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The fuel pump is the part that makes pressure. Injectors only meter what that pressure allows. Petrol engines and diesels both have a low-pressure supply from the tank. Direct-injection petrol engines and common-rail diesels add a high-pressure pump, mechanically driven, because the injectors need tens or hundreds of bar rather than the few bar an in-tank pump can make. The faults are loss of supply, air on the suction side, and a high-pressure pump that cannot hold the rail. The injectors that then suffer are on injector faults. How the controller’s quantity command relates to the fuel that arrives is on fuelling faults.
Common petrol engine problems and common diesel engine problems both link here.
Two circuits, not one pump
Bosch’s current gasoline direct-injection layout delivers fuel to the high-pressure pump at up to 6 bar. That pump then supplies the rail at up to 350 bar. Bosch’s aftermarket description of the same split puts the low-pressure circuit at about 4 bar and the high-pressure circuit at about 40 to 350 bar. Those are ranges for systems Bosch is describing, not the specification of every engine. An early Bosch MED 7 system, in Volkswagen’s SSP 253, ran the in-tank pump at about 3 bar in normal operation and up to 5.8 bar on a hot start, with 50 to 100 bar in the rail. A gauge reading means nothing until it is compared with the procedure for that pressure sensor and that relief valve.
A common-rail diesel makes the same split. A lift pump, in the tank or on the engine, feeds the high-pressure pump through a filter. The high-pressure pump fills the rail. The injectors take from the rail. Older distributor-pump diesels, and unit-injector engines, do not have this rail. A test written for a common-rail pressure sensor does not transfer to them. EDC15 injection systems records the layouts that were actually used with that controller.
Low-pressure supply
The in-tank pump on a petrol car is cooled and lubricated by the fuel around it. Running the tank dry, or leaving it permanently low, lets it run hot. Ageing pumps still spin and still make idle pressure, then fall away when the injectors demand volume. The symptom is a hesitation or a lean misfire under load, often worse when the fuel is hot and already close to vapour. A filter that is blocked imitates a weak pump. Measure pressure and, where the procedure asks for it, volume. Pressure alone, with no flow, is a restriction.
On a diesel the low-pressure side fails in two additional ways. Air leaks on the suction side, at a perished hose, a loose filter cap or a cracked pickup, let the lift pump draw air. The high-pressure pump then loses its charge. A transparent section in the suction line shows the bubbles; they are the leak, not a failed high-pressure pump. A blocked filter or a failing lift pump shows as rail pressure that is adequate at idle and collapses when fuel quantity rises. Water in the filter bowl is a separate finding. It is drained and the reason it arrived is fixed before a new high-pressure pump is asked to run on it.
A petrol pump that is loud is not automatically failed, and a quiet pump is not automatically healthy. Current draw, pressure under load and a scan-tool pressure against a mechanical gauge are the tests. On a returnless petrol system the pump is controlled, so a relay-and-fuse check from an older diagram can miss a module that has simply been told to run slowly.
High-pressure pumps
The petrol high-pressure pump is cam-driven. In the MED 7 example the camshaft turns an eccentric, the piston draws fuel from the low-pressure side on the downstroke, and it delivers to the rail on the upstroke. A pressure-control valve spills the excess. Later pumps move that control around, some metering on the inlet so that the pump only compresses the fuel the rail needs, but the failure modes stay recognisable:
The low-pressure supply is too weak, so the high-pressure pump cannot fill. Replacing the high-pressure pump leaves the fault in the tank.
The pump’s own valve or piston leaks, so rail pressure does not follow the setpoint once injection quantity rises. Misfires under load and a lean condition on a direct-injection engine follow.
The pressure sensor and the control valve disagree. Compare the sensor with a gauge using the high-pressure procedure. Do not use a low-pressure petrol gauge on a high-pressure rail.
A diesel high-pressure pump fails by not building pressure during cranking, by not holding it, or by wearing internally until debris reaches the rail. The last of those is the expensive pattern. Metal from a worn cam, roller or piston in the pump is carried into the rail and the injectors, and a new pump fitted to a contaminated rail fails again. Any diesel pump replacement that follows metal in the filter or in the rail includes cleaning or replacing the rail, the lines and the injectors the debris has already reached. That is a contamination repair, not a pump-only repair. Which pumps shed metal, and at what mileage, is an application question. It is not a property of every common-rail engine.
Cranking pressure is compared with the figure in that system’s test, not with a universal bar number. Measuring at the pump outlet, with the equipment that procedure specifies, separates the pump from a rail-pressure sensor that is wrong. The injector return test is the other half, because an injector that is emptying the rail looks like a pump that cannot fill it.
Separating the pump from the injectors
A pump supplies every cylinder. An injector usually spoils one. If every cylinder goes lean together, or a diesel loses rail pressure as soon as more than one injector is commanded, the supply is the first suspect. If one cylinder is down and the rail pressure holds, the injector, the spark or the compression of that cylinder is the first suspect. Excess return from one diesel injector can pull the rail down enough to look like a dead pump. The return-quantity comparison on the injector page is the test that prevents a pump being replaced for an injector that is emptying it.
High-pressure systems stay at pressure after the engine stops. The Vauxhall common-rail return procedure used above tells the technician to wait at least 30 seconds after stopping the engine before opening a joint. Follow the depressurising procedure for the vehicle in front of you, which may demand longer. Diesel common-rail pressures will inject through skin. Petrol direct-injection rails are lower and still not a job for an open fitting and a rag.
After the repair
Prime the low-pressure side so the high-pressure pump is not cranked dry. Clear the adaptations that were compensating for low pressure, where the controller has them. Then repeat the test that failed: a hot restart, or a loaded pull, with rail pressure recorded against the setpoint. Idle pressure after a new pump does not prove the pump can deliver volume.
Sources
Bosch Mobility, gasoline direct injection. Supply to the high-pressure pump at up to 6 bar, and rail pressure up to 350 bar.
Bosch Aftermarket, gasoline direct injection. Low-pressure circuit about 4 bar and high-pressure circuit about 40 to 350 bar, with the in-tank pump feeding the mechanical pump.
Volkswagen SSP 253, MED 7. One early high-pressure pump: cam-driven pistons, about 3 to 5.8 bar on the low side, 50 to 100 bar in the rail.
Vauxhall Astra H workshop procedure, Bosch solenoid injector return quantity. The depressurising wait before opening a common-rail system, and a return test that explains a rail that will not stay full.
EDC15 injection systems and timing feedback on this wiki, for distributor-pump, unit-injector and common-rail layouts that do not share one test.
Open each version to inspect formatting, tables and photographs.
Revision 2: Fuel pump faults
The fuel pump is the part that makes pressure. Injectors only meter what that pressure allows. Petrol engines and diesels both have a low-pressure supply from the tank. Direct-injection petrol engines and common-rail diesels add a high-pressure pump, mechanically driven, because the injectors need tens or hundreds of bar rather than the few bar an in-tank pump can make. The faults are loss of supply, air on the suction side, and a high-pressure pump that cannot hold the rail. The injectors that then suffer are on injector faults. How the controller’s quantity command relates to the fuel that arrives is on fuelling faults.
Common petrol engine problems and common diesel engine problems both link here.
Two circuits, not one pump
Bosch’s current gasoline direct-injection layout delivers fuel to the high-pressure pump at up to 6 bar. That pump then supplies the rail at up to 350 bar. Bosch’s aftermarket description of the same split puts the low-pressure circuit at about 4 bar and the high-pressure circuit at about 40 to 350 bar. Those are ranges for systems Bosch is describing, not the specification of every engine. An early Bosch MED 7 system, in Volkswagen’s SSP 253, ran the in-tank pump at about 3 bar in normal operation and up to 5.8 bar on a hot start, with 50 to 100 bar in the rail. A gauge reading means nothing until it is compared with the procedure for that pressure sensor and that relief valve.
A common-rail diesel makes the same split. A lift pump, in the tank or on the engine, feeds the high-pressure pump through a filter. The high-pressure pump fills the rail. The injectors take from the rail. Older distributor-pump diesels, and unit-injector engines, do not have this rail. A test written for a common-rail pressure sensor does not transfer to them. EDC15 injection systems records the layouts that were actually used with that controller.
Low-pressure supply
The in-tank pump on a petrol car is cooled and lubricated by the fuel around it. Running the tank dry, or leaving it permanently low, lets it run hot. Ageing pumps still spin and still make idle pressure, then fall away when the injectors demand volume. The symptom is a hesitation or a lean misfire under load, often worse when the fuel is hot and already close to vapour. A filter that is blocked imitates a weak pump. Measure pressure and, where the procedure asks for it, volume. Pressure alone, with no flow, is a restriction.
On a diesel the low-pressure side fails in two additional ways. Air leaks on the suction side, at a perished hose, a loose filter cap or a cracked pickup, let the lift pump draw air. The high-pressure pump then loses its charge. A transparent section in the suction line shows the bubbles; they are the leak, not a failed high-pressure pump. A blocked filter or a failing lift pump shows as rail pressure that is adequate at idle and collapses when fuel quantity rises. Water in the filter bowl is a separate finding. It is drained and the reason it arrived is fixed before a new high-pressure pump is asked to run on it.
A petrol pump that is loud is not automatically failed, and a quiet pump is not automatically healthy. Current draw, pressure under load and a scan-tool pressure against a mechanical gauge are the tests. On a returnless petrol system the pump is controlled, so a relay-and-fuse check from an older diagram can miss a module that has simply been told to run slowly.
High-pressure pumps
The petrol high-pressure pump is cam-driven. In the MED 7 example the camshaft turns an eccentric, the piston draws fuel from the low-pressure side on the downstroke, and it delivers to the rail on the upstroke. A pressure-control valve spills the excess. Later pumps move that control around, some metering on the inlet so that the pump only compresses the fuel the rail needs, but the failure modes stay recognisable:
- The low-pressure supply is too weak, so the high-pressure pump cannot fill. Replacing the high-pressure pump leaves the fault in the tank.
- The pump’s own valve or piston leaks, so rail pressure does not follow the setpoint once injection quantity rises. Misfires under load and a lean condition on a direct-injection engine follow.
- The pressure sensor and the control valve disagree. Compare the sensor with a gauge using the high-pressure procedure. Do not use a low-pressure petrol gauge on a high-pressure rail.
A diesel high-pressure pump fails by not building pressure during cranking, by not holding it, or by wearing internally until debris reaches the rail. The last of those is the expensive pattern. Metal from a worn cam, roller or piston in the pump is carried into the rail and the injectors, and a new pump fitted to a contaminated rail fails again. Any diesel pump replacement that follows metal in the filter or in the rail includes cleaning or replacing the rail, the lines and the injectors the debris has already reached. That is a contamination repair, not a pump-only repair. Which pumps shed metal, and at what mileage, is an application question. It is not a property of every common-rail engine.
Cranking pressure is compared with the figure in that system’s test, not with a universal bar number. Measuring at the pump outlet, with the equipment that procedure specifies, separates the pump from a rail-pressure sensor that is wrong. The injector return test is the other half, because an injector that is emptying the rail looks like a pump that cannot fill it.
Separating the pump from the injectors
A pump supplies every cylinder. An injector usually spoils one. If every cylinder goes lean together, or a diesel loses rail pressure as soon as more than one injector is commanded, the supply is the first suspect. If one cylinder is down and the rail pressure holds, the injector, the spark or the compression of that cylinder is the first suspect. Excess return from one diesel injector can pull the rail down enough to look like a dead pump. The return-quantity comparison on the injector page is the test that prevents a pump being replaced for an injector that is emptying it.
High-pressure systems stay at pressure after the engine stops. The Vauxhall common-rail return procedure used above tells the technician to wait at least 30 seconds after stopping the engine before opening a joint. Follow the depressurising procedure for the vehicle in front of you, which may demand longer. Diesel common-rail pressures will inject through skin. Petrol direct-injection rails are lower and still not a job for an open fitting and a rag.
After the repair
Prime the low-pressure side so the high-pressure pump is not cranked dry. Clear the adaptations that were compensating for low pressure, where the controller has them. Then repeat the test that failed: a hot restart, or a loaded pull, with rail pressure recorded against the setpoint. Idle pressure after a new pump does not prove the pump can deliver volume.
Sources
- Bosch Mobility, gasoline direct injection. Supply to the high-pressure pump at up to 6 bar, and rail pressure up to 350 bar.
- Bosch Aftermarket, gasoline direct injection. Low-pressure circuit about 4 bar and high-pressure circuit about 40 to 350 bar, with the in-tank pump feeding the mechanical pump.
- Volkswagen SSP 253, MED 7. One early high-pressure pump: cam-driven pistons, about 3 to 5.8 bar on the low side, 50 to 100 bar in the rail.
- Vauxhall Astra H workshop procedure, Bosch solenoid injector return quantity. The depressurising wait before opening a common-rail system, and a return test that explains a rail that will not stay full.
- EDC15 injection systems and timing feedback on this wiki, for distributor-pump, unit-injector and common-rail layouts that do not share one test.
Revision 2: Fuel pump faults
The fuel pump is the part that makes pressure. Injectors only meter what that pressure allows. Petrol engines and diesels both have a low-pressure supply from the tank. Direct-injection petrol engines and common-rail diesels add a high-pressure pump, mechanically driven, because the injectors need tens or hundreds of bar rather than the few bar an in-tank pump can make. The faults are loss of supply, air on the suction side, and a high-pressure pump that cannot hold the rail. The injectors that then suffer are on injector faults. How the controller’s quantity command relates to the fuel that arrives is on fuelling faults.
Common petrol engine problems and common diesel engine problems both link here.
Two circuits, not one pump
Bosch’s current gasoline direct-injection layout delivers fuel to the high-pressure pump at up to 6 bar. That pump then supplies the rail at up to 350 bar. Bosch’s aftermarket description of the same split puts the low-pressure circuit at about 4 bar and the high-pressure circuit at about 40 to 350 bar. Those are ranges for systems Bosch is describing, not the specification of every engine. An early Bosch MED 7 system, in Volkswagen’s SSP 253, ran the in-tank pump at about 3 bar in normal operation and up to 5.8 bar on a hot start, with 50 to 100 bar in the rail. A gauge reading means nothing until it is compared with the procedure for that pressure sensor and that relief valve.
A common-rail diesel makes the same split. A lift pump, in the tank or on the engine, feeds the high-pressure pump through a filter. The high-pressure pump fills the rail. The injectors take from the rail. Older distributor-pump diesels, and unit-injector engines, do not have this rail. A test written for a common-rail pressure sensor does not transfer to them. EDC15 injection systems records the layouts that were actually used with that controller.
Low-pressure supply
The in-tank pump on a petrol car is cooled and lubricated by the fuel around it. Running the tank dry, or leaving it permanently low, lets it run hot. Ageing pumps still spin and still make idle pressure, then fall away when the injectors demand volume. The symptom is a hesitation or a lean misfire under load, often worse when the fuel is hot and already close to vapour. A filter that is blocked imitates a weak pump. Measure pressure and, where the procedure asks for it, volume. Pressure alone, with no flow, is a restriction.
On a diesel the low-pressure side fails in two additional ways. Air leaks on the suction side, at a perished hose, a loose filter cap or a cracked pickup, let the lift pump draw air. The high-pressure pump then loses its charge. A transparent section in the suction line shows the bubbles; they are the leak, not a failed high-pressure pump. A blocked filter or a failing lift pump shows as rail pressure that is adequate at idle and collapses when fuel quantity rises. Water in the filter bowl is a separate finding. It is drained and the reason it arrived is fixed before a new high-pressure pump is asked to run on it.
A petrol pump that is loud is not automatically failed, and a quiet pump is not automatically healthy. Current draw, pressure under load and a scan-tool pressure against a mechanical gauge are the tests. On a returnless petrol system the pump is controlled, so a relay-and-fuse check from an older diagram can miss a module that has simply been told to run slowly.
High-pressure pumps
The petrol high-pressure pump is cam-driven. In the MED 7 example the camshaft turns an eccentric, the piston draws fuel from the low-pressure side on the downstroke, and it delivers to the rail on the upstroke. A pressure-control valve spills the excess. Later pumps move that control around, some metering on the inlet so that the pump only compresses the fuel the rail needs, but the failure modes stay recognisable:
- The low-pressure supply is too weak, so the high-pressure pump cannot fill. Replacing the high-pressure pump leaves the fault in the tank.
- The pump’s own valve or piston leaks, so rail pressure does not follow the setpoint once injection quantity rises. Misfires under load and a lean condition on a direct-injection engine follow.
- The pressure sensor and the control valve disagree. Compare the sensor with a gauge using the high-pressure procedure. Do not use a low-pressure petrol gauge on a high-pressure rail.
A diesel high-pressure pump fails by not building pressure during cranking, by not holding it, or by wearing internally until debris reaches the rail. The last of those is the expensive pattern. Metal from a worn cam, roller or piston in the pump is carried into the rail and the injectors, and a new pump fitted to a contaminated rail fails again. Any diesel pump replacement that follows metal in the filter or in the rail includes cleaning or replacing the rail, the lines and the injectors the debris has already reached. That is a contamination repair, not a pump-only repair. Which pumps shed metal, and at what mileage, is an application question. It is not a property of every common-rail engine.
Cranking pressure is compared with the figure in that system’s test, not with a universal bar number. Measuring at the pump outlet, with the equipment that procedure specifies, separates the pump from a rail-pressure sensor that is wrong. The injector return test is the other half, because an injector that is emptying the rail looks like a pump that cannot fill it.
Separating the pump from the injectors
A pump supplies every cylinder. An injector usually spoils one. If every cylinder goes lean together, or a diesel loses rail pressure as soon as more than one injector is commanded, the supply is the first suspect. If one cylinder is down and the rail pressure holds, the injector, the spark or the compression of that cylinder is the first suspect. Excess return from one diesel injector can pull the rail down enough to look like a dead pump. The return-quantity comparison on the injector page is the test that prevents a pump being replaced for an injector that is emptying it.
High-pressure systems stay at pressure after the engine stops. The Vauxhall common-rail return procedure used above tells the technician to wait at least 30 seconds after stopping the engine before opening a joint. Follow the depressurising procedure for the vehicle in front of you, which may demand longer. Diesel common-rail pressures will inject through skin. Petrol direct-injection rails are lower and still not a job for an open fitting and a rag.
After the repair
Prime the low-pressure side so the high-pressure pump is not cranked dry. Clear the adaptations that were compensating for low pressure, where the controller has them. Then repeat the test that failed: a hot restart, or a loaded pull, with rail pressure recorded against the setpoint. Idle pressure after a new pump does not prove the pump can deliver volume.
Sources
- Bosch Mobility, gasoline direct injection. Supply to the high-pressure pump at up to 6 bar, and rail pressure up to 350 bar.
- Bosch Aftermarket, gasoline direct injection. Low-pressure circuit about 4 bar and high-pressure circuit about 40 to 350 bar, with the in-tank pump feeding the mechanical pump.
- Volkswagen SSP 253, MED 7. One early high-pressure pump: cam-driven pistons, about 3 to 5.8 bar on the low side, 50 to 100 bar in the rail.
- Vauxhall Astra H workshop procedure, Bosch solenoid injector return quantity. The depressurising wait before opening a common-rail system, and a return test that explains a rail that will not stay full.
- EDC15 injection systems and timing feedback on this wiki, for distributor-pump, unit-injector and common-rail layouts that do not share one test.