Ghost in the machine

[Rogcapital]

I have a 2007 rz2 with a marine power 340 5.7 engine. I have an intermittent problem that no one can figure out. At around 3000 rpms I lose forward momentum boat slows down a lot, engine goes down to 2000 rpms and I hear a backfire noise from the engine. Of course this is very dangerous while surfing. It happens every 3rd day or so. Very intermittent.

I have changed throttle body, throttle potentiometer, reflashed ECM, put in two new batteries, new fuel filter, new plugs good tune up, put in GPS instead of paddle wheel.

Nothing seems to fix this program. Any ideas?


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[boatwakes]

Fuel pressure. Keep a gauge on the rail and investigate when it goes whacky. Not always the fuel pump, could be anti-siphon valve, but leaning out the fuel injection will make backfires, loss of power etc...

[Rogcapital]

I had the boat out just now and kept my eye on the psi number on the gauge. It was normally around 400 but went down to 250 when the boat lost speed and started backfiring. Does that tell you it’s fuel pump related?


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[chpthril]

Can you post a pic of what gauge you were reading 400, then 250? A fuel pressure test gauge would have to be connected to the fuel rail for diagnostics and is not part of the helm display, and 400 PSI is a tad high for fuel pressure.

[Rogcapital]

I haven’t attached a gauge to the rail yet. This is just the readings from the speedo display. Attached are the pics.


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[Rogcapital]

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[boatwakes]

Your oil pressure is reading in kPa, kilopascal and 390 kPa equals 56.8psi, which is normal. As Chpthrl said, the fuel pressure is read off the fuel rail test port with an aftermarket gauge you must purchase and connect. Are you in Canadia?

[Rogcapital]

Right but when the boat starts backfiring the oil pressure is at 250 kpa which I believe is 36.26 psi. Isn’t that low? Yes I’m in Canada.


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