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High Fuel Pressure, Misfires and Low Idle

Old 02-08-2019, 11:18 PM
  #31  
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When you get a chance to look at it. If it won't start...then turn the key to the on position. Note the position of the voltmeter and the fuel level gauge. If they are "dead"...Then this list of symptoms is a hallmark for a failing crankshaft position sensor. Particularly on your year of XJ. (No computer comms, dead voltmeter and fuel gauge, ignition problems after warming up, and misfires). The fact that it is not starting now, might just mean it has finally completely failed.

I wonder if the mechanic scoped the cam and crank sensor signals while the problem was present.

Any crankshaft or camshaft sensor replacements in the recent past?
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Old 02-09-2019, 12:42 AM
  #32  
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Originally Posted by jordan96xj View Post
When you get a chance to look at it. If it won't start...then turn the key to the on position. Note the position of the voltmeter and the fuel level gauge. If they are "dead"...Then this list of symptoms is a hallmark for a failing crankshaft position sensor. Particularly on your year of XJ. (No computer comms, dead voltmeter and fuel gauge, ignition problems after warming up, and misfires). The fact that it is not starting now, might just mean it has finally completely failed.

I wonder if the mechanic scoped the cam and crank sensor signals while the problem was present.

Any crankshaft or camshaft sensor replacements in the recent past?
Yes I recently did a crank sensor change with the mopar part. I think it's electrical, I've been doing a ton of research. Will let you all know tomorrow
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Old 02-09-2019, 07:17 AM
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Sorry for your headache and frustration and what was a seemingly bad pressure gauge and subsequent misdiagnosis. Jordan gives solid advice and knows his crap. His recent post on troubleshooting a bad ignition coil is particularly helpful. Sorry again.
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Old 02-09-2019, 08:44 AM
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When you went through all that trouble to replace the pump, but it also had the high pressure, that fact is pretty strange. Did you ever get a chance to confirm that pressure with another gauge? or the mechanic?

If they were both truly high, that would be well worth troubleshooting first.
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Old 02-09-2019, 04:27 PM
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FINAL UPDATE (I hope). Went to the mechanic today and started running electrical tests. After a stressful amount of research I learned that no PCM connection usually means a short somewhere in the 5v circuit. The previous owner had replaced the PCM along with a multitude of parts trying to chase down this issue that everyone in the Cherokee forum seems to have. When I purchased it, I noticed the oil pressure sensor was broken which was not a concern since I went through 3 of them on my other XJ. This doesn't effect the performance of the vehicle.......usually. Turns out there is a 5v line going to the oil pressure sensor and guess what? That's where my short was. Removed the connecting harness and the Jeep fired right up. I'm guessing the mechanic must have hit the sensor when installing the new coil rail. So, I'm still not sure if the rail was actually faulty, but I'm just so relieved to have fixed the issue, I do not care. The mechanic was nice enough, despite what happened and let me use some of his tools and helped me relocate a ground.

No more misfire, no more rough idle, purrs like a kitten. RPM is still a bit low (in my opinion) but the engine does not sound like it's struggling whatsoever.

My advice to anyone with the famous XJ misfire problem. CHECK YOUR ELECTRICAL!!!! IT"S PROBABLY A SHORT SOMEWHERE!!!!!!!! I've probably read every single XJ misfire forum out there. Nobody ever solves it because they keep throwing parts at the problem. Nobody ever tests their electrical which is stupid because it's free to do. I should have done that from the beginning but do not be like me. DO NOT THROW PARTS AT THE PROBLEM!!!!!

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Old 02-09-2019, 05:03 PM
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Originally Posted by mattheww97 View Post
My advice to anyone with the famous XJ misfire problem. CHECK YOUR ELECTRICAL!!!! IT"S PROBABLY A SHORT SOMEWHERE!!!!!!!! I've probably read every single XJ misfire forum out there. Nobody ever solves it because they keep throwing parts at the problem. Nobody ever tests their electrical which is stupid because it's free to do. I should have done that from the beginning but do not be like me. DO NOT THROW PARTS AT THE PROBLEM!!!!!
Agreed. Many people get very intimidated by electrical.

As a follow up, do you think the high fuel pressure thing was just a bad pressure gauge? Now that the comms are back online, you could use a scanner to check your fuel trims and double check that it isn't doing anything strange.

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Old 02-09-2019, 05:17 PM
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Originally Posted by mattheww97 View Post
Turns out there is a 5v line going to the oil pressure sensor and guess what? That's where my short was.:
uses 2 different sensors, one for the gauge the other for the dash lamp, make sure you purchase the one correct for the application.

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Old 02-09-2019, 05:46 PM
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Originally Posted by jordan96xj View Post
Agreed. Many people get very intimidated by electrical.

As a follow up, do you think the high fuel pressure thing was just a bad pressure gauge? Now that the comms are back online, you could use a scanner to check your fuel trims and double check that it isn't doing anything strange.
I am also intrigued by this as well... I know that the regulator is not linked to the PCM; however, is the voltage from the pressure sensor tied back to the voltage across the pump?
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Old 02-09-2019, 05:49 PM
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Also, that is awesome that it sounds like you got her fixed.
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Old 02-09-2019, 06:38 PM
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Originally Posted by jburg318 View Post
I am also intrigued by this as well... I know that the regulator is not linked to the PCM; however, is the voltage from the pressure sensor tied back to the voltage across the pump?
If we are talking about a 5v line, then definitely not. Voltage to the pump can easily be measured at the fuel pump relay, just as the amperage can.
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Old 02-11-2019, 03:36 PM
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Originally Posted by jordan96xj View Post
Agreed. Many people get very intimidated by electrical.

As a follow up, do you think the high fuel pressure thing was just a bad pressure gauge? Now that the comms are back online, you could use a scanner to check your fuel trims and double check that it isn't doing anything strange.
I have yet to check the pressure again. I've just been driving it around making sure everything is fine. My wallet is still crying a bit from my money spent on parts, so I didn't want to pull another $150 for the rental in case I break the gauge or something stupid. Once I get my paycheck I think I'm just gonna invest in one. I will let you guys know the readings when I get the chance to check them
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Old 02-11-2019, 05:10 PM
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Originally Posted by jordan96xj View Post
If we are talking about a 5v line, then definitely not. Voltage to the pump can easily be measured at the fuel pump relay, just as the amperage can.
I looked at the wiring diagrams. For anyone wondering on the 00 XJ the 5v line runs through the:

TPS
MAP Sensor
crankshaft position sensor
Camshaft position sensor
Oil Pressure Sensor
Vehicle Speed Sensor

I think these were two unrelated issues or perhaps the short was messing with my PCM and throwing everything off. I haven't gotten the chance to check the fuel pressure again.

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