The fuel system of a vehicle is a complex network of components, and when faults occur within it, troubleshooting can be a rather intricate and time-consuming process. There are numerous potential fault points in the fuel system, and the situation where there is no fuel or only a minimal amount of fuel coming from the low-pressure fuel circuit of a diesel engine can be caused by a multitude of factors. Trying to identify these faults can often lead one down various detours, wasting precious time. So, is there a relatively simple and quick method to check for fuel system faults? Today, let’s listen to truck-mounted crane driver Lao Li as he discusses the hand primer pump, a valuable tool in diagnosing fuel system issues.
I. Construction and principle of the hand primer pump
The manual oil pump is composed of four main parts: the pump body part, the handle part, the fuel tank, and the rear seat part.
(1) Pump body part: This is the core component of the oil pump. It houses a multitude of essential elements, including the high-pressure working chamber, low-pressure working chamber, two check valves, high-pressure valve (safety valve), low-pressure valve, unloading valve, two non-return valves, oil inlet, and more. The various holes and passages within the pump body are organically connected, forming a complex yet functional system.
The two check valves play a crucial role in preventing pressure oil from flowing back. They have the same specifications and functions. The low-pressure valve and high-pressure valve are responsible for pressure control. The pressures are precisely controlled at 1.5 MPa and 63 MPa respectively. The two steel balls located at the oil inlet serve as non-return valves. Once the work is completed, by loosening the unloading valve, the pressure oil can flow back to the oil storage pipe, thus completing the unloading process.
(2) Handle part: Primarily composed of the pressure rod and pressure handle. These components are connected to the pump body and plunger by two pins. Manual force is applied to the pressure rod, which in turn drives the plunger to reciprocate. This reciprocating motion generates the pressure necessary for pumping the fuel.
(3) Rear seat or fuel tank: There are oil filling and vent holes on the rear seat or fuel tank. Loosening the screws on these holes allows for venting and intake of air. It is important to open the vent valve when in use to ensure proper functioning of the pump.
When there is air in the oil passage of a diesel engine, such as in a newly launched vehicle or when the fuel in the fuel tank is completely exhausted, starting the engine can be a challenge. The diesel fuel pump can only operate when the engine is running. At such times, the hand primer pump becomes essential. It is used to pump the fuel from the fuel tank to the oil passage. Before starting the engine, it is necessary to loosen the exhaust bolt on the fuel injection pump until all bubbles are expelled.
For example, imagine a newly purchased truck that is being started for the first time. Due to air in the fuel system, the engine may not start immediately. By using the hand primer pump, the operator can manually pump the fuel through the system, expelling the air and enabling the engine to start smoothly.
The working principle of the hand primer pump is based on the reciprocating motion of the piston. When the piston moves upward, the volume in the pump chamber expands, creating a vacuum. Under the action of the spring, the one-way valve 2 is closed. At the same time, due to the atmospheric pressure outside, the one-way valve 1 is opened, allowing fuel to enter the pump chamber through the oil inlet pipe.
When the piston moves downward, the oil pressure in the pump chamber rises. This causes the one-way valve 1 to close and the one-way valve 2 to open. Fuel is then discharged from the oil outlet. This continuous cycle of upward and downward movement of the piston ensures the continuous delivery of fuel by the traveling pump.
For instance, think of a truck that has been sitting idle for a long time. As the engine is started, the hand primer pump begins to work, pumping fuel from the tank to the engine. The piston’s movement creates the necessary pressure to move the fuel through the system, ensuring a steady supply for the engine to run.
II. Maintenance skills
Now, let’s share some practical tips. By comparing the different feelings and changes of the hand primer pump compared to when there is normal fuel supply, it is possible to identify some fault causes relatively quickly.
Suction in the fuel circuit:
Loosen the vent screw of the fuel circuit. When pumping oil with the hand primer pump and gently pulling up the handle, if one slightly feels suction in the fuel circuit and when pressing the handle, only a small amount of fuel is discharged at the vent screw. This indicates that the suction resistance of the fuel transfer pump is too large.
Possible reasons for reduced fuel supply include a blocked vent hole on the fuel tank cap. When this happens, a negative pressure is created in the fuel tank, which can affect the fuel supply. Another reason could be the use of diesel with too high viscosity in winter, resulting in poor fluidity.
For example, if a truck is being used in extremely cold weather and the diesel fuel thickens, it can cause difficulties in the fuel transfer pump’s operation. The blocked vent hole on the fuel tank cap can further exacerbate the problem, reducing the flow of fuel to the engine.
Mixed gas in the discharged fuel:
When pumping oil, if the fuel overflowing from the vent screw is mixed with gas and one feels resistance when pressing the handle while only a small amount of fuel is discharged, it means that the filter element or fuel circuit between the fuel transfer pump and the high-pressure pump is blocked, reducing the fuel supply.
For instance, over time, the filter element can become clogged with dirt and debris, restricting the flow of fuel. This can lead to a mixture of gas and fuel being discharged from the vent screw, indicating a problem in the fuel system.
Vehicle won’t start. Pumping oil with the hand primer pump can start it, but it stalls immediately:
If the vehicle won’t start on its own but can be started by pumping oil with the hand primer pump, only to stall immediately after stopping, it is generally due to a pressure relief point in the fuel system. This results in insufficient pressure and inability to deliver fuel to the engine. In such cases, it is necessary to check where in the fuel circuit there is pressure relief.
For example, a truck that has been sitting for a long time may develop a leak in the fuel system, causing a pressure loss. When the engine is started with the help of the hand primer pump, the fuel pressure is temporarily restored. However, as soon as the pump stops working, the pressure drops again, causing the engine to stall.
Output fuel mixed with gas:
When pumping oil, if the fuel output from the drain screw is mixed with gas, there could be several reasons. The tubing connection between the drain screw and the oil pump may not be tightened properly. A damaged joint sealing gasket or damaged fuel circuit can also cause air leakage. Additionally, air can be sucked in from the threaded connection on the fuel transfer pump or from the gap between the piston of the hand primer pump and the outer wall. A stuck injector nozzle pair in the open position, poor outlet valve seal, and high-pressure gas leaking from the fuel chamber into the low-pressure fuel circuit can also lead to this problem. Another possible cause is a very low fuel level in the fuel tank, which can result in air being sucked in. Severe blockage in the fuel circuit from the fuel tank to the oil pump can also make it easy to suck in air.
For instance, if the tubing connection between the drain screw and the oil pump is loose, air can enter the system, mixing with the fuel and causing problems in the engine’s operation. Similarly, a damaged injector nozzle can allow high-pressure gas to leak into the low-pressure fuel circuit, affecting the fuel supply and engine performance.
Low resistance at the handle:
If one doesn’t loosen the vent screw and always feels low resistance when pressing the handle for a long time, it indicates that the return valve on the high-pressure fuel circuit or the inlet and outlet valves in the fuel transfer pump are not well sealed, resulting in fuel leakage.
Possible reasons for this include severe wear on the valve surface, impurities on the valve surface, or a damaged valve spring.
Engine’s own auxiliary devices:
When checking for faults, we can detect the general fault points and fault phenomena of the engine through some of the engine’s own auxiliary devices. As long as the scope is determined, it becomes easier to handle the specific reasons. Therefore, we should be good at using effective methods to find and solve engine faults more quickly.
For example, by monitoring the engine’s sensors and gauges, we can get an indication of potential problems in the fuel system. If the fuel pressure sensor shows low pressure, it can help us narrow down the search for the fault.
III. Fault case
Consider the case of a HOWO A7 heavy-duty vehicle that couldn’t start after changing the engine oil and three filters. It could start momentarily by spraying starting fluid. Pumping oil and exhausting air and attempting to start the engine more than ten times still didn’t result in a successful start. The dashboard showed fault code 639, but no corresponding fault code table was found.
Check the fine filter and low-pressure fuel circuit pressure:
Loosen the drain screw of the fine filter. There is no air present at all. After consulting the service station, it was determined that the pressure in the high-pressure fuel circuit was not reached. In this case, multiple starts were attempted. The thinking was that after the high-pressure fuel circuit reached the working pressure, the fuel injector could inject fuel and the engine could start. However, after multiple attempts, the engine still couldn’t start.
For instance, the operator may be frustrated by the repeated failures to start the engine. The lack of pressure in the high-pressure fuel circuit is a significant clue to the problem, but it is not immediately clear what is causing the issue.
Check the low-pressure fuel circuit again:
After emptying the coarse filter and fine filter, there is still no air. There are two non-return valves between the fine filter and the high-pressure oil pump. These valves are designed to establish a certain pressure in the low-pressure fuel circuit. When the first non-return valve is removed, it is found that the seal is not tight. Blowing on it with the mouth reveals that it actually leaks air. After cleaning, the sealing function is restored. When the second non-return valve, which is closer to the high-pressure oil pump, is removed, a small hard piece is found at the sealing place between the steel ball and the pipeline. After inspection, it is determined to be the adhesive shedding from the joint between the paper core inside the filter and the end cover. Judging from the disassembly and inspection process of these two non-return valves, it is clear that the filter is severely damaged. At this point, the fault code 639 on the dashboard still persists. Originally, it was thought that finding the problem with the non-return valves could start the vehicle, but the fault still remains. The service station personnel are also at a loss. An experienced master judges that it is still a fuel system problem.
For example, the discovery of the damaged filter and non-return valves is a significant finding. However, the fact that the engine still won’t start indicates that there may be other issues in the fuel system.
Check the fuel circuit for the third time:
This time, the high-pressure fuel circuit is checked. The high-pressure fuel pipe screw is opened. When pumping oil, it is found that there is oil and no air at the injector end of the high-pressure fuel pipe, but there is not much pressure. This is the case for each cylinder. All the pipeline connection screws are tightened and the engine is started again, but the result is still the same. Upon closer inspection, at the front end of the high-pressure common rail, there is a fuel pipe connection screw. Loosening it slightly and pressing the oil pump a few times results in a very small amount of air being discharged from the loosened screw. Then, after tightening it, the fault code 639 on the dashboard disappears. Turning the key successfully starts the engine.
Summary: From the initial situation of low pressure in the low-pressure fuel circuit, through checking the non-return valves and finding foreign objects, it is clear that the fault is mainly caused by a poor-quality filter. Then, during the exhaust process, a small amount of air enters the common rail system. Don’t underestimate such a small amount of air entering the common rail system. It is this that makes the high-pressure fuel circuit unable to reach the working pressure and causes the engine not to start.
In conclusion, understanding the construction and working principle of the hand primer pump, as well as being familiar with the various maintenance skills and fault cases, can be extremely helpful in quickly diagnosing and resolving fuel system faults. By using the hand primer pump as a diagnostic tool and carefully observing the symptoms and changes in the fuel system, it is possible to identify problems more efficiently and get the vehicle back on the road in a shorter time.
