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High Fuel Pressure, Misfires and Low Idle

Old 01-30-2019, 04:54 PM
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Default High Fuel Pressure, Misfires and Low Idle

Hey guys I've been trying to track down a random cylinder misfire (p0300, the usual Cherokee issue) on my 00 Cherokee sport. I did all the basics, spark plugs, crankshaft position sensor, throttle position sensor, seafoamsand only fixed the problem for a little while.

i didntn want to keep throwing parts at it so i figured I should test the fuel pressure. When I released the pressure at the shrader valve, a ton of gas came shooting out for a good 10 seconds. I tested the pressure and it is 55-58 psi at idle and doesn't change when given gas. I read that 49 psi is normal so is this a red flag? I'm not sure what it could be. I was gonna take the throttle body apart and clean it out to see if it helps with the idle. I'm also gonna test the coil packs but this high pressure leads me to believe it's gas related But I'm at a loss much like many people with the misfire problem. I need to get it inspected but it won't pass due to the misfire

Thank you!
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Old 01-30-2019, 04:57 PM
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Perform a compression/leak down test.
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Old 01-30-2019, 05:08 PM
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Originally Posted by Turbo X_J View Post
Perform a compression/leak down test.
What is the procedure for this? I have the regular fuel pressure rental kit from AutoZone. I'm gonna keep it over the weekend
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Old 01-30-2019, 06:50 PM
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Originally Posted by mattheww97 View Post
Hey guys I've been trying to track down a random cylinder misfire (p0300, the usual Cherokee issue) on my 00 Cherokee sport. I did all the basics, spark plugs, crankshaft position sensor, throttle position sensor, seafoamsand only fixed the problem for a little while.

i didntn want to keep throwing parts at it so i figured I should test the fuel pressure. When I released the pressure at the shrader valve, a ton of gas came shooting out for a good 10 seconds. I tested the pressure and it is 55-58 psi at idle and doesn't change when given gas. I read that 49 psi is normal so is this a red flag? I'm not sure what it could be. I was gonna take the throttle body apart and clean it out to see if it helps with the idle. I'm also gonna test the coil packs but this high pressure leads me to believe it's gas related But I'm at a loss much like many people with the misfire problem. I need to get it inspected but it won't pass due to the misfire

Thank you!
Yes, that high of fuel pressure is a red flag. It shouldn't change all that much when you give it gas, as the fuel pressure is supposed to remain relatively constant. But the fuel pressure regulator is probably not doing its job.

Having fuel pressure that is above spec can cause other issues in this case. Most importantly, your PCM (computer) actually has no information regarding fuel pressure, so it makes all of its decisions based on what it is "supposed" to be (around 49). Your computer controls the amount of fuel entering the combustion chamber by holding injectors open for shorter/longer periods of time. So right now your computer is probably trimming back considerably, because based on its normal programming, quite a bit more fuel is getting into the combustion every time it pulses the fuel injectors. So if the vehicle's air/fuel mixture is bad enough that the computer can't compensate for it, it could lead to driveability problems, including misfires. The fact that you are seeing only the general misfire code (meaning misfire detected on all 6 cylinders) makes me think this might be related.

So if I was in your position, I would resolve the fuel pressure problem before chasing anything else. I think the compression/leak-down test recommended above is to get ahead of the possibility that the head is cracked due to the year of your vehicle (that year is prone to head cracks). Which may or may not be the case. So do the test if you can, but regardless the fuel pressure problem is going to cause mixture/fuel trim problems until it is resolved.

On my 96, the fuel pressure regulator is part of the fuel pump assembly inside the fuel tank. I'm not certain that is the same on your year. Perhaps someone with more 00/01 knowledge can provide guidance for fuel pressure regulator on your specific vehicle. I think yours might be accessible separately from the fuel pump.
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Old 01-30-2019, 07:29 PM
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Originally Posted by jordan96xj View Post
Yes, that high of fuel pressure is a red flag. It shouldn't change all that much when you give it gas, as the fuel pressure is supposed to remain relatively constant. But the fuel pressure regulator is probably not doing its job.

Having fuel pressure that is above spec can cause other issues in this case. Most importantly, your PCM (computer) actually has no information regarding fuel pressure, so it makes all of its decisions based on what it is "supposed" to be (around 49). Your computer controls the amount of fuel entering the combustion chamber by holding injectors open for shorter/longer periods of time. So right now your computer is probably trimming back considerably, because based on its normal programming, quite a bit more fuel is getting into the combustion every time it pulses the fuel injectors. So if the vehicle's air/fuel mixture is bad enough that the computer can't compensate for it, it could lead to driveability problems, including misfires. The fact that you are seeing only the general misfire code (meaning misfire detected on all 6 cylinders) makes me think this might be related.

So if I was in your position, I would resolve the fuel pressure problem before chasing anything else. I think the compression/leak-down test recommended above is to get ahead of the possibility that the head is cracked due to the year of your vehicle (that year is prone to head cracks). Which may or may not be the case. So do the test if you can, but regardless the fuel pressure problem is going to cause mixture/fuel trim problems until it is resolved.

On my 96, the fuel pressure regulator is part of the fuel pump assembly inside the fuel tank. I'm not certain that is the same on your year. Perhaps someone with more 00/01 knowledge can provide guidance for fuel pressure regulator on your specific vehicle. I think yours might be accessible separately from the fuel pump.
Thank you for all of the info. I was afraid I might have to do the fuel pump. On the 00, the regulator is in fact part of the pump. I heard it's a ***** to take down the tank, so that should be fun. It's currently 0 degrees here. Is there any other way to test this? Or is there anything else I should test before taking the gas tank down? Thank you
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Old 01-30-2019, 07:31 PM
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Originally Posted by jordan96xj View Post
Yes, that high of fuel pressure is a red flag. It shouldn't change all that much when you give it gas, as the fuel pressure is supposed to remain relatively constant. But the fuel pressure regulator is probably not doing its job.

Having fuel pressure that is above spec can cause other issues in this case. Most importantly, your PCM (computer) actually has no information regarding fuel pressure, so it makes all of its decisions based on what it is "supposed" to be (around 49). Your computer controls the amount of fuel entering the combustion chamber by holding injectors open for shorter/longer periods of time. So right now your computer is probably trimming back considerably, because based on its normal programming, quite a bit more fuel is getting into the combustion every time it pulses the fuel injectors. So if the vehicle's air/fuel mixture is bad enough that the computer can't compensate for it, it could lead to driveability problems, including misfires. The fact that you are seeing only the general misfire code (meaning misfire detected on all 6 cylinders) makes me think this might be related.

So if I was in your position, I would resolve the fuel pressure problem before chasing anything else. I think the compression/leak-down test recommended above is to get ahead of the possibility that the head is cracked due to the year of your vehicle (that year is prone to head cracks). Which may or may not be the case. So do the test if you can, but regardless the fuel pressure problem is going to cause mixture/fuel trim problems until it is resolved.

On my 96, the fuel pressure regulator is part of the fuel pump assembly inside the fuel tank. I'm not certain that is the same on your year. Perhaps someone with more 00/01 knowledge can provide guidance for fuel pressure regulator on your specific vehicle. I think yours might be accessible separately from the fuel pump.
00/01 has the fuel filter/pressure regulator combo unit on top of the fuel pump (return less fuel system) sticking out of the top of the tank. Only way to change it is to drop the tank. Not an hard job unless the tank is full of fuel. 95 and older had the rail mounted vacuum assist regulator with a return line.
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Old 01-30-2019, 07:58 PM
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I just replaced my tank, pump, filter, straps, hangers, and hoses a couple months ago. It wasn't as bad as I thought it would (the job I mean). You can empty the tank easily by running the fuel pump line into a container, and then using a paper clip to jump the fuel pump relay so that it pumps the fuel right into the container. The fuel tank can be held up by a ratchet strap while you deal with the hangers and straps. Once its empty, it can be easily man handled. Its not that heavy. When I was ready to put it back in, I just got the ratchet strap out again and used it to hold it generally in place while I got the hanger bolts and straps in place.

Yours is probably plastic. Mine was 25 year old rusty mess. Nice to see it gone.



ratchet strap holding empty tank while I cut out rusted straps with dremel.


After getting it out.


Wasn't actually leaking...yet. But it was about to for sure. Yes, the 96 has the assembly on the front of tank. Which is pretty awesome. Take that 97+ers.....
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Old 01-30-2019, 08:40 PM
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Originally Posted by downs View Post
00/01 has the fuel filter/pressure regulator combo unit on top of the fuel pump (return less fuel system) sticking out of the top of the tank. Only way to change it is to drop the tank. Not an hard job unless the tank is full of fuel. 95 and older had the rail mounted vacuum assist regulator with a return line.
Do you know where I can find a good replacement pump module? I heard the OEM ones are impossible to find..
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Old 01-30-2019, 08:41 PM
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Originally Posted by mattheww97 View Post
Thank you for all of the info. I was afraid I might have to do the fuel pump. On the 00, the regulator is in fact part of the pump. I heard it's a ***** to take down the tank, so that should be fun. It's currently 0 degrees here. Is there any other way to test this? Or is there anything else I should test before taking the gas tank down? Thank you
Also where did you buy your new pump assembly from?
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Old 01-30-2019, 08:47 PM
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96ers are special in that regard. There aren't many assemblies to choose from. It uses an assembly that is not common to any other year XJ. One of the few one-off parts on the vehicle. So I'm afraid my XJ is not a good guide for you in this regard. Also my assembly is mounted in the front of the tank, where as yours is positioned on top of the tank (sorry for that. lol). Have you tried Amazon yet? I find about 95% of my parts there. If you add your year and vehicle to their garage feature, it can help you search for vehicle specific parts quickly.

I added a 00 XJ to my vehicle list and searched for fuel pressure regulator:

Mopar Fuel Pressure Regulator
by Amazon.com
Learn more:
Amazon Amazon

This is a mopar unit (good). Looks like it can be serviced separately from the entire assembly. Maybe someone can correct me if I'm mistaken (on my fuel pump assembly the regulator is somewhat "built-in" and not so easy to service separately - though some have managed to do it).

Last edited by jordan96xj; 01-30-2019 at 08:53 PM.
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Old 01-30-2019, 09:53 PM
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Woah woah woah .... test the fuel pressure again with another gauge before ordering parts .
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Old 01-30-2019, 10:10 PM
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Originally Posted by GreaseMonkey17 View Post
Woah woah woah .... test the fuel pressure again with another gauge before ordering parts .
What exactly should I be looking for? I attached the gauage to the shrader valve. Turn the key to on, 57psi. Started it, 57si Gave it gas, 57 psi. I left the gauge attached for 10 minutes with the Jeep of and came back to around 55 psi.

I saw a video where someone stuck the gauge into their fuel line?? I'm not really too familar with the fuel systems so how to diagnose it without throwing parts
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Old 01-31-2019, 02:02 PM
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Update

While removing the throttle body I noticed a broken hose coming off of the cruise control module. Could this be causing a vacuum leak, thus a misfire? I have attached a picture

Fuel rail looks pretty busted, and a few fuel injectors are cracked. I did the screwdriver test and they all seemed to be operating normally.

The throttle body has a lot of carbon buildup. I'm going to clean it and reinstall to see if it helps with anything. (Worth cleaning anyway. Once cleaned I'll take before after pics and test fuel pressure again.


Fuel rail looks rusted and some fuel injectors are cracked




Last edited by mattheww97; 01-31-2019 at 02:09 PM.
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Old 01-31-2019, 05:14 PM
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Update #2 Throttle body cleaning made little to no difference but still worth cleaning. I decided to read the fuel pressure again and decided to take a

​​​​​Notice how the needle constantly vibrates but briefly stills from time to time. Would this be a sign of a bad fuel pressure regulator?

​​​​​​
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Old 01-31-2019, 06:32 PM
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Originally Posted by mattheww97 View Post
Update #2 Throttle body cleaning made little to no difference but still worth cleaning. I decided to read the fuel pressure again and decided to take a Video

​​​​​Notice how the needle constantly vibrates but briefly stills from time to time. Would this be a sign of a bad fuel pressure regulator?

​​​​​​
From the 2000 FSM
"Fuel Pressure Regulator Operation:
The pressure regulator is a mechanical device that is not controlled by engine vacuum or the Powertrain Control
Module (PCM).
The regulator is calibrated to maintain fuel system operating pressure of approximately 339 kPa
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."

Cleaning the throttle body shouldn't change a thing I would put my money on a bad fuel pressure regulator. Your fuel trims should be showing rich as well and you should be able to smell it in your exhaust fumes. Although it looks like its cold where you are located which will also make the vehicle run a bit richer.
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