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misfile cylinders 1 and 6 with full tank of gas

1K views 13 replies 5 participants last post by  CCKen 
#1 · (Edited)
misfire cylinders 1 and 6 with full tank of gas

I think have has some type of fuel tank venting or check valve issue. If anyone could help point me into some troubleshooting pointers.
I have a 2000 Jeep Cherokee with a straight 6. When I fill the fuel tank up with more than 3 gallons of gas the engine runs like garbage and I get engine codes of P300, P301, P306.
I recently purchase the jeep about 2 months ago and just found out where the misfire was coming from but I haven't been able to find what part of the fuel system is causing it
I have done the following:
I have ran with full tanks of gas with 87 oct and 91 oct(ethanol free) and misfires still happened
Replaced Spark plugs with Champion RC12ECC
Replaced Fuel injectors with BOSCH but it was also doing it prior with stock injectors.
I have checked the spark on the spark plugs with spark plug checker and that is fine.
just to make sure the spark plugs or Fuel injectors are still not the issue I swap spark plug 1 and 2, problem still existed on cylinder 1. Then I swapped injector 5 with 6 and problem still existed on cylinder 6.

Just to repeat the engine runs fine with only 3 gallons of fuel no misfires occur. I can repeatedly fill up with 3 gallons of gas and not a single misfire. Once I fill up with 4 or more gallons, when I start the engine right at the gas station it starts engine sounds bad and about a mile down the road I will get misfires.

Any help would be appreciated.
 
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#2 ·
Have you tested the fuel pressure ? It should have 49 +/- 5 psi of fuel pressure.
 
#4 ·
A misfire is usually either ignition related or fuel related. Since the symptoms come and go based on fuel level, the fuel pressure should be checked. If there is a fuel delivery problem such as a clogged filter, tank contamination, or a loose pick-up tube the pressure will likely be out of specification.

Most auto parts stores will rent a fuel pressure gauge.
 
#5 ·
I agree with Tim that fuel pressure is next up. Yes, there are a few possibilities, but this is a "process of elimination" and fuel pressure is very easily checked. I'll guarantee you it is one of the first thing that would be tested if you were to take your XJ into a good technician.

Call around to your local parts store to see who will rent you a fuel pressure test kit. It will be very inexpensive to rent this tool and they are easy to use.

Keep us updated with what you find.......
 
#6 · (Edited)
Ok. So I did the fuel pressure and check valve test. They both passed. When I had the pressure gauge connected and engine was idling the pressure was at approx. 49. I then increased the idle and it didn't drop below 48. I then turned off the engine and let it set for 5 minutes, after the 5 minutes the pressure was still at 49. It didn't start to lower until 10 minutes and when I came back after 30 minutes, it was down at 30. Which according the service manual is normal.

My friend pointed out I don't get the misfires unless I am driving. I haven't seen misfires while sitting at idle even if I rev the engine. So he was wondering maybe if the fuel pump is still and issue but not until normal usage.

I have some pictures and video if anyone wants to see them.
 
#10 ·
misfire cylinders 1 and 6 with full tank of gas

You may find this interesting - read about the OBD II Emissions Monitor under Fuel Gauge Sending Unit. From the 2000 FSM.

You fuel quantity sending unit may be dicking up your PCM/CCD Bus [?]

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This does sound like why it might not throw any error codes because the fuel being low but when it does have the misfire I can actually feel the performance difference in the drive of the engine. So even if it isn't sending error codes because of the ODB II Emission Monitor it drives as it should.

If I reset the battery on a full tank it drives like is should and I don't have any misfires recorded. I know that is because the computer is figuring out the proper air to fuel ratio and once it figures that out it starts to misfire. So could I have a vacuum leak which causes a improper calculation of fuel to air ratio? I've thought about that but if that is cause would having less than 3 gallons of gas actually cause the engine to not misfire. I can see what you found here to point out why it might not display the errors but the actual performance of the engine is as it should be. I purchased a vacuum gauge and I am going to plug that in to see what I is displayed. I will take a video of the results and upload to help troubleshoot.
 
#9 ·
The coil rail is new and I actually tested both the ohm's of it and the length of the spark. I purchased one of the those spark testers that plugs into the connector boot and then you ground it and change the threaded screw to test the spark distance. The Ohm's of the coil rail was in specs and the spark distances was correct.
 
#11 ·
What part number do you have on your PCM?

2000 PCM w/4.0 and AW4, Federal emissions is 56041 634AE (or higher rev). w/California emissions it's 56041 635AE (or higher rev).

If you have a scanner that displays live data, check the O2S operations and the short term fuel trim, when it runs smoothly and when it runs poorly. This will tell you if it is running rich or is running lean.
 
#13 ·
Soundline - Plugs are brand new Champion RC12ECC.

Here is the video of the vacuum test. https://1drv.ms/v/s!Anj41Soe0dYOo4pffyNnmaJubSWxfg

also while I was looking around next to the air intake I saw this rubber line not connected to anything but I traced it back to rear of the car but it had a weird plug in in it. Not sure if that plug is suppose to be in it or not. https://1drv.ms/i/s!Anj41Soe0dYOo4phg_KGMu76GqfXCQ

ccken - I know this Jeep was made in 99 and does not follow the Cali emissions. Do you still need me to look at the PCM model?
 
#14 ·
Soundline - Plugs are brand new Champion RC12ECC.

What were these plugs gapped to - .035"? Chrysler recommends using NGK ZFR5N plugs due to the waste spark ignition system used on the 2000/2001 4.0. See Chrysler tech tip below.

Here is the video of the vacuum test. https://1drv.ms/v/s!Anj41Soe0dYOo4pffyNnmaJubSWxfg

That looks like leaking valve guides, bad head gasket, or weak valve springs. If you're inclined to pull the head (for any reason) you should consider replacing it with a new Clearwater head.

also while I was looking around next to the air intake I saw this rubber line not connected to anything but I traced it back to rear of the car but it had a weird plug in in it. Not sure if that plug is suppose to be in it or not. https://1drv.ms/i/s!Anj41Soe0dYOo4phg_KGMu76GqfXCQ

What's the diameter of the tube - 5/8"? If so, it could be the vacuum tube to the Evap canister from a Leak Detection Pump (which you may not have). If it's a smaller diameter tube going to the Evap canister I have no idea what it would be. The breather fitting on the end of the tube is not standard equipment. The only time I've seen that in that area it was the front axle vent tube breather fitting.

ccken - I know this Jeep was made in 99 and does not follow the Cali emissions. Do you still need me to look at the PCM model?

2000 is the calendar year model and will be equipped for 2000 emissions even though it was assembled in 1999. Check the PCM part number to verify you have the correct PCM. It's just a matter of raising the hood and looking at the decal on the side of the PCM.
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