Working on an NTA 855 C2, CPL is 1383. It is on a 300 kw genset 1800 RPM fixed with a simple electronic control to the PT pump. It has a low grade hunt at no load and it won't take much more than 50-100 kw load before it falls on its face. The supply/return lines were old and cracked and replaced but we put clear on the return and it is returning a whole lot of air, way more air than fuel. The supply side was pressured with air to check for leaks since it comes out of a convault with a foot valve so anything up to the PT pump is negative pressure. It runs about 4" Hg vacuum running and will hold 2-3" after having stopped for half an hour or more. Last thing I did was run it directly out of a fuel can, no change in behavior. One interesting thing is that the air in the clear return seems to increase when it is on the upside of its hunt. Are injectors capable of returning compression to the fuel? Or maybe it is something in the PT pump sucking air. Hopefully someone with more specific experience on this old platform can offer advice. Thanks
Platform?
Very simple test---loosen the fuel supply steel line coming out of the top of fuel shut off solenoid {arrow in image} just a hair {barely loose}. Yours probably has a dual governed PT pump with load sensing gov on top, still the same procedure. Start it running at idle fuel should just be enough to form a crown around top of fitting at steel line. If air is getting into system you will see very small bubbles {foam} if it doesn't and it's clear it means air is getting in after pump. Yes air can be introduced at injectors. *
Thanks for the picture. This one has a tap off the pressure port that is plugged on yours that goes to the STC. They teed a gauge off there which is inaccurate and leaking so we have it plugged. So a handy pipe plug. I just read to check for air to install a needle valve and a clear return hose, will do that next. Need to pick up a needle valve.
The reason-- checking for air the way I described, tells you whether it's a before the pump including pump issue or the air is induced after pump. It cuts down chasing ones tail. Because it could possibly be pulling air past throttle shaft. I say possibly. * Unless that gauge was liquid filled it would be very inaccurate at measuring rail psi. * Being STC depending hours, those injectors are probably worn because those might have been set using the old method using plunger locking tool. Which has been up-dated to X amount of inch lbs--as much as 90 inch lbs. But I don't have actual data for that CPL. That you will have to find out from the big c. * One thing to always check with STC is engine oil in the sensing fuel line running from STC control box to top of pump, remove line a control box to see if fuel is dark. If so control box needs attention. * The other issue is the injector oil control lines leaking on top of heads, inside rocker boxes. Common.