The diesel engine is not reaching its maximum rated speed under load. Less engine speed means less compressor output. The compressor side is fine -- the engine just cannot deliver full power. Could be fuel, air, or speed control issues.
What you'll see
If the engine is at full RPM but pressure is still low, the problem is on the compressor side (dirty inlet filter, intake valve, air leaks) or the demand exceeds capacity. If the engine is struggling with black smoke, check the engine air filter and fuel system first. See: Air Leaks, Intake Valve Not Fully Opening.
How to diagnose
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Check engine RPM under load
On machines with electronically controlled engines, the controller usually displays the RPM. On mechanical engines, use a tachometer. Compare the actual RPM under full load to the rated maximum speed on the engine nameplate. If the engine is running slower than rated, investigate why.Result: RPM at or near rated maximum = engine speed OK, problem is elsewhere. RPM significantly below rated = engine speed issue. -
Check the speed control actuator
On mechanically governed engines, the speed actuator (pneumatic or electronic) controls the fuel injection pump speed setting. Check that the actuator is moving the fuel rack to the maximum position when the compressor is loaded. The mechanical stop (maximum speed screw) may be set too low, or the linkage may be binding. Try moving the fuel lever by hand (engine off) to check for free movement.Result: Lever moves freely to maximum = actuator or control air issue. Lever binds = lubricate or repair linkage. -
Check for engine power loss
If the speed actuator is commanding maximum but the engine cannot reach it, the engine has lost power. Check the most common causes: dirty fuel filters, dirty engine air filter, air in fuel system, restricted exhaust. Also check if the compressor is creating excessive internal resistance (dirty separator element, too-high pressure setting) that overloads the engine.Result: Dirty filters = replace. Clean filters and engine still underpowered = deeper engine issue.
How to fix it
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Adjust the maximum speed setting
If the speed control is simply set too low, adjust it. On mechanical engines, there is usually a maximum speed adjustment screw on the fuel injection pump or on the speed actuator linkage. Adjust while monitoring the RPM -- do not exceed the rated maximum. On electronic engines, the maximum speed is set in the controller software.
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Fix engine power issues
Replace dirty fuel and air filters. Bleed air from the fuel system. Check exhaust for restrictions. Replace the oil separator element if it has excessive pressure drop. Lower the pressure setpoint if it was set higher than the rated maximum. These maintenance items, taken together, often restore full engine performance.
Operators sometimes set the compressor pressure higher than the rated maximum, thinking they will get more air. This just overloads the engine -- it cannot reach full speed because it is working against excessive pressure. The result is LESS output, not more. Always operate within the rated pressure range on the nameplate. Also: do not adjust the engine speed beyond the nameplate rating to try to get more air -- this overspeeds the compressor element too, which is dangerous.
Tachometer (if engine does not have a display). Fuel filters. Engine air filter. Screwdriver for speed adjustment screw. Compressor nameplate data for rated RPM and pressure.
Do not adjust engine maximum speed beyond the rated RPM shown on the nameplate. Over-speeding the engine causes catastrophic damage. Most engines have a mechanical stop that prevents overspeeding -- do not tamper with it.
This issue can also cause
- Engine Won't Rev Up Under Load Engine stays at idle RPM when compressor should be loading—speed actuator or control air issue.