729945-51390 Common Rail Pump – Flow-Curve Sourcing, Yanmar Platform, Duty-Cycle Data

Rated pressure is one number. The flow curve is the map. 729945-51390 is specified by how delivered volume changes across pump speed and control duty - not by a single peak figure. A pump can hit 2,000 bar on a test bench and still mismatch an injector whose demand curve rises faster at low speed. The selection question is whether the supply curve and the demand curve cross in the right operating window.

729945-51390 targets Yanmar common rail diesel platforms in industrial, marine, generator and agricultural duty. Instead of checking only the part number, compare three curve-relevant identifiers: OEM pump label, engine rating plate and ECU calibration ID. Two engines sharing a block family can carry different injector flow classes, which shifts the demand curve. Confirm actuator connector keying, flange orientation and metering-valve part number against the original. Interchange charts filter candidates; they do not confirm curve compatibility.

The metering valve does not operate at a fixed position. It sweeps a duty band as the ECU chases the pressure setpoint. A pump whose flow curve sits above the engine's demand forces the valve toward the low-flow edge, where response becomes non-linear. A pump whose curve sits below demand pushes the valve toward the high-flow edge, raising return flow and fuel temperature. Both conditions show up as pressure deviation the ECU tries to mask with injection timing and EGR changes. The visible symptom may be rough idle, torque limiting or elevated fuel temperature - not a clean pump fault code.

Low-pressure starvation flattens the supply curve at high speed, mimicking a worn pump. Return-line restriction raises housing pressure and distorts valve response near the low-flow edge. Air ingress compresses under pressure and adds oscillation that looks like curve instability. Each is cheaper to rule out than a pump swap. Check lift-pump volume at operating temperature, return restriction and suction-side sealing before ordering.

Before fitting 729945-51390, record the original pump's behaviour: pressure rise time at cranking, steady-state deviation at idle and under load, and return-flow trend. After installation, repeat the same measurements. A new pump that matches the old deviation pattern suggests the fault was upstream. Complete any ECU pump-learning routine before load testing. Verify high-pressure line torque and prime the circuit per OEM procedure - trapped air adds oscillation that reads as curve error.

Why does fuel temperature rise after pump replacement?
A flow curve above engine demand pushes the metering valve toward the low-flow edge, increasing return flow and heat. Check duty band and return temperature against the original pump.

Can a pump pass a pressure test and still mismatch the engine?
Yes. Peak pressure is a single point. Curve shape across speed and duty determines whether the ECU can control pressure without compensation strategies.

What causes rough idle only when the engine is warm?
Warm fuel shifts viscosity and alters valve response near the low-flow edge. If the pump curve is marginal, the duty band widens and idle stability drops. Compare hot-idle duty data before replacing the pump.

How do I tell curve mismatch from a worn pump?
Worn pumps lose volume at all speeds, especially cranking. Curve mismatch shows normal cranking but abnormal steady-state duty. Record both before deciding.

Is a higher-flow pump ever a safe upgrade?
Only if the ECU calibration and injector flow class support it. A higher curve forces the metering valve into a correction range it was not mapped for, producing torque and emission side effects.

What data should a rebuilt 729945-51390 supplier provide?
Delivered volume at three speeds, steady-state duty band, return-flow stability and internal leakage at operating temperature. Pressure-only sheets are insufficient.