98 ZJ - Rough idle, cylinder misfire, loss of power, etc. - JeepForum.com

 
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post #1 of 3 Old 07-10-2013, 10:16 AM Thread Starter
HartDave
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1998 ZJ 
 
Join Date: Jul 2013
Location: Macomb
Posts: 2
98 ZJ - Rough idle, cylinder misfire, loss of power, etc.

Hey guys,

Went to drive my jeep the other day and when I would accelerate it would die and seemed to be running rough. Finally a check engine light came on and it read with 3 cylinder misfires and fuel level sensor was bad. I pulled the plugs and the 3 that were misfiring are white (air to fuel ratio is off). Also checked the fuel rail and when i pushed the bleed off nothing came out (no fuel in the fuel rail). So my question I guess is; Would the fuel level sensor cause the cylindar misfires or is it something else? Any help would be appreciated!

(Plugs and wires are brand new I also cleaned the injectors as well)

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post #2 of 3 Old 07-10-2013, 11:04 AM
Kingr98
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1998 ZJ 
 
Join Date: Dec 2012
Location: Madison
Posts: 472
What is the "bleed off" that you referred to? Are you talking about the fuel pressure test port, which is a schrader valve? You might want to hook up a fuel pressure gauge to that test port and see what pressure it reads with the key turned to on.

The fuel gauge sending unit is simply that. It is connected to a float in the gas tank that moves an arm and changes the resistance that is sent to the PCM and then PCM sends a signal to the fuel gauge. Fuel gauge sending units go bad in ZJs all the time. Mine is bad now.

Apparently, it also is used for OBD II emission monitoring requirements. It is to "prevent the OBD II system from recording/setting false misfire and fuel system monitor trouble codes." (from FSM) "the feature is activated if the fuel level in the tank is less than approximately 15 percent of its rated capacity. If equipped with a leak detection pump (EVAP system monitor), this feature will also be activated if the fuel level in the tank is more than approximately 85 percent of its rated capacity."

I am not sure what all of that means, but I do know that the PCM also controls the electrical pulse delivered to the fuel injectors. The FSM only says that the "PCM determines injector pulse width based on various inputs."

Maybe some of the more experienced members will be able to shed more light...
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post #3 of 3 Old 07-10-2013, 12:41 PM Thread Starter
HartDave
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1998 ZJ 
 
Join Date: Jul 2013
Location: Macomb
Posts: 2
I appreciate the imput, yes I was referring to the schrader valve on the fuel rail.

Originally when I bought the jeep not too long ago it would take a while to start, I would have to typically turn the key over twice in order to get it to fire up. So after some discussion with a buddy I replaced the fuel pressure regulator to try and fix the problem. Which in the end didnt cure the problem and now I have this new addition of issues that I listed earlier.

Im going to replace the whole fuel pump assembly and see what that does. Hopefully that fixes the fuel pressure and maybe fix the misfiring problem. My thought is that if the fuel pressure is not right than that could be why the cylinders are misfiring. At least I'm hoping that will fix the issue and its not something bigger.
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