So while troubleshooting a drop in gas milage and some other symptoms, I checked the fuel pressure on my 01 xj. The factory service manual calls for ~49psi +/-5psi and mine was at 47psi, so good there.

The problem is that after shutting the engine off, the fsm says "Pressure should not fall below 30 psi for five minutes". Mine dropped slowly to 17psi after 5 minutes (performed multiple tests same number each time).

I tried swapping my injectors because I have the stock one laying around and figured it might be a leak in the injectors. The pressure now drops down to 22psi (multiple tests) after 5 minutes since shutting the engine off.

So I am just wondering if maybe the fsm is stating that the total drop should not be less than 30 psi (47psi-17psi=30psi drop meaning I am good), or am I out of luck and in need of a new fuel pump assembly?

 

Your fine it still starts and runs good don't worry.Dropping mileage can stem from colder weather to type of gas and where you get it,all gas isn't created equal you know.Plus how you drive also.

 

Yea that is what I am probably going to do. I have been trying to go through a full list of possible causes to about 7 different symptoms I started having about 2 months ago though including: difficulty starting sometimes, loss of power in acceleration, decreased gas mileage (17mpg every single fillup to 11mpg every time), and a ticking noise. I have solved most of the lack of power by sealing a small vacuum leak and the ticking/difficulty starting was fixed by replacing a bad cam position sensor/oil pump drive and re-gapping my spark plugs. Gas mileage is up to about 14mpg every fillup, but I just want to cover every base to keep this thing running in top condition as I only have 88k miles on it and want to keep it for another 200k.

 

The check valve is located on top of the gas tank and is part of the fuel pump assembly on 95.5-2001 vintage 4.0 XJs. When the check valve starts to fail, it reduces fuel pressure to a point where an extended crank is needed to start the engine. It is a very common failure. I experienced it on my 99 at around 75K.

A good (and simple) way to troubleshoot this problem is called “the poor mans prime”

1. Turn key to ON position (do not crank the engine!)
2. The fuel pump will energize and run for a couple of seconds
3. Turn key to OFF position
4. Repeat above a two more times
5. NOW crank the engine over

If the engine starts quickly and cleanly after performing this procedure, you may have the check valve issue. If it starts and runs poorly for a few seconds, it could also be a leaky fuel injector resulting in the bleed down and the stumble upon startup is the engine clearing the excess fuel that has leaked into the cylinder because of the faulty injector.

Also be aware that a bad battery can give you this symptom. Sounds strange but it definitely does happen. Have battery load tested if you have any questions; your XJ does NOT like low available battery voltage. Be sure battery connections and posts are CLEAN.

Some troubleshooting tips for extended crank times:

Hook up fuel pressure gauge on the schrader valve on the fuel rail

Start engine and bring to normal operating temperature.

Observe test gauge. Normal operating pressure should be 49.2 psi (plus or minus 5 psi)

Shut engine off.

Pressure should not fall below 30 psi for five minutes.

If pressure falls below 30 psi, it must be determined if a fuel injector, the check valve within the
fuel pump module, or a fuel tube/line is leaking. An adaptor tool/hose included with the fuel pressure gauge can help you with this. Consult gauge manual for more information on this but here is basically how it works:

*Turn the engine off and immediately clamp the fuel line at the adaptor hose. Watch the pressure gauge and see how long it takes to lose pressure.

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

You can limp a check valve problem along indefinitely if you wish as it doesn't mean the fuel pump itself is going bad. Normally, the check valve is replaced as an entire assembly as you have to drop the gas tank to access the assembly. Fuel pump assembly consists of fuel pump, regulator, check valve, filter. If you do replace the fuel pump assembly, purchase a high quality OEM style unit. Some cheap aftermarket assemblies don’t hold up and fail prematurely. I specifically seek out Bosch fuel pumps as they are the best in the business. Stay away from Airtex, as IMHO, they have a bigtime reliability problem.

 

Yup right from the book is what he wrote and it's all true,save up for the parts and do the swap if it does get worse or prove's to be the problem,like he typed from the book the fuel filter and fuel pressure regulator is a replaceable part of the fuel pump assembly , you don't need to pull the whole assembly to remove this part on the 1996-01 xj's good luck.

 

As you mentioned tjwalker the fsm's wording is, "If pressure falls below 30psi" there is a problem. Mine fell exactly 30psi and no less when doing the test, so that is why I thought maybe the manual worded the statement incorrectly. But yea that is highly improbable. So most likely I do have either a check valve problem or fuel pump issue. I will probably replace that whole unit like tjwalker mentions since I have also heard its a bad idea to replace single components and my fuel level indicator is off so its probably part of that unit. Just to be sure though:

-I will do the "poor man's prime" to test the injectors in there and also clean/test the 4 hole injectors I used to have in there before putting them back in.

-I will get under the jeep and check out all the fuel lines with the jeep running to look for leaks.

Thanks guys.