Low Fuel Pressure, Won't Start 2001 XJ

I've got one of the troublesome 2001 XJ's (226k miles, 4.0L, 4WD, AW4 auto, ABS, A/C) with the "heat soak" P303 issue. I've installed the heat insulator sleeve on injectors 3 and 4, as recommended in the TSB, but it never solved the vapor lock on hot startup trouble. I had always assumed that my fuel pump check valve was leaking more pressure than normal, and always turned the ignition ON, OFF, ON, OFF, ON, START as a poor man's workaround.

This morning, after a long weekend of driving with no problems, including 2+ hours last night in very heavy rainstorm, it refused to start; turned and cranked but refused to start. I've been searching up and down and similar symptoms point to fuel pump/filter/valve, or possibly crank position sensor.

I put a fuel pressure gauge on the fuel rail schrader valve, and it shows that when the fuel pump primes, pressure goes up to 42PSI with the Jeep off.

I fed it some ether in the throttle body, gave it some throttle while trying to start and managed to get it started, albeit sounding "starved". Fuel pressure at idle showed a solid 49PSI as expected; turn the Jeep off and it drops instantly down to 42PSI. I cannot get it to start without giving it gas, but with some accelerator pedal on start, I can get it going.

I fear that I need to replace the fuel pump, which is located inside the fuel tank on the '01 XJ. Can anyone recommend some good, easy troubleshooting for a not-so-skilled mechanic to diagnose? I'm not sure where to go next to figure out if the fuel pump is the culprit... no obvious fuel line leaks. Possible red herring: my oil pressure gauge recently started reading MAX, though I figure I just need to replace the pressure sending unit (again!).

If I do need to replace the fuel pump (of if I have to drop the fuel tank, I figure I should replace it while I'm there), is this Bosch unit all I need to do the job? BSH N69316

THANKS!

I can't help with the problem but the fuel pump I just put into my 97 was Bosch N67658. I had the leaky check valve also so I replace the whole pump assemble.

Start with the simple and obvious solutions:

Begin with basic trouble shooting of the start and charge systems. Remove, clean, and firmly reconnect all the wires and cables to the battery, starter, and alternator. Look for corroded or damaged cables or connectors and replace as needed. Do the same for the grounding wires from the starter to engine block, and from the battery and engine to the Jeep's frame/body. You must remove, scrape, and clean until shiny, the cable/wire ends, and whatever they bolt to. Jeeps do not tolerate low voltage, bad connections, or poor grounds.

Have the battery Load Tested. Low voltage from poor connectors or a weak battery can cause the symptoms you describe.

Test the CPS. Crankshaft Position Sensor Connector (CPS/CKP)


.



TESTING PROCEDURE 1991 – 2001 4.0L H.O. engines

1. Near the rear of intake manifold, disconnect sensor pigtail harness connector from main wiring harness.
2. Place an ohmmeter across terminals B and C (See Image). Ohmmeter should be set to 1K-to-1OK scale for this test.
3. The meter reading should be open (infinite resistance). Replace sensor if a low resistance is indicated.


Test the fuel pressure and ohms test the fuel injectors:
Testing of the fuel injectors should show if any are faulty and are allowing fuel to drain into the cylinder. Either problem can allow heat soak to vaporize the remaining fuel in the fuel rail and you may be dealing with a bit of vapor lock as well, especially in warmer weather.

1. Connect the gauge at the fuel rail and start the engine. The gage should read 49 psi plus or minus 5 psi.

2. Turn the engine off and immediately clamp the fuel line just ahead of the fuel tank. Watch the pressure gage and see how long it takes to loose pressure.

If the pressure remains at 49 psi for an extended period of time then the problem is in the tank - probably the check valve. If the pressure falls below 49 psi fairly rapidly then the problem is probably a leaky injector.

Tim_MN said:

Start with the simple and obvious solutions:

Click to expand...

Thanks Tim! Your advice is sage, and helps to bring some sanity back to tracking these problems down. I've already bookmarked lots of great stuff from you that turned up from searching the forum!

Tim_MN said:

Begin with basic trouble shooting of the start and charge systems. Remove, clean, and firmly reconnect all the wires and cables to the battery, starter, and alternator. Look for corroded or damaged cables or connectors and replace as needed. Do the same for the grounding wires from the starter to engine block, and from the battery and engine to the Jeep's frame/body. You must remove, scrape, and clean until shiny, the cable/wire ends, and whatever they bolt to. Jeeps do not tolerate low voltage, bad connections, or poor grounds.

Click to expand...

I need to go back to square one and double-check these basics. Less than a month ago, I discovered that the braided ground strap from engine block to firewall had disintegrated, and I replaced it with a 12ga battery cable. I'll verify all my grounds... just as soon as the onslaught of rain dies down!

Tim_MN said:

Test the fuel pressure and ohms test the fuel injectors:

Click to expand...

I don't see a specified resistance for the fuel injectors in the factory service manual, does anyone have tips on testing fuel injectors? Any way to test an injector to see if it is leaking?

Tim_MN said:

If the pressure remains at 49 psi for an extended period of time then the problem is in the tank - probably the check valve. If the pressure falls below 49 psi fairly rapidly then the problem is probably a leaky injector.

Click to expand...

I'm seeing an immediate drop to 42PSI when I kill the engine; can I get away with clamping the fuel line just before killing it, or will that change the line pressure as the injectors continue trying to pull fuel?

Many thanks to everyone for the help - I need it!

driggs said:

.... I cannot get it to start without giving it gas, but with some accelerator pedal on start, I can get it going.

Click to expand...

This points to an electrical issue.

For 1996+ Jeeps, the OBD-II engine computer (ECU/PCM) re-boots, and the idle settings are deleted, when you: (1) let the battery run down by leaving the lights or radio on, (2) disconnect the battery for an extended period of time, (3) when the battery goes bad and needs replacing, (4) when the alternator in not properly charging the battery, (5) or when poor battery connections result in a voltage drop at the PCM.



Your symptoms are: (1) having difficulties starting the engine without depressing the gas pedal, and (2) the engine will have a low idle and probably stall unless you keep your foot on the gas pedal. The ECU will relearn the idle settings after a short period of driving and the engine will start and idle normally.

Loose, corroded, or damaged battery cables or ground wires may also cause or contribute to the problem. Begin with basic trouble shooting of the start and charge systems. Remove, clean, and firmly reconnect all the wires and cables to the battery, starter, and alternator. Look for corroded or damaged cables and replace them as needed. Do the same for the grounding wires from the battery and engine to the Cherokee's frame/body.

If the symptoms reoccur, you need to have your battery and/or the alternator load tested at a repair shop or auto parts store. This is usually a free service they offer to bring in potential customers. Even with low battery voltage, the engine will start, but the ECU can re-boot multiple times and a new battery may be required.

Cleaning the Idle Air Controller (IAC), regular tune-ups, and using fuel injector cleaner should be part of your routine preventative maintenance.

driggs said:

I don't see a specified resistance for the fuel injectors in the factory service manual, does anyone have tips on testing fuel injectors? Any way to test an injector to see if it is leaking?

Click to expand...

Answering my own question about testing fuel injectors; it is listed in the Fuel System section of the manual, not the Engine section!

Resistance reading should be approximately 12 ohms +/-1.2 ohms at 20°C (68°F).

So my fuel rail reads 42PSI at the schrader valve when the pump is primed (key ON), at idle it reads 49PSI, and when I shut the engine off it immediately drops back down to 42PSI. What does this mean?

1. Connect the gauge at the fuel rail and start the engine. The gage should read 49 psi plus or minus 5 psi (FSM specification).

2. Turn the engine off and immediately clamp the fuel line just ahead of the fuel tank. Watch the pressure gage and see how long it takes to loose pressure.

If the pressure remains at 49 psi for an extended period of time then the problem is in the tank - probably the check valve. If the pressure falls below 49 psi fairly rapidly then the problem is probably a leaky injector.

There are a few solutions to the problem –

1. Turn the key to ON for 5 seconds and let the fuel pump prime. Turn the key to OFF, turn the key back to ON for 5 seconds, start the engine.

2. Crank the engine for 3-5 seconds, pause, and crank the engine again. It will almost always start on the second try.

3. Replace the fuel pump assembly in the gas tank. The check valve is not a separate part and cannot be serviced. This is both expensive and time consuming because you have to remove any hitches and skid plates and then remove the gas tank from the vehicle.

You can use solutions 1 or 2 for as long as you want to, they do not harm anything and the fuel pump will continue to function for many more years.

If you do replace the fuel pump assembly, purchase a high quality OEM style unit. Some cheap aftermarket fuel pump assemblies, like Airtex, don’t hold up and fail completely within 12-18 months.

Tim_MN said:

1.
3. Replace the fuel pump assembly in the gas tank. The check valve is not a separate part and cannot be serviced. This is both expensive and time consuming because you have to remove any hitches and skid plates and then remove the gas tank from the vehicle.

You can use solutions 1 or 2 for as long as you want to, they do not harm anything and the fuel pump will continue to function for many more years.

If you do replace the fuel pump assembly, purchase a high quality OEM style unit. Some cheap aftermarket fuel pump assemblies, like Airtex, don’t hold up and fail completely within 12-18 months.

Click to expand...

A while back had a problem with the fuel gauge producing error codes, caused both running and starting problems that got worse over time ... dropped the tank found the fuel gauge resistance was giving false reading ... these can sometime be repaired ... but after ~20 years thought it was time for a new pump, filter and guage ..

So replaced the entire assembly with a brand new OEM from Jeepsareus ...

Ever since the install have had to primed the engine each time from a cold start ... ...

Tested with a fuel pressure tester ... clamped off just prior to the fuel rail ... and again with a clamp near the fuel tank

... appears the leak is in the fuel pump / check valve or perhaps a fuel line connection ... when have the time to drop the tank will test the install with another fuel pump ..

Some reference notes (for testing fuel delivery & start-up problems)

(Notes: from 99/2000/2001 XJ factory service manuals)

__ After making fuel line pressure gauge test connections, clamping, etc.

** A quick loss of pressure usually indicates a defective check valve in the filter/regulator.

** A slow loss of pressure usually indicates a defective check valve in the electric fuel pump.

---------------------------

__ Description: check valve operation (fuel pump / regulator-filter)

The regulator is calibrated to maintain fuel system operating pressure of approximately 339 kPa 6 34 kPa (49.2 psi +/- 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.

If fuel pressure at the pressure regulator exceeds approximately 49 psi, an internal diaphragm closes and excess fuel is routed back into the tank through the pressure regulator. A separate fuel return line is not used.


Other notes: Engine startup - fuel system ..

Engine Start-up Mode — This is an open loop mode.

The following actions occur when the starter motor is engaged:

1. The auto shutdown and fuel pump relays are energized. If the PCM does not receive the camshaft and crankshaft signal within approximately one second, these relays are deenergized.

2. The PCM energizes all fuel injectors until it determines crankshaft position from the camshaft and crankshaft signals. The PCM determines crankshaft position within one engine revolution. After the crankshaft position has been determined, the PCM energizes the fuel injectors in sequence. The PCM adjusts the injector pulse width and synchronizes the fuel injectors by controlling the fuel injectors’ ground paths.

----------------

Fuel Pressure − Fuel pressure is controlled by the fuel pressure regulator. The PCM cannot detect a clogged fuel pump inlet filter, clogged in-line filter, or a pinched fuel supply.(*)

-----------

Jeep XJ factory manuals have complete fuel delivery testing / diagnostic procedures and descriptions

There are OBD tests that can be performed through the PCM , usually requires an expensive or Chrysler scan tool as well as other test tools.

*-

driggs said:

I fed it some ether in the throttle body, gave it some throttle while trying to start and managed to get it started, albeit sounding "starved". Fuel pressure at idle showed a solid 49PSI as expected; turn the Jeep off and it drops instantly down to 42PSI. I cannot get it to start without giving it gas, but with some accelerator pedal on start, I can get it going.


!

Click to expand...

The problem I suppose could be only with the fuel delivery system ..

Have you already considered other things that effect engine startup?

Is the engine cold or already warmed up ..?

=========================

Engine modes of operation .. "Open Loop" & "Closed Loop"

Open Loop Modes: ____

__ Engine Warm-Up Mode − This is an open loop mode. The PCM adjusts injector pulse width and controls injector synchronization by controlling the fuel injectors’ ground paths. The PCM adjusts ignition timing and engine idle speed. The PCM adjusts the idle speed by controlling the idle air control

__ Wide Open Throttle Mode − This is an open loop mode. The throttle position sensor notifies the PCM of a wide open throttle condition. The PCM adjusts injector pulse width to supply a predetermined amount of additional fuel.

Closed Loop Modes: ____

__ Cruise or Idle Mode
__ Acceleration Mode
__ Deceleration Mode

======================

When starting the engine the PCM needs to see a signal from the Crankshaft and Camshaft within approx. one second or the fuel pump relay and auto shutdown relays are de-energized ...

after energizing the relays .. the PCM then takes input from other sensors such as ..

– engine coolant temperature
– manifold absolute pressure
– intake air temperature
– engine revolutions
– throttle position

I have remedied similar types of (intermittent) startup and idling problems by making adjustments to the TPS sensor ..