The starting and operating efficiency of the engine depends largely on high-quality and correct fuel injection. Loss of fuel-system sealing, air ingress and clogged lines lead to reduced power, uneven running and difficult starting. The procedures below cover checking for and removing air, checking inlet restriction, and checking fuel supply and return for the Cummins ISF3.8 engine.
Checking for air in the fuel system
After replacing fuel pipes, fuel filters, the high-pressure fuel pump and injectors, air remains in the fuel system.
This causes poor engine starting, uneven idling, misfires, reduced power and uneven fuel delivery.
A few air bubbles found in the fuel line during checking are normal.
Numerous small bubbles (foam) indicate air ingress into the system, high inlet restriction causing cavitation, or insufficient filling of the fuel system.
If fuel inlet restriction is within specification, identify the source of air ingress among the following components:
- fuel suction lines;
- fuel lines of auxiliary equipment;
- fuel filter inlet unions;
- fuel tank suction tubes.
- Disconnect the fuel supply line from the fuel supply pump.
- Install the pressure gauge adapter (No. 4918462) between the ECM cooler plate and the fuel pump.
Fig. 1. Installing the pressure gauge adapter. - Start the engine.
- Raise and lower the idle speed several times to remove air.
- Allow the engine to idle while watching the fuel flow from the transparent hose.
Fig. 2. Watching the fuel flow.
If air continues to bubble in the hose for several minutes, air is entering the fuel system.
- Check the fuel system for damaged fuel lines and loose connections.
- Replace damaged lines and tighten loose connections.
Removing air
Models with a manual priming pump
Note: before starting the engine, there is no need to remove air from the high-pressure circuit of the fuel system. The system fills with fuel while the engine is cranked by the starter.
- If there is no fuel in the filter and lines, operate the manual priming pump 120 times before starting the engine.
- If there is fuel in the filter and lines, operate the manual priming pump 40 times before starting the engine.
Start the engine and check for leaks.
Checking the fuel inlet line for clogging
- Unscrew the banjo bolt from the fuel pump inlet.
Fig. 3. Unscrewing the banjo bolt from the fuel pump inlet. - Measuring fuel inlet restriction requires the use of the pressure gauge installation tool kit (No. 4918462).
- Install the M12 (male thread) × M12 (female thread) adapter included in the pressure gauge installation tool kit on the fuel inlet union.
- Install the bolt included in the pressure gauge installation tool kit on the fuel union and the adapter.
- Install a vacuum gauge with a measuring range of at least 0–508 mm Hg on the pressure gauge adapter.
Fig. 4. Installing the vacuum gauge on the adapter. - Set a high idle speed and measure fuel inlet restriction. Permissible fuel inlet restriction at high idle: 50.8 kPa.
Fig. 5. Measuring fuel inlet restriction. - If fuel inlet restriction is high, check that the dimensions of the auxiliary equipment fuel lines from the tank are within specification and that they are free from kinks and clogging.
Fig. 6. Checking the auxiliary equipment fuel lines. - Make sure the coarse and fine filters are not clogged and the non-return valves are not faulty.
- Check the fuel filter restriction.
- Check that there is no debris (plastic bags, paper, etc.) in the auxiliary equipment fuel tanks that could periodically block the suction lines.
- Check the auxiliary equipment fuel lines for internal damage, such as lining defects on the pipe walls, that could intermittently block fuel flow.
- If the problem is intermittent and its causes cannot be identified during engine servicing, install the Fleetguard fuel filter restriction indicator (No. 3892576) in the connector between the fuel lines and the engine.
Fig. 7. Fleetguard fuel filter restriction indicator.
The indicator will show the maximum pressure drop in mm Hg. When the problem occurs, the fuel filter restriction indicator will show the presence of its cause on the faulty equipment side.
Removing and installing the fuel return line
- Mark the position of all P-clips and the routing of the fuel return lines so that they can be installed in the places from which they were removed.
Fig. 8. Marking the position of the P-clips. - Unscrew all cap screws from the P-clips securing the fuel return lines.
Fig. 9. Unscrewing the cap screws from the P-clips. - Disconnect the fuel return line connector from the injector fuel return union at the rear of the cylinder head.
Fig. 10. Disconnecting the fuel return line connector. - Disconnect the fuel return line from the high-pressure fuel pump.
Fig. 11. Disconnecting the fuel return line from the high-pressure fuel pump. - Disconnect the fuel return line from the return manifold.
Installing the fuel return line
- Route the fuel return line in accordance with the marks made during removal.
- Connect the fuel return line connector to the injector fuel return union at the rear of the cylinder head.
- Connect the fuel return lines to the manifold.
- Connect the fuel return line to the high-pressure fuel pump.
Checking fuel supply
Note: clean all connections before dismantling. Ingress of dirt can damage the fuel system.
- Disconnect the high-pressure fuel pipe from the high-pressure fuel pump to the fuel rail.
Fig. 12. Disconnecting the fuel pipe from the high-pressure fuel pump. - Connect a clean hose to the high-pressure fuel pump outlet union.
- Place the other end of the hose in a measuring container.
- Crank the starter for 30 seconds and measure the high-pressure fuel pump delivery.
Rated delivery:
- at 125 rpm: 45 ml;
- at 150 rpm: 53 ml.
If the fuel delivery differs from the specified value, carry out the following checks:
- make sure there is no air in the fuel system;
- make sure the fuel inlet line is not clogged;
- check the fuel metering valve.
If, after replacing the metering valve, the fuel delivery is below the minimum value, replace the high-pressure fuel pump.
Connect the metering valve connector and clear all fault codes using a scanner.
Checking fuel return flow
Warning! The high-pressure fuel pump outlet fuel pipes contain fuel at very high pressure. To avoid injury and equipment damage, do not loosen any connections with the engine running.
If the engine does not start, check the fuel flow through the return line. High fuel flow through the return line may indicate a faulty return-line bypass valve or high-pressure fuel pump.
- Disconnect the fuel return quick-release coupling from the high-pressure fuel pump.
- To reduce the risk of fuel leakage from the combined fuel rail and injector return line, the flexible return line must be closed with a fuel line plug (No. 24918464).
Fig. 13. Installing the plug on the fuel return line. - Fit the return hose with quick-release coupling (No. 4918434) onto the fuel pump return union.
- Place the end of the fuel return hose in a measuring container.
- Start the engine and let it idle.
- Measure the fuel flow for 30 seconds after it starts entering the measuring container.
Maximum flow through the return line:
- at 800 rpm: 317 ml.
If the flow is above the maximum value, check the fuel system for air and make sure the fuel inlet line is not clogged. If the results of these checks meet the requirements, replace the high-pressure fuel pump.
The materials presented cover checking the fuel system of the Cummins ISF3.8 engine: checking for air in the fuel system, removing air on models with a manual priming pump, checking the fuel inlet line for clogging, removing and installing the fuel return line, checking fuel supply, and checking fuel return flow. The material is intended for diagnosis, troubleshooting and repair of the injection system.
