cruising then it wouldn't go more than 2 mph. Turned the engine off (ran the kicker motor for 30 minutes) then it went fine for 90 minutes to get back home. Still no error codes. No difference whether we pump the primer bulb or not... it stays pretty firm the whole time FYI. It's a new bulb and works very well.

Well at this point I removed the low and high pressure fuel filters. Got an extra 6 gallon fuel tank. Got an extra primer bulb. Going to try one more sea trial in the bay and try all of these extra parts and see if we get closer to narrowing things down.

Anyway... if all else fails I got an appointment with the shop in 2 weeks so they can dig in deeper.

Thanks for your continued help!

[redlowrey]

If the primer is staying hard and there was no restriction between the primer and the intake of the vst it would indicate the high pressure pump was not making much pressure.

Run a separate power and ground to the pump and try it, that way will run the pump at its max pressure all the time, under normal conditions the computer controls the ground depending on load, you could have a high resistance on either the power or ground supply to the pump.