Engine drop in rpm

[yann83]

Hi, I have a df115 2015 with 1100 hrs. Always serviced, running in saltwater.

Months ago the low pressure pump fail and it was changed.

Now when I run the boat for about 15 min, sometimes even longer, I have a drop in rpm, 1000 or more. Sometime when I keep the engine in idle the engine run rough and won't go over 2500 or 3000 (not safety mode). To temporary fix the issues seems to be to switch off the engine, turn a couple of times the key without starting the engine, and then start the engine.

So now I changed all the fuel line, the primer bulb was leaking a bit of air. I'm still waiting the new bulb meanwhile I connect the fuel hose directly. I sent the VST to Suzuki and they replaced the fuel pressure regulator, the needle and the filters.

Injectors are clean and working properly

There still a couple of bubble per minute, but I guess when I'll connect primer and water separator should fix it?

Another issues I'm having is with the engine interface, my simrad is connected via nmea2000 to the engine and I'm getting weird reading, engine hours increase, rpm increase, fuel rate changes. All data going crazy...

Can be that the ECM is faulty and causing both issues?

[Moonlighter]

There should be NO bubbles in the fuel line. If there are then air is leaking in and you need to find the source and fix it. Your fuel tank pickup or the fuel line connection to the tank should be checked. If you have an anti syphon valve on the tank (most US built boats do) then I would remove it and replace with an open barb fitting) because they are just problems waiting to happen IMO.

Re the crazy data from the engine, it is far more likely that other things are the cause, not the ecu:

- the engine interface cable is failing. They can throw feedback to the ecu and cause running problems. For a 2015 engine you will use the latest Version 3.5.7 cable. Or 3.5.4 is fine too, it was superseded by 3,5.7 only about a year ago. One clear sign of a failing cable apart from crazy data is that the module in the cable gets quite hot. Or shows signs of eater intrusion where the cables enter.

- check the plugs from the SDS adapter cable to the network. A plug that has damaged wires or pins can cause such issues.

- or the NMEA2000 backbone is not set up correctly and is causing data issues. It is ABSOLUTELY CRITICAL that the backbone is set up right, with T’s lined up and connected top to top like this TTTT and it must have a terminating resistor on each end to close off the open end of the end T’s in the backbone. All devices must only be connected to the leg part of the T’s, never to the top. I have helped numerous people with network issues and some of the weirdest problems have been when data was perfect for a year or more then suddenly wasn’t - and the backbone setup was the cause. So make 100% sue the backbone is correct before you replace the interface cable.

[yann83]

No bubble now, but still the the issues
At few hours slow trolling the problem was even worse... Could be the high pressure pump inside the VST? or again the low pressure pump?

[Murray]

A fuel pressure check would confirm or rule out your high pressure fuel pump.