I have a 1989 Jeep Cherokee XJ 4.0. a couple weeks ago it idled for about 10 minutes and when I went to leave it's sputtered like it wasn't firing or lack of gas. I pulled over put it neutral brought the RPMs up a little bit everything was fine went on about my business drove it a couple more times and it's sat for about a week. I fired it up a couple days ago to let it warm up for about 15 minutes and I went to leave and do the same thing and I pulled back in and it won't stop sputtering. I unplugged spark plug wires and the only one that didn't make the engine run any more rough was number five. I pulled the plugs out and they all look bad so I did plugs, distributor cap, rotor button and wires. It's still doing the same thing. I put a spark plug tester on there and one two three four and six are all sparking but number five was very erratic and the light in the tester was kind of dim. I apologize for the long message but I figure I need to describe it well. Any help is greatly appreciated

 

Weak compression generally causes this. Unplug the injector to #5 and shoot a little oil in the cylinder to build compression back up. Keep the injector unplugged and try the ignition test again.

 

Or a compression check. That would make more sense.

 

Yeah a compression testis going to happen after work today. What should the pressure be?

 

120-150 psi with no more than a 30 psi variation between cylinders.

 

And just to be clear, you are not testing just #5, you're testing all of them.

Not a bad idea to have a battery charger handy....

 

Keep in mind that fuel dumping in the cylinder will cause low compression. That's why I said to unplug the injector.

 

not sure how you tested your spark

If you used a true external "spark tester", the internal compression of the engine will not affect the spark ?

spark to each plug is controlled by an individual transistor in the PCM, they are known to fail eventually

not to say you shouldnt do a compression test

all my old tune up books, the procedure starts with the words " perform a compression test"

 

Sorry for not getting back. Past couple weeks has been hectic around here I'm just getting around today to doing my compression tests but I just tested spark with one of the regular spark plug testers from the auto parts store. One that plugs from the cap to the spark plug

 

Compression test came out good. All 6 were 120-150. numbers 4 and 5 were in between 125 and 130, numbers 1 2 3 and 6 we're all around 135

 

Swap injectors around and see if the problem follows the injector.

 

I was in the process of doing that yesterday evening but the rain got me. I'd love to find out that it's just an injector

 

Well a single dead cylinder can only be one of a handful of things. Plug, wire, cap, compression, or injector (to include injector pulse). That's really all it could be if it's specific to one cylinder.

 

Just now had time to get back around to it. I got a handful of other things I'm working on as well. I switched to injectors around and the problem did follow the injector so at least I know what needs to be changed now. I really appreciate all the help