JP2003083190A - Diagnosing device in fuel feed system of engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To diagnose the opening and fixing failure of a fuel bypass valve for opening and closing a passage for returning the fuel from the downstream of a fuel distribution pipe to a fuel tank. SOLUTION: A flow rate sensor 56 is mounted for detecting the fuel flow passing through the fuel bypass valve 52, and the opening and fixing failure of the fuel bypass valve 52 is determined when the flow detected in a state of a closing (OFF) control of the fuel bypass valve 52 is more than a predetermined value. Further a pressure sensor is mounted for detecting the fuel pressure at the upstream side of the fuel bypass valve 52, and the opening and fixing failure of the fuel bypass valve 52 is determined when a width of the pressure pulsation generated in the opening and closing switch control of the fuel bypass valve 52 is smaller than a predetermined value, or when the falling gradient of the fuel pressure after the switching-off of an ignition switch 54 is sharp over a predetermined value.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a diagnostic device for an engine fuel supply system, and more particularly to a fuel bypass valve for switching whether to return fuel from a fuel distribution pipe to which a fuel injection valve is connected to a fuel tank. Related to fault diagnosis technology.

[0002]

2. Description of the Related Art Conventionally, a pressure regulator is interposed between a fuel distribution pipe to which a fuel injection valve for injecting fuel into an engine is connected and a fuel pump, and a fuel tank is installed from the downstream side of the fuel distribution pipe. A fuel bypass valve is provided in the fuel return pipe for returning the fuel to the fuel return pipe, and at the time of start-up, the fuel bypass valve is opened to circulate the fuel in the fuel distribution pipe to discharge the air in the fuel distribution pipe. There is known a fuel supply device configured to close the fuel bypass valve and prevent the high temperature fuel in the fuel distribution pipe from returning to the fuel tank (see Japanese Patent Laid-Open No. 9-280132).

[0003]

By the way, when the fuel bypass valve is mechanically opened and fixed and the high temperature fuel in the fuel distribution pipe is returned to the fuel tank even when the fuel bypass valve is controlled to be closed during normal operation. The fuel temperature in the tank becomes high, and a large amount of fuel vapor is generated, which is supplied to the engine through the canister, which deteriorates the exhaust emission of the engine.

Therefore, a failure of the fuel bypass valve,
In particular, it has been desired to provide a diagnostic device that can diagnose open fixation. The present invention has been made in view of the above circumstances, and a failure (open stuck failure) of a fuel bypass valve that opens and closes a fuel return pipe that returns fuel from a downstream side of a fuel distribution pipe to a fuel tank.
An object of the present invention is to provide a diagnostic device for diagnosing.

[0005]

Therefore, according to the first aspect of the present invention, a fuel supply pipe connecting a fuel distribution pipe to which a fuel injection valve for injecting fuel to an engine is connected and a fuel pump are provided, and a fuel tank is provided. Pressure regulator that controls the fuel pressure by controlling the amount of fuel returned to the fuel tank, and a fuel bypass that is installed in the fuel return pipe that returns fuel from the downstream side of the fuel distribution pipe to the fuel tank and that opens and closes the fuel return pipe. A fuel supply device for an engine, comprising: a valve; The fuel bypass valve is diagnosed based on the correlation with.

According to the above construction, if the fuel bypass valve is opened, the fuel is returned to the fuel tank through the valve, and if the fuel bypass valve is closed, there is no flow to the fuel tank through the valve. The failure of the fuel bypass valve is diagnosed depending on whether or not the flow rate state corresponds to the open / close control state of the fuel bypass valve. According to the second aspect of the invention, in the closed control state of the fuel bypass valve, the open sticking failure of the fuel bypass valve is determined when the fuel flow rate flowing through the fuel bypass valve is equal to or higher than a predetermined flow rate.

According to the above structure, the fuel flow rate above the predetermined flow rate is detected when the fuel bypass valve is in the closed control state and the flow of the fuel to be originally returned to the fuel tank through the valve should not be present. At times, it is determined that the fuel bypass valve is in the open stuck state in which the fuel bypass valve is actually open despite the closing control. According to the third aspect of the present invention, a fuel pressure detecting means for detecting the fuel pressure is provided on the upstream side of the fuel bypass valve, and the failure diagnosis of the fuel bypass valve is based on the behavior of the fuel pressure during the opening / closing switching control of the fuel bypass valve. It is configured to perform.

According to the above configuration, whether the behavior of the fuel pressure generated when the fuel bypass valve is opened / closed is actually opened / closed in response to the opening / closing switching control based on whether or not the fuel pressure actually occurs. Determine whether or not. In the invention according to claim 4, when the pulsation width of the fuel pressure when the fuel bypass valve is switched from the open control state to the close control state is less than or equal to a predetermined value, the open stuck failure of the fuel bypass valve is determined. It was configured.

According to the above structure, when the fuel bypass valve is switched from open to closed, pressure pulsation occurs due to interruption of the fuel flow passing through the valve up to that time. Therefore, whether or not the pressure pulsation actually occurs is determined. If the occurrence of pressure pulsation is not recognized, it is determined that the valve has an open-sticking failure in which the valve does not actually close in response to the open-to-close switching control.

According to a fifth aspect of the present invention, fuel pressure detection means for detecting the fuel pressure on the upstream side of the fuel bypass valve is provided, and the failure diagnosis of the fuel bypass valve is performed based on the decreasing gradient of the fuel pressure after the engine is stopped. It was configured to do. According to the above configuration, if the fuel bypass valve is closed after the engine is stopped and the fuel pump is stopped, the space between the pressure regulator and the fuel bypass valve becomes a closed space to maintain the fuel pressure. However, when the fuel bypass valve is opened, the fuel pressure is drastically decreased by returning the fuel to the fuel tank, and the actual valve open / closed state can be determined from the difference in the falling gradient of the fuel pressure.

According to a sixth aspect of the present invention, the fuel bypass valve is kept closed when the engine is stopped. When the fuel pressure drops at a predetermined rate or more after the engine is stopped, the fuel bypass valve is closed. The configuration is such that the open sticking failure of the valve is determined. With the above configuration, the fuel bypass valve is controlled to be closed when the engine is stopped, so if the valve is actually closed, the space between the pressure regulator and the fuel bypass valve becomes a closed space to maintain the fuel pressure. However, if open valve sticking occurs and the valve is opened after the engine is stopped, the fuel pressure will drop rapidly with the engine stopped (fuel pump stopped), and the valve stuck open. The state is estimated.

According to the seventh aspect of the present invention, the pressure regulator and the fuel bypass valve are integrated, and the fuel return pipe downstream of the fuel bypass valve is shared by the pressure regulator and the fuel bypass valve. According to the above configuration, the pressure regulator and the fuel bypass valve are integrated, and the same pipe is used to return the fuel to the fuel tank.

[0013]

According to the first aspect of the present invention, the generation / stop of the fuel flow due to the open / closed state of the fuel bypass valve is detected, and it is judged whether or not this is in accordance with the control state. There is an effect that it is possible to reliably diagnose a failure that is stuck in the closed state.

According to the second aspect of the present invention, the state in which the valve is stuck in the open state can be reliably diagnosed by detecting the fuel flow passing through the valve in the closed control state of the fuel bypass valve. The effect is that you can do it. According to the third aspect of the invention, the behavior of the fuel pressure that is predicted to be generated with the opening / closing switching operation of the fuel bypass valve is
By detecting whether or not the valve actually occurs, it is possible to reliably diagnose a failure in which the valve is stuck in the open or closed state.

According to the fourth aspect of the present invention, the state in which the valve is fixed in the open state is based on whether or not the fuel pulsation that should occur when the control for closing the fuel bypass valve is actually performed. There is an effect that it can be surely diagnosed. According to the invention described in claim 5, the valve opening is based on the fact that the decreasing gradient of the fuel pressure after the fuel pump is stopped due to the stop of the engine greatly differs depending on whether the fuel bypass valve is closed or open. Alternatively, there is an effect that it is possible to reliably diagnose a failure that is stuck in the closed state.

According to the sixth aspect of the present invention, there is an effect that the open sticking failure of the fuel bypass valve can be reliably diagnosed from the rapid decrease of the fuel pressure to be held after the engine is stopped. According to the invention described in claim 7, since the pressure regulator and the fuel bypass valve are integrated with each other, the assembling and arrangement of the parts are facilitated, and the fuel is returned to the fuel tank by sharing the same pipe. This has the effects of reducing the number of parts and simplifying the routing of the piping.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below. FIG. 1 shows a fuel supply device for a vehicle engine according to an embodiment. In FIG. 1, an electric fuel pump 2 is internally provided in a fuel tank 1.
Sucks the fuel into the fuel tank 1 and discharges it.

One end of a fuel supply pipe 3 is connected to the discharge port of the fuel pump 2, and the other end of the fuel supply pipe 3 is
It is connected to the inlet side of the pressure regulator 4. The pressure regulator 4 includes a diaphragm 41, a reference pressure chamber 42 and a fuel pressure chamber 43 that are separated by the diaphragm 41, a valve body 44 fixed to a central portion of the diaphragm 41, and an open end of the valve body 44. Is configured to include a fuel return pipe 45 that is opened and closed, and a coil spring 46 that urges the diaphragm 41 in a direction in which the valve body 44 closes the fuel return pipe 45.

The pressure in the reference pressure chamber 42 and the fuel pressure chamber 4
When the pressure difference with the pressure of 3 becomes a predetermined value or more, the diaphragm 4
1 bends to the left side in the drawing, and the valve element 44 separates from the open end of the fuel return pipe 45, so that the fuel is returned to the fuel tank 1 via the fuel return pipe 47 whose one end is connected to the fuel return pipe 45, Reduce fuel pressure. The pressure regulator 4
The other end of the fuel supply pipe 48 connected to the outlet side of is connected to the fuel distribution pipe 49.

A plurality of fuel injection valves 51 for injecting fuel into the engine 50 are connected to the fuel distribution pipe 49. When the fuel injection valves 51 are opened, the fuel whose pressure is adjusted by the pressure regulator 4 is connected. Are injected and supplied to the engine 50. One end of a fuel return pipe 39 is connected to the downstream end of the fuel distribution pipe 49, and the other end of the fuel return pipe 39 is connected to the inlet side of the fuel bypass valve 52.

The fuel bypass valve 52 is a normally-closed electromagnetic valve that opens and drives the valve body by a magnetic attraction force of an electromagnet. The fuel bypass valve 52 is constructed integrally with the pressure regulator 4 and is controlled to be opened (ON). In some cases, the fuel that has passed through the valve body is caused to flow into the fuel return pipe 45, and the fuel is returned to the fuel tank 1 through the fuel return pipe 47.

When the closing control (OFF) is performed,
The fuel return pipe 39 is closed, and the fuel supplied to the fuel distribution pipe 49 is not returned to the fuel tank 1. still,
When the fuel bypass valve 52 is opened, the fuel bypass valve 52 functions as an orifice to maintain the fuel pressure in the fuel distribution pipe 49.

The fuel bypass valve 52 is opened / closed by a control unit 53. The control unit 53 includes a microcomputer, and controls opening / closing (ON / OFF) of the fuel bypass valve 52 according to a program stored in advance.

A signal from the ignition switch 54 and an output signal of a water temperature sensor 55 for detecting the temperature of the cooling water of the engine 50 are input to the control unit 53, and based on these signals, a flow chart shown in FIG. Then, the fuel bypass valve 52 is controlled to open and close. In the flowchart of FIG. 2, step S1
Then, it is determined whether the ignition switch 54 is ON or OFF, and if it is ON, the process proceeds to step S2.

In step S2, the fuel bypass valve 52 is turned on to open the fuel bypass valve 52. In step S3, start (end of cranking)
It is determined whether or not a predetermined time corresponding to the temperature of the cooling water has elapsed, and the fuel bypass valve 52 is kept in the ON state until the predetermined time elapses.

When the fuel bypass valve 52 is turned on and opened, the fuel supplied to the fuel distribution pipe 49 is returned to the fuel tank 1 and the fuel in the fuel distribution pipe 49 is circulated. The air in 49 is discharged, and the deterioration of the injection performance due to the mixing of air is avoided. When it is determined in step S3 that the predetermined time has elapsed, step S4
Then, the fuel bypass valve 52 is turned off and closed.

When the fuel bypass valve 52 is closed, the downstream side of the fuel distribution pipe 49 is closed, and the fuel distribution pipe 4 is closed.
In 9, the fuel heated by the heat of the engine 50 is not returned to the fuel tank 1, the temperature rise in the fuel tank 1 is avoided, and the generation of fuel vapor is suppressed. When the ignition switch 54 is off, the fuel bypass valve 52 is held off.

Further, the control unit 53 is
The failure diagnosis of the fuel bypass valve 52 is performed as shown in the flowchart of FIG. Here, in order to perform the diagnosis shown in the flowchart of FIG. 3, a flow rate sensor 56 (flow rate detection means) is provided in the fuel return pipe 39 on the upstream side of the fuel bypass valve 52 as shown in FIG.

In the flowchart of FIG. 3, in step S11, it is determined whether or not the engine 50 is operating, and if it is, the process proceeds to step S12. Step S
At 12, it is determined whether the fuel bypass valve 52 is in the OFF state (closed control state). After a predetermined time has passed from the start, the fuel bypass valve 52 is turned off.
When in the FF state (closed control state), step S1
3, the output signal of the flow rate sensor 56 is read.

In the next step S14, it is determined whether or not the fuel flow rate detected by the flow rate sensor 56 is equal to or higher than a predetermined flow rate. Then, when the fuel flow rate is smaller than the predetermined flow rate, the routine proceeds to step S15, where it is determined that the fuel bypass valve 52 is in the normal state in which it is actually closed with respect to the OFF control (close control).

On the other hand, when the fuel of a predetermined flow rate or more is flowing,
When it is determined that the fuel is returned to the fuel tank 1 through the fuel bypass valve 52, step S1
In step 6, it is determined that the fuel bypass valve 52 is in the open stuck failure state in which the fuel bypass valve 52 actually holds the open state with respect to the OFF control (close control). When the open sticking failure state is determined, a warning lamp provided in the driver's seat of the vehicle is lit to prompt maintenance and inspection.

In the above embodiment, the flow rate sensor 56 is used as the flow rate detecting means, but a flow rate switch that switches between ON and OFF at a predetermined flow rate may be used. When using the flow rate switch,
During the OFF control (close control) of the fuel bypass valve 52, diagnosis is performed based on whether or not the flow rate switch shows the output on the side with a large flow rate.

The threshold value of the flow rate for diagnosing the open sticking failure may be a minute flow rate such that the failure is immediately judged when the state is not completely closed. However, after starting, the fuel tank is connected via the fuel bypass valve 52. The configuration may be set based on the allowable value of the fuel flow rate returned to 1. Figure 5
2 shows a second embodiment of the failure diagnosis of the fuel bypass valve 52.

In the diagnosis shown in the flow chart of FIG. 5, as shown in FIG.
2 A fuel pressure sensor 57 (fuel pressure detection means) is provided in the upstream fuel return pipe 39 in place of the flow rate sensor 56, and the fuel pressure sensor 57 is used for diagnosis. Step S
In 21, the ignition switch 54 is turned ON → OFF
The ignition switch 54 is turned from ON to OFF in order to stop the engine 50 by determining whether it is time to switch.
When switched, the process proceeds to step S22.

In step S22, the fuel pressure sensor 57
The pressure detected by is read and set to the fuel pressure P1. In the next step S23, it is determined whether or not a predetermined time (10 to 100 ms) has elapsed after sampling the fuel pressure P1. When the predetermined time has elapsed, the process proceeds to step S24, the pressure detected by the fuel pressure sensor 57 is read, and it is set to the fuel pressure P2.

In step S25, the fuel pressure change amount ΔP = P1-P2 at the predetermined time is calculated. Step S
At 26, it is determined whether or not the fuel pressure change amount ΔP (falling gradient of the fuel pressure) is a predetermined value SL or more. When the engine is stopped with the ignition switch 54 turned off,
Since the fuel bypass valve 52 is held in the OFF state, the space between the pressure regulator 4 and the fuel bypass valve 52 is closed, and the fuel pressure gradually changes depending on the leakage amount.

However, the fuel bypass valve 52 is turned off.
When the fuel bypass valve 52 is opened without being closed, a large amount of fuel flows out through the fuel bypass valve 52, so that the fuel pressure sharply drops as compared with the case where the fuel bypass valve 52 is closed. (See Figure 6). Therefore, when the fuel pressure change amount ΔP is equal to or greater than the predetermined value SL and it is determined that the fuel pressure is decreasing at a rate equal to or more than the predetermined value, the open and stuck state of the fuel bypass valve 52 is estimated. Advances to step S27 to determine an open sticking failure.

The determination result of the open sticking failure is stored, and the warning lamp is turned on when the ignition switch 54 is turned on next time. On the other hand, when the fuel pressure change amount ΔP is smaller than the predetermined value SL, the fuel bypass valve 52
Is estimated to be normally closed, the process proceeds to step S28 without immediately determining normality, and the counter CNT is incremented by 1.

In step S29, the counter CNT is
Is determined to be equal to or greater than the predetermined value n, and until the value is equal to or greater than the predetermined value n, the process returns to step S22 to repeat the determination of the fuel pressure decrease speed. When it is determined in step S29 that the value of the counter CNT is equal to or greater than the predetermined value n, the process proceeds to step S30, and it is determined that the fuel bypass valve 52 is normal.

Therefore, the fuel pressure change amount ΔP is the predetermined value SL.
If it is smaller than the above, the normality determination is made only after the determination is repeated a predetermined number of times n. When the open sticking failure or the normality is determined, the counter CNT is reset to zero in step S31, and this routine is ended. In the second embodiment, a fuel pressure switch is used instead of the fuel pressure sensor 57 and the ignition switch 5 is used.
It is also possible to make a diagnosis as to whether or not the fuel pressure has dropped at a predetermined rate or more, based on the state of the pressure switch at the time when a predetermined time has elapsed after 4 was turned off.

The flowchart of FIG. 7 shows a third embodiment of the failure diagnosis of the fuel bypass valve 52, and the fuel pressure sensor 57 shown in FIG. 4 is used for the diagnosis in the diagnosis shown in the flowchart of FIG. Step S4
In step 1, it is determined whether the fuel bypass valve 52 has been switched from the open state to the closed state by the opening / closing control based on the flowchart of FIG.

When the above switching control is performed, the process proceeds to step S42, and the maximum fuel pressure and the minimum fuel pressure within a predetermined time after the switching control are sampled.
In step S43, the pulsation width of the fuel pressure is calculated as the deviation between the maximum fuel pressure and the minimum fuel pressure. In step S44,
It is determined whether the pulsation width is a predetermined value or more.

If the pulsation width is greater than or equal to the predetermined value, the routine proceeds to step S45, where it is determined that the fuel bypass valve 52 is normal, and if the pulsation width is smaller than the predetermined value, the routine proceeds to step S46. , Fuel bypass valve 52
Judgment of open sticking failure of. The fuel bypass valve 52
Is switched from the open state to the closed state, and when the flow of the fuel that passes through the fuel bypass valve 52 and is returned to the fuel tank 1 is blocked, the fuel pressure pulsates due to the influence (see FIG. 6). ), When the fuel bypass valve 52 is stuck open and is kept open for the closing control, the fuel is continuously returned to the fuel tank 1 through the fuel bypass valve 52. Therefore, even if the open control state is switched to the closed control state, there is no change in the fuel path,
The fuel pulsation as described above does not occur.

Therefore, when the pulsation of the fuel pressure commensurate with the open-to-close operation does not occur due to the switching from the open control state to the close control state, it corresponds to the switching from the open control state to the close control state. It is possible to judge the occurrence of a failure in which the state of the fuel bypass valve does not change, that is, a stuck failure. The open / close switching for detecting the pulsation width is not limited to the control based on the flowchart of FIG. 2, and the open / close switching control may be forcibly performed for diagnosis.

Further, in the above embodiment, the pressure regulator 4 and the fuel bypass valve 52 are integrated, and the fuel return pipe 47 is shared to return the fuel to the tank 1. However, the pressure regulator 4 and the fuel bypass valve 52 are also provided. It may be configured such that the fuel tank 1 and the fuel tank 2 are separated from each other, and a dedicated fuel return pipe is connected to each of them to return the fuel to the tank 1.

As described above, the pressure regulator 4
In the configuration in which the fuel bypass valve 52 and the fuel bypass valve 52 are separately provided, the flow sensor or the flow switch may be provided on the downstream side of the fuel bypass valve 52 to perform the failure diagnosis shown in the flowchart of FIG. it can.

[Brief description of drawings]

FIG. 1 is a system configuration diagram of a fuel supply device for a vehicle engine according to an embodiment.

FIG. 2 is a flowchart showing opening / closing control of a fuel bypass valve in the embodiment.

FIG. 3 is a flowchart showing a first embodiment of failure diagnosis of a fuel bypass valve.

FIG. 4 is a system configuration diagram of a fuel supply device common to the second and third embodiments of the fuel bypass valve failure diagnosis.

FIG. 5 is a flowchart showing a second embodiment of fuel bypass valve failure diagnosis.

FIG. 6 is a time chart showing the correlation between opening and closing of the fuel bypass valve and changes in fuel pressure in the embodiment.

FIG. 7 is a flowchart showing a third embodiment of fuel bypass valve failure diagnosis.

[Explanation of symbols]

1 ... Fuel tank 2 ... Fuel pump 3 ... Fuel supply piping 4 ... Pressure regulator 39 ... Fuel return pipe 47 ... Fuel return piping 48 ... Fuel supply piping 49 ... Fuel distribution pipe 50 ... Engine 51 ... Fuel injection valve 52 ... Fuel bypass valve 53 ... Control unit 54 ... Ignition switch 55 ... Water temperature sensor 56 ... Flow rate sensor 57 ... Fuel pressure sensor

Continued front page    F term (reference) 3G066 AA02 AA07 AC09 AD05 BA23                       BA28 BA56 BA61 CB07U                       CB15 CB16 CE22 DC11 DC18                 3G084 DA27 EA11 EB22 FA00 FA20

Claims (7)

[Claims] 1. A fuel supply pipe that connects a fuel distribution pipe to which a fuel injection valve for injecting fuel to an engine is connected and a fuel pump, and controls the amount of fuel returned to the fuel tank. A fuel supply device for an engine, comprising: a pressure regulator for adjusting pressure; and a fuel bypass valve, which is interposed in a fuel return pipe for returning fuel from the downstream side of the fuel distribution pipe to the fuel tank, and opens and closes the fuel return pipe. In the fuel bypass valve, the flow rate detecting means for detecting the flow rate of the fuel passing through the fuel bypass valve is provided, and the fuel bypass is determined based on the correlation between the fuel flow rate detected by the flow rate detecting means and the opening / closing control state of the fuel bypass valve. A diagnostic device in a fuel supply device for an engine, which performs a valve failure diagnosis. 2. In the closed control state of the fuel bypass valve, when the flow rate of the fuel flowing through the fuel bypass valve is equal to or higher than a predetermined flow rate, an open sticking failure of the fuel bypass valve is determined. The diagnostic device for a fuel supply system for an engine according to claim 1. 3. A fuel supply pipe connecting a fuel distribution pipe to which a fuel injection valve for injecting fuel to an engine is connected and a fuel pump, and controlling the amount of fuel returned to the fuel tank. A fuel supply device for an engine, comprising: a pressure regulator for adjusting pressure; and a fuel bypass valve, which is interposed in a fuel return pipe for returning fuel from the downstream side of the fuel distribution pipe to the fuel tank, and opens and closes the fuel return pipe. A fuel pressure detecting means for detecting a fuel pressure on the upstream side of the fuel bypass valve, and performing a failure diagnosis of the fuel bypass valve based on a behavior of the fuel pressure during opening / closing switching control of the fuel bypass valve. A diagnostic device in a fuel supply device for an engine. 4. An open sticking failure of the fuel bypass valve is determined when a pulsation width of the fuel pressure when the fuel bypass valve is switched from an open control state to a close control state is below a predetermined value. The diagnostic device for a fuel supply system for an engine according to claim 3. 5. A fuel supply pipe, which connects a fuel distribution pipe to which a fuel injection valve for injecting fuel to the engine is connected, and a fuel pump, is provided, and the amount of fuel returned to the fuel tank is controlled to control the fuel. A fuel supply device for an engine, comprising: a pressure regulator for adjusting pressure; and a fuel bypass valve, which is interposed in a fuel return pipe for returning fuel from the downstream side of the fuel distribution pipe to the fuel tank, and opens and closes the fuel return pipe. The fuel bypass valve upstream side is provided with a fuel pressure detecting means for detecting a fuel pressure, and the failure diagnosis of the fuel bypass valve is performed based on a descending gradient of the fuel pressure after the engine is stopped. Diagnostic device in engine fuel supply system. 6. The fuel bypass valve is configured to be kept closed when the engine is stopped, and the fuel bypass valve is opened when the fuel pressure drops at a predetermined rate or more after the engine is stopped. The diagnostic device for a fuel supply system for an engine according to claim 5, wherein a sticking failure is determined. 7. The pressure regulator and the fuel bypass valve are integrated with each other, and the fuel return pipe on the downstream side of the fuel bypass valve is configured to be shared by the pressure regulator and the fuel bypass valve. Item 7. A diagnostic device in a fuel supply device for an engine according to any one of items 1 to 6.