Yanmar ViO55-6A Loses Power Under Load
Fault Codes & Symptoms
Engine bogs down and loses power when tracking under load. Related symptoms: - Engine runs rough under load. - Intermittent low power issue. - Engine difficult to restart after running out of fuel. The machine runs fine on flat ground, but when I try to climb a steep grade or track out of a trench, the engine loses power and almost stalls.
Confirmed cause: A small, partially cracked o-ring on the suction-side fitting of the primary fuel filter housing. The crack was not visible on initial inspection and only allowed air to be pulled in when the fuel pump was demanding a high volume of fuel. Repair performed: Replaced the o-ring on the primary fuel filter housing suction fitting. Verification: Installed the sight glass again and performed a second hydraulic stall test. No air bubbles were present in the fuel. The machine was then tested by tracking up a steep grade and it was able to maintain engine power and RPM without issue. Technician notes: - Air leaks on the suction side of a fuel system often don't leak fuel externally, as they are under a vacuum. They only show up as a performance problem under high demand. - A clear sight glass is an essential tool for diagnosing fuel system air intrusion. - Always replace o-rings and seals when servicing fuel filter housings.