wenglish

My son has a 2000 Cherokee 4.0.
The issue is it takes a few seconds to start when cranking the engine.
I checked the fuel pressure when running and it is good, however when I shut off the engine the pressure starts to go down immediately and within about a minute is well below 20psi.
I understand it could be a check valve in the fuel pump or it could be a leaking injector but how can you check for either one?

MtnHermit

Use a "Poor Man's Prime", turn the ignition on a few times before engaging the starter motor, should start quickly. My 1997 XJ does the same thing.

Clearly a check valve is not working properly, but replacing that fuel pump is way too expensive.

wenglish

Tried the "Poor Man's Prime" and that didn't seem to help at all.

tjwalker

You definitely have a fuel pressure leakdown. Pressure should not drop to below 30 psi for at least 5 minutes. Your engine doesn't meet that spec.

When the engine does start, does it start and run "cleanly" or is there a stumble/miss for a few seconds and then runs clean? If it stumbles upon starting, that is a big clue that you have a leaking fuel injector and the stumble is the engine clearing out excess fuel.

I still think that one of the check valves may still be in play here. Try the poor mans prime THREE times, then crank the engine. Sometimes a single prime is not enough to generate enough pressure (49 psi) to start the engine. More on this.
----------------------------------------
The check valves (there are two of them; one on the fuel pressure regulator and another on the fuel pump) are part of the fuel pump assembly on 95.5-2001 vintage 4.0 XJs. When a check valve starts to fail, it can reduce fuel pressure to a point where an extended cranking time 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) first step in helping you 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 steps 1-3 two more times
5. NOW crank the engine over

If the engine starts quickly and cleanly after performing this procedure, you may have a check valve issue. If it starts and runs poorly for a few seconds, it could also be a leaky fuel injector resulting in the fuel pressure leaking down and the subsequent 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 starting symptoms as well. Have your battery load tested (any parts store will do this for free) if you have any doubt; your XJ does NOT like low available battery voltage. Be sure battery connections and posts are CLEAN.

Some troubleshooting tips for extended crank times. You will need a fuel pressure gauge. Many parts stores will rent you this tool inexpensively.

*Hook up fuel pressure gauge on the schrader valve on the fuel rail under the hood

*Start engine and bring to normal operating temperature.

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

*Shut engine off.

*Observe pressure on gauge. Pressure should not fall below 30 psi for five minutes.

If pressure falls below 30 psi, it must be determined if a fuel injector, a check valve within the
fuel pump assembly, or a fuel tube/line is leaking. An adaptor tool/hose included with the fuel pressure gauge can help you with this. Consult the manual that comes with the fuel pressure gauge and the Factory Service Manual for your year XJ for more information on this exact procedure, 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 – most likely a check valve. If the pressure falls below 49 psi fairly rapidly then the problem is probably a leaky fuel injector.

**Note. Where check valve is suspect, a quick loss of fuel pressure is often the fuel pressure regulator check valve and a slow loss of pressure is often the fuel pump check valve. But keep in mind that either or both check valves can be to blame.

You can limp a check valve problem along indefinitely if you like, as it doesn't mean the fuel pump itself is going to fail. Perform the “poor mans prime” before cranking to speed up your starts. However, if you have a check valve issue and want to resolve it, it is recommended that you replace the entire fuel pump assembly for two reasons. First, you have to drop the gas tank to access the assembly. Secondly, with two check valves, replacing just the fuel pressure regulator where one of the check valve resides may or may not resolve your problem; remember there are two check valves; the other one is on the fuel pump.

The fuel pump assembly consists of fuel pump, fuel pressure regulator, fuel gauge sending unit, fuel gauge float, pickup filter and pigtail wiring harness. If you do replace the fuel pump assembly, purchase a high quality unit; you do get what you pay for here. Some cheap aftermarket assemblies have been known to fail prematurely. I personally prefer Bosch fuel pumps as they manufacture the best fuel pump assemblies for the XJ. Carter is my second choice. Airtex is my least favorite choice and I will not recommend or install them.

macasylum

You need a little patience when you turn the key on and off. The fuel pump runs for about 2 seconds when you first turn the key on. Takes a little while to get the fuel pressure back up. My 2000 it was both the check valve in the fuel module and a leaky injector. Also can be a leaky fuel line. You need to isolate the fuel rail from the fuel supply line during the pressure bleed down test and you need adapters and a test gauge because all the lines are steel and they dont pinch very well. If you want to fix it start with the injector upgrade and then the fuel pump module. There is some question on where exactly the check valve for the bleed back is so replace the complete fuel pump module if you do it.

macasylum

and all that other stuff TJ said.