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Unread 01-21-2014, 01:04 PM   #1
uga84
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Location: hope mills, nc
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Fuel Pressure Pictures

Everything seems to be normal...But what do you guys think about the pressure after I key it off?

This is with key turned on.

This is while engine is running.

After running then Key off. After 1 minute it drops to this.

After 5 minutes.

After 10 minutes.


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Unread 01-21-2014, 03:23 PM   #2
Holder350
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That actually pretty normal, do you have issues starting in the mornings?
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Unread 01-21-2014, 04:40 PM   #3
tjwalker
It's the crank sensor!
 
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Quote:
Originally Posted by Holder350 View Post
That actually pretty normal, do you have issues starting in the mornings?
Agreed, you need to tell us what your concern/symptoms are.
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Unread 01-21-2014, 05:03 PM   #4
uga84
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Just after reading it needs to hold atleast 30 psi after being turned off I assumed since mine only holds steady at 22 to 24 psi after shut off and at 20 psi after 10 min..didn't think this was normal? I thought it dropping below 30 psi in a 5 min span was an issue.
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Unread 01-21-2014, 05:09 PM   #5
tjwalker
It's the crank sensor!
 
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Quote:
Originally Posted by uga84 View Post
Just after reading it needs to hold atleast 30 psi after being turned off I assumed since mine only holds steady at 22 to 24 psi after shut off and at 20 psi after 10 min..didn't think this was normal? I thought it dropping below 30 psi in a 5 min span was an issue.
Still haven't answered if you have any actual starting or driveability symptoms?? If not, don't worry about it. Choose your battles when it comes to a 14 year old vehicle and with no symptoms.

You are at about 45 psi with engine running and that is the one data point that is the most important and it is within factory spec. The spec. for the 2000 is 49 psi, plus or minus 5 psi.

You are likely on the front end of a check valve problem within your fuel pump assembly. Like everyone with your vintage XJ will experience. If it gets worse you will eventually experience an "extended cranking time" to start the engine. At this time, you can make a decision on whether to chase it or not. Here's more.

In the meantime, enjoy your XJ!
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The check valves (there are two of them; the primary one is on the fuel pressure regulator and a secondary one can be found on the fuel pump itself) 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 about 2 seconds. Be sure to wait until it stops running.
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 normally 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 check valve is part of 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 assembly. Know this….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. A search will show you why.
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Unread 01-21-2014, 05:15 PM   #6
Mud Pilot
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The only concern I see that if it drops that much in 10 mins then after hours of down time I would see it dropping to 0 psi. Now with it dropping this much it would lead me to believe that your check valve inside the fuel pump is failing. As already asked do you have any starting issues after hours of down time. My 1998 Cherokee was doing the same thing and i let it go as no big deal but after a years time of this my fuel pump failed but I'm not sure if it was the continuous cranking of a dry pump to prime that may have finally taken a toll on it or the mud I found in the bitten of the tank and in the fuel pump pickup filter. I'm going with mud but you never know.

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Unread 01-21-2014, 05:33 PM   #7
uga84
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No issues starting..drive it all day and it fires right up in the morning. It probably is the check valve going bad...mine as well replace the fuel pump assembly while I'm at it. It's not that I am chasing anything...I bought this as a project truck and enjoy working on it. Most people like golf or bowling..I like drinking beer and working on the xj in the garage.
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Unread 01-21-2014, 05:52 PM   #8
Mud Pilot
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Yeah I understand that. I see your in NC too. I'm in Nashville, NC. Both of us are right off 95. Just at different ends.

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Unread 01-21-2014, 05:57 PM   #9
Kalali
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I'm totally with you on the beer angle but don't poke too much into things that are going down their merry way. That said, one area you may want to watch closely or fiddle with is the cooling system, especially if you notice any loss of coolant with no obvious/external leaks. That's when you want to get yourself a case of cold beer and yank that cylinder off before it takes your engine with it....
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