Dropping fuel pressure
 

Hello everyone, I tried searching and came up with no luck. I'm working on my friends 2000 Cherokee with the 4.0. She is having a delayed start after it sits a little while. So I rented a fuel pressure tester and I am getting 47 psi which is in range. Now here is my problem, after I shut it off the pressure drops off slowly and within 5 minutes it is almost down to zero. Now is this the fuel pump going or a injector leaking? Is there any way to test for a leaky injector? Thanks for any help. I just want to be sure before I have her drop a lot of money on a good fuel pump.

You could try this:


Here's more:

https://duckduckgo.com/?q=leaky+inje...ffnt&ia=videos

Thanks for the reply. I watched the videos and can give it a try. I'll just have to drink up some water to have some empty bottles.

Or you could just lay out some paper towel. A leak is a leak.

You can also perform the pressure test by priming the pump then clamping off the fuel line to the pump. It helps to isolate whether the drop is coming from the injectors or the pump.

How much of a "delayed" start is the vehicle experiencing?

Thanks for the great ideas. If the jeep sits overnight it will crank for 6 seconds and normally she will turn the key off and try again. It starts up the second try. But if you just shut it off and start her back up with in a hour or so it starts right up.

I have a similar problem in my '00. Since the day I bought it. 3 years now. I believe my issue is the regulator on top of the fuel pump.

https://www.napaonline.com/napa/en/p...?interchange=1

Thing is what you have to do to replace just that is the same as replacing the fuel pump. People run for years with it like that. I most likely will do likewise unless I get bored and the Jeep behaves herself for awhile.

the 2000's xj should have a regulator on the rail not the pump IIRC only the 1996 xj had a pump mounted regulator

On the OP's XJ I would say it's either a leaky fuel injector or more likely a bad fuel pressure regulator.

I thought I read somewhere that it is not a regulator on the rail but something else.

I do have the fuel dampner on the rail but I replaced the fuel rail too when I did a 4 hole injector upgrade. Brand new injectors, O-Rings, and clips. Had the problem before and still have it. Did not think that doing that would solve my problem just wanted to upgrade. I have been through it, my buddy (who wrenches for a living) also agrees after he checked. Are we 100% sure that is the problem? Course not. But pretty sure.

Thanks for the link and the help. The thing is the regulator is almost as much money as a Bosch fuel pump on ebay. At that point I would just replace the whole pump.

That is why I said I am just going to live with it. Seems stupid to do all that work for the regulator, assuming that is even the problem, and on the other hand replace a fuel pump that, with the exception of the leak down, appears to operate fine?

Yeah I understand, for the cost of the pump, it is cheaper just to leave it. But I have had other cars that you would have to do the prime trick and eventually the pump gave up and left you stranded on the side of the road.

i just looked it up in the manual and it turns out you are correct the thing on the rail is a dampener for dampening fuel pulsations from the fuel pump, the fuel pressure regulator is part of the pump itself.

FUEL PUMP
DESCRIPTION
The fuel pump is located inside of the fuel pump
module. A 12 volt, permanent magnet, electric motor
powers the fuel pump.

OPERATION
Voltage to operate the electric pump is supplied
through the fuel pump relay.
Fuel is drawn in through a filter at the bottom of
the module and pushed through the electric motor
gearset to the pump outlet.
Check Valve Operation: The pump outlet contains a one-way check valve to prevent fuel flow back
into the tank and to maintain fuel supply line pressure (engine warm) when pump is not operational. It
is also used to keep the fuel supply line full of gasoline when pump is not operational. After the vehicle
has cooled down, fuel pressure may drop to 0 psi
(cold fluid contracts), but liquid gasoline will remain
in fuel supply line between the check valve and fuel
injectors. Fuel pressure that has dropped to 0
psi on a cooled down vehicle (engine off) is a
normal condition. Refer to the Fuel Pressure Leak
Down Test for more information.
FUEL GAUGE SENDING UNIT

DESCRIPTION
The fuel gauge sending unit (fuel level sensor) is
attached to the side of the fuel pump module. The
sending unit consists of a float, an arm, and a variable resistor track (card).

OPERATION
The fuel pump module has 4 different circuits
(wires). Two of these circuits are used for the fuel
gauge sending unit for fuel gauge operation, and for
certain OBD II emission requirements. The other 2
wires are used for electric fuel pump operation.

For Fuel Gauge Operation: A constant input
voltage source of about 12 volts (battery voltage) is
supplied to the resistor track on the fuel gauge sending unit. This is fed directly from the Powertrain
Control Module (PCM). NOTE: For diagnostic purposes, this 12V power source can only be verified with the circuit opened (fuel pump module
electrical connector unplugged). With the connectors plugged, output voltages will vary from
about.6 volts at FULL, to about 8.6 volts at
EMPTY (about 8.6 volts at EMPTY for Jeep
models, and about 7.0 volts at EMPTY for
Dodge Truck models). The resistor track is used to
vary the voltage (resistance) depending on fuel tank
float level. As fuel level increases, the float and arm
move up, which decreases voltage. As fuel level
decreases, the float and arm move down, which
increases voltage. The varied voltage signal is
returned back to the PCM through the sensor return
circuit.
Both of the electrical circuits between the fuel
gauge sending unit and the PCM are hard-wired (not
multi-plexed). After the voltage signal is sent from
the resistor track, and back to the PCM, the PCM
will interpret the resistance (voltage) data and send
a message across the multi-plex bus circuits to the
instrument panel cluster. Here it is translated into
the appropriate fuel gauge level reading. Refer to
Instrument Panel for additional information.
For OBD II Emission Monitor Requirements:
The PCM will monitor the voltage output sent from
the resistor track on the sending unit to indicate fuel
level. The purpose of this feature is to prevent the
OBD II system from recording/setting false misfire
and fuel system monitor diagnostic trouble codes.
The feature is activated if the fuel level in the tank
is less than approximately 15 percent of its rated
capacity. If equipped with a Leak Detection Pump
(EVAP system monitor), this feature will also be activated if the fuel level in the tank is more than
approximately 85 percent of its rated capacity.

FUEL FILTER/FUEL PRESSURE REGULATOR
DESCRIPTION
The combination fuel filter and fuel pressure regulator is located on the top of fuel pump module (Fig.
1).
OPERATION
A combination fuel filter and fuel pressure regulator is used on all engines. A separate frame mounted
fuel filter is not used with any engine.
Fuel Pressure Regulator Operation: The pressure regulator is a mechanical device that is not controlled by engine vacuum or the Powertrain Control
Module (PCM).
The regulator is calibrated to maintain fuel system
operating pressure of approximately 339 kPa 6 34
kPa (49.2 psi 6 5 psi) at the fuel injectors. It contains a diaphragm, calibrated springs and a fuel
return valve. The internal fuel filter is also part of
the assembly.
Fuel is supplied to the filter/regulator by the electric fuel pump through an opening tube at the bottom of filter/regulator (Fig. 3).
The regulator acts as a check valve to maintain
some fuel pressure when the engine is not operating.
This will help to start the engine. A second check
valve is located at the outlet end of the electric fuel
pump.

Thanks for the video but that must be for a older cherokee because I know both her 2000 and my 2001 don't have a fuel rail with a return line just the feed line. And there is no regulator on the end of the rail like in the video.