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Misfire & rpm drop HELP

Old 05-01-2019, 05:36 PM
  #1  
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Default Misfire & rpm drop HELP

I have been having a problem with my XJ for about month. It has a random misfire along with when I'm at speed (anywhere from 45-55mph) the rpm will drop from about 2200 to 1400-1500rpms and the motor will have heavy vibration/shake. I've replaced a lot of stuff including pugs, wires, cap, rotor, pick up, coil, iac, tps, map sensor and both O2 sensors. I'm lost with whats going on, wondering if anyone has any input.
Thanks

It is a 96 4.0

Last edited by tslice; 05-01-2019 at 06:14 PM.
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Old 05-01-2019, 06:11 PM
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Use a "testing based strategy" to get to the root cause.

1. Has your check engine light ever been on? If so, you must retrieve codes for clues and guidance on where to look.

2. In the absence of codes, job #1 absolutely is to test fuel pressure. You need to note the year/engine of your XJ with your threads as there are differences in specs through the years. If you post the year, we can tell you the fuel pressure spec. Then get a hold of a fuel pressure gauge (this MUST be done with a gauge) and see where you are at compared to spec. If you don't have access to a fuel pressure gauge, they can often be rented from local parts stores; call around.

I am not saying fuel pressure is your problem. What I am saying is that it must be ruled in or out before moving onto more complex possibilities. Oftentimes, you have to prove out what is GOOD before you can figure out what is FAULTY. And fuel pressure is a low hanging fruit that can easily be tested to rule it in or out.

Last edited by tjwalker; 05-01-2019 at 06:50 PM.
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Old 05-01-2019, 06:12 PM
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Sorry it is a 1996 4.0
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Old 05-01-2019, 06:16 PM
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Originally Posted by tslice View Post
Sorry it is a 1996 4.0
1996 was a year of change for the fuel system of the XJ.

Early 96 XJs have the fuel pressure regulator under the hood (and fuel filter along frame rail below drivers door). The fuel pressure spec for those is 31 psi.Then disconnect the vacuum line leading to the fuel pressure regulator and the pressure should jump to approximately 39 psi. Check for the condition of that vacuum line and for any raw fuel in that vacuum line. If there is any, you have a ruptured fuel pressure regulator diaphragm and must replace it.

If you have a later 1996 model, (without the fuel pressure regulator under the hood and fuel filter along the frame rail), the fuel pressure spec is 49 psi, plus or minus 5 psi.

Test your fuel pressure, compare it to your spec and report back. Good luck!

Last edited by tjwalker; 05-01-2019 at 06:51 PM.
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Old 05-06-2019, 07:29 PM
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I checked the pressure and it seemed to be reading low by about 5-7 psi and after having it running and shut off it dropped 20 psi in less then 10 minutes and with the key on drops from 47ish psi to 38 in about a minute.
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