96 ZJ 4.0 - Misfiring and runs rough at idle and acceleration - JeepForum.com
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post #1 of 3 Old 08-21-2013, 12:15 PM Thread Starter
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1996 ZJ 
Join Date: Dec 2009
Location: Columbus
Posts: 51
96 ZJ 4.0 - Misfiring and runs rough at idle and acceleration

Greetings. 96 limited with the 4.0. I'm having misfiring issues. It all started after I neglected to replace my worn motor mounts. I have an eBay APN style header and it would hit the motor mount bracket. I have since replaced the driver side mount and at first, I thought that was main reason for all of the vibration inside the jeep. At one point, I had p0304 code but it hasn't came back. All of the following relevant parts have been replaced in the last year:

- New Reman Distributor
- Cap, rotor button, plugs and wires.
- MSD Coil
- Injector at #4 (This was replaced out of desperation)

A few more things to point out. I have compression tested the #3 and #4 and got 40psi. Today, I'm going to check the rest of the cylinders to see if its a universal 40, so I'll let you know. Today, I ran it up to temp and noticed bubbling at the intake/exhaust manifold gasket. Would it be plausible that my intake gasket is toast causing the loss in compression? I'm thinking that when that exhaust manifold was hitting the motor mount bracket, it may have caused stress points on the gasket. I could be wrong, but just throwing it out there.

I have also noticed a loss of coolant; however, I think that may have to do with the water pump as I noticed dripping near it. I also fear it could be the valves or piston rings. Can anyone shed some light on my situation? This has been the worst ZJ I have ever owned. Always something with this one.

Thanks guys!

1996 Grand Cherokee Limited (ZJ) | 4.0 L6 | 42RE | NP242 Transfer Case Swap | 2" Rough Country BB
1996 Grand Cherokee Laredo (ZJ) | 4.0 L6 | 42RE | NP242 Transfer Case
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post #2 of 3 Old 08-21-2013, 06:10 PM
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1997 ZJ 
Join Date: Dec 2008
Posts: 8,997
compression has several ways to escape depending on what is leaking. into the coolant system, into the cylinder next door, into thin air, out the exhaust, into the intake, or into the crank case.

no need to chase any problem until you get the compression questions solved

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ZeeJay1997 is offline  
post #3 of 3 Old 08-21-2013, 10:34 PM
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1997 ZJ 
Join Date: Aug 2008
Location: NH
Posts: 293
To be honest you shouldn't blame the machinery for poor owner maintenance. With a loose motor mount the vibrations could have caused many problems with various components under the hood including the water pump. Unfortunately it sounds as if the vibration may have damaged your PCM, their pretty damn fragile (I've replaced two already). I am currently repairing a very similar problem. I had the misfire on injector 4 code as well (still do) and it WAS running very rough. I spliced the signal wire (wire coming from PCM) to injector 5 after performing various troubleshooting. To troubleshoot it, simply unplug the misfiring injector plug, if the engine doesn't bog down, remove injector 5's plug (it will bog down) then plug it into injector 4. If the engine Idle improves, the injector is good. If so, take injector 4's plug and now plug it in injector 5. If no improvement the PCM isn't properly sending signal to the injector. At this point, you have two options. You can try to replace the injector plug (sometimes the go bad) but I would suggest tracing the signal wire to the computer and putting a volt meter from the signal pin on the PCM, and to a ground. Now compare with a working injectors terminal. If different....you have a short/bad PCM. As a temporary fix splice the injector 4 plug signal wire with injector 5's signal wire. (This will make the Jeep run MUCH better but not its best. Also CEL will remain on). This only works in I6 motors since the fire in-line. The timing will be off slightly so she will still run sluggish but much better. To fix the PCM simply go to junkyard and match your PCM's model number with another computer and do a bolt on replacement. Note that 1998 and later models need to be brought to the dealer to be reprogrammed/flashed to suite your vehicle and update VIN# information. Older models do not require this. Good luck, and in the future if something as important as a motor mount is going make sure to replace it right away otherwise you run the risk of damaging other parts. Jeeps are resilient but not invincible.

1997 Jeep Grand Cherokee 4.0L I-6 150,600
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