Guinea-Bissau Mechanical Diesel Engine Accelerates Normally Without Load but Lacks Fueling Under Boost as Injection-Pump Boost Compensator and Pressure-Reference Line Are Investigated

The Engine Responded Normally Until Real Load Was Applied

A representative commercial diesel case in Guinea-Bissau involved a mechanically injected turbocharged engine that could accelerate reasonably well while stationary but became noticeably weak when the vehicle was placed under real load.

Because the injectors had already shown acceptable opening pressure and basic spray behavior, the workshop investigated whether the injection pump was actually increasing permitted fuel delivery as boost pressure developed.

The focus shifted to the injection-pump boost compensator, sometimes referred to as an aneroid or boost-pressure compensating device depending on pump design.

Why No-Load Acceleration Can Be Misleading

An engine revving freely in the workshop does not require the same torque as a loaded commercial vehicle.

Under road load, the engine requires:

• more air;
• more fuel;
• increased turbocharger output;
• and an injection-pump response appropriate to boost pressure.

A mechanical pump can therefore appear acceptable without load while failing to provide the expected fueling response when boost develops.

The Boost Reference Became the First Check

On pumps equipped with a boost compensator, manifold or charge pressure may be routed to the pump through a reference hose.

The workshop inspected:

• pressure-reference hose;
• loose connections;
• splits or restrictions;
• contamination;
• and whether boost pressure actually reached the compensator.

A perfectly functional compensator cannot respond if its reference line does not transmit pressure correctly.

The Compensator Mechanism Was Evaluated Separately

Depending on pump design, the boost compensator may contain:

• diaphragm;
• spring;
• control pin;
• lever mechanism;
• or related fuel-limiting components.

If the diaphragm is damaged or the mechanism sticks, pump fueling may remain limited even though the turbocharger is producing boost.

No universal adjustment was applied because these settings are pump-specific.

Why Injector Replacement Would Not Correct the Problem

The injectors can only deliver the fuel supplied by the injection pump.

If the pump remains in a reduced-fueling position under boost, installing different injectors does not correct the missing fuel command.

The diagnostic chain became:

No-load acceleration acceptable → Loaded power weak → Boost confirmed → Pressure reference checked → Compensator movement evaluated → Pump delivery reassessed

Smoke Behavior Added Supporting Evidence

The workshop also considered whether the engine produced:

• unusually little smoke despite heavy load;
• excessive smoke;
• or normal exhaust behavior.

This information helped compare air supply with fuel quantity but was not treated as a stand-alone diagnosis.

Case Insight

The Guinea-Bissau case demonstrates why mechanical diesel diagnosis should include load-dependent pump control.

The useful distinction was:

Can the injector deliver fuel?
versus
Is the injection pump allowing enough fuel when boost rises?

These are different questions.

FAQ

Can a mechanical diesel engine run well without load but still have a pump-control problem?

Yes. A boost-compensation fault may become noticeable only when fuel demand rises under load.

Do all mechanical injection pumps use a boost compensator?

No. The system must be confirmed from the specific pump and engine design.