Fuel pressure/stumbling issue

Finally got around to doing a fuel pressure test on the 98 XJ. Symptoms have been lower mileage, and stumbling on startup for about 15 seconds when sat overnight. Here's what I tested:


1. Connected fuel pressure gauge in the morning without turning key on. 0psi
2. Turned on key, pressure jumped to 32psi.
3. Watched as pressure slowly dropped by 1-2psi in about 2 minutes.
4. Turned off key, turned it back on.
5. Pressure up to 45psi. Down to 44psi in about 2 minutes.
6. Started, slight stumbling for 15-18s, then fine. Pressure oscillating rapidly between 48 and 50psi.
7. Shutdown. Psi at 46psi, dropping slowly as before.


Do I have a leaky injector? Bad check valve in the tank assembly? What should I check next? I have the fuel tester for the next 48 hours.

My XJ ran with 10psi.
I don't think this is your issue

Check your grounds. Change fluids.
Fuel filter if ur really unsure.

I installed a perm gauge for PSI on my fuel rail. Best $20 I Ever spent.

It shouldn't be dropping to 0 though overnight. I definitely have adequate pressure to run it, but want to get to the bottom of the other issues.

Something is leaking down when the pump is not running. Is there a soft line you can clamp from the tank to isolate whether it's the checkvalve in the pump or a leaky injector?

blu3fan said:

Check your grounds. Change fluids.
Fuel filter if ur really unsure.

I installed a perm gauge for PSI on my fuel rail. Best $20 I Ever spent.

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The fuel filter is part of the pump and inside the gas tank.

RCP Phx said:

The fuel filter is part of the pump and inside the gas tank.

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1. Connected fuel pressure gauge in the morning without turning key on. 0psi

He is not saying its at 0 overnight.
He hooked up the gauge. indicating he removed the Schrader.

unless I misunderstood?

I connected the test gauge to the shraeder, being careful not to release fuel. Very very little dribbled out a couple drops) before the gauge was fully on, so the 0psi reading was probably correct

lawsoncl said:

Something is leaking down when the pump is not running. Is there a soft line you can clamp from the tank to isolate whether it's the checkvalve in the pump or a leaky injector?

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I'm not sure I have a soft section of the fuel line, but that's a good one to check. Thanks.

Sadly no, the fuel line is a hard plastic or metal the whole way.

A faulty check valve will cause the starting symptoms described. Test the fuel pressure after 8 hours of not running.


I can't remember if the fuel line near the gas tank is all metal or if I installed a short section of rubber line for the testing.



When a 1997 to 2001 Cherokee won’t start on the first try day after day, the first thing to suspect is a failed check valve on the fuel pump. The fuel pump check valve is supposed to hold pressure in the fuel line while the engine is off. The check valve is located on the fuel pump and located inside the gas tank. A failed check valve allows the fuel to drain back to the tank. This results in one or more of the following symptoms:

SYMPTOMS

• Longer than normal cranking times
• Never starts on first try, almost always starts on the second try
• Rough idle for a few moments of idling
• Rough running for first ½ block of driving
• Little or no fuel pressure at the fuel rail test valve similar to this -

1- Key in on position; fuel pump primes for 2 seconds then stops. 0 PSI
2- Cranking over the engine. 2-3 PSI
3- Engine starts and idles for 20 seconds. < 5 PSI
4- Engine idles and fuel pressure creeps up to 45-50 PSI.

POSSIBLE CAUSES

• Failed check valve on the fuel pump in the gas tank.
• Leaky fuel injector(s) may be the true cause of your problem, but more likely they are simply contributing to the check valve issue.

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 one or more leaky injectors.

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 pump assembly such as Bosch or Carter. Some cheap aftermarket fuel pump assemblies, like Airtex, and Precision don’t hold up and will fail completely within 12-18 months

You must have installed a rubber section, mine is metal all the way, at least as far as I can get my hand near the pump assembly.

No there's about a foot of soft between the frame tube and the fuel pump.

The braided one in your photo? I felt the one on the left and it seemed to be plastic.

Yes, but I wouldn't recommend trying to clamp either back by the fuel tank or in the engine bay. The chances of getting debri in your injectors is really high specially if the hoses are old. Remember the only filter is "in" the gas tank, anything past that could clog the injectors.

Good advice. In the end I didn't bother with that test. I did read the FSM again which states that 0psi overnight is normal. The pressure shouldn't drop below 30psi within 5 minutes of shutdown, which was definitely fine on mine.



The search continues...

asymptonic said:

Good advice. In the end I didn't bother with that test. I did read the FSM again which states that 0psi overnight is normal. The pressure shouldn't drop below 30psi within 5 minutes of shutdown, which was definitely fine on mine.



The search continues...

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Issues on startup for me over the years.


causes I have resolved which fixed it.
Bad grounds
Spark plugs.
Wires.
CPS.
Fuel pump not getting 12v due to bad ground.

I've got a 98 and have successfully (jinx) fixed a stumbling at start up problem by replacing the TPS sensor. Having said this, my stumble lasted longer than 15 secs and seemed to coincide with it warming up. For this reason, I also installed a new CTS (no effect) and any time there's idle problems, at least cleaning your IAC is a good idea (cleaned, then replaced. No effect). I then replaced the TPS and have been gtg since. In terms of it being a fuel pressure issue, my fuel pressure bleeds off more significantly than yours, and in terms of figuring out whether that was injector or check valve related I visually inspected the area around the injectors for leakage (none), excessive fuel smell (none), and checked the exaust at startup for smoke/smell. I'd start here. If no visual/smell fuel related cues, I'd start looking at sensors starting with the IAC valve. Hope this helps!

You can check for leaky injectors fairly easily. I can't remember if the xj has clips to keep them on the rail, but if not you can just zip tie them to the rail. I normally do that regardless. Pull all the injectors out with the rail attached and jumper the fuel pump relay. Do this with the key off. Sometimes the ECM has a feature to "prime" injectors when the key is turned on, so it could be mistaken as a leaker. If one is looking a little drippy, replace them. A hard start/stumbling on initial start isn't always a bad fuel pump check valve. Matter of fact, mine has a bad check valve and it starts up every time without stumbling. This test is easy and might cost you some zip ties and o-rings at worst.

If I do find a leaker, do I need to replace all the injectors to make sure their flow rates match?