Cannot reach full rpm under load

[Mystical]

Hey guys,
Our 2012 Ranger 29 is not reaching full rpm under load. We have been working on this since last year. Bottom cleaned, prop tuned, filters changed, fuel is good. We can reach 4200 with no load on the engine, but underway, can only get around 3700. Yesterday I ran the boat with my scan tool hooked up to it. No codes present by the way. With 100% load, the yanmar gauge is showing 29psi boost while the scan tool is showing only 22. For starters, which do I believe?
Thanks
Brian

 

[knotflying]

Brian,

On my 4BY2 180 my turbo is supposed to be between 22 to 26 PSI. at WOT. My experience has been that getting full RPM on my boat, which is 4,000 has a lot of variables such as weight, sea conditions, temperature of water etc. However I have only seen a 50 RPM difference.

 

[Mystical]

we always were able to run at 4200 wot before last year without any issues and cruise in the high teens, low twenties, now 3700 and 15 is about all she will do.

 

[knotflying]

I don't know, it could be the turbocharger. However, I would think you would get an alarm and error code. I had an issue with mine. There is a small vacuum hose that connects to the waste gate undernieth it. The hose had slipped off several times and I finally found a small enough screw clamp to hold it on. If that hose disconnects then you will loose a good 500 RPM's .
See if you can locate that hose and make sure it is connected. It connects on the bottom of the turbo.

 

[Mystical]

I figured we would get an alarm as well. I will check the hose.

 

[HRowland]

Per the BY2 service manual the 6BY2-260 boost pressure should be 29.0-31.9 PSI at WOT. To connect a mechanical gauge it suggests removing the charge air temp sensor from the charge air intake pipe and using a M12 X 1.5 adapter. From the picture this is at the top of the engine near the connection of the intake pipe and intake manifold.

For others the 4BY2-150 boost pressure at WOT is 23.2-25.4 PSI and the 4BY2-180 is 23.9-26.1 PSI.

Howard