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Unread 06-21-2013, 05:28 PM   #1
RxWatcher
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Tune up caused problems?

Ok, so it's been about 3 years since I did spark plugs on my '94 cherokee 4.0 so I figured it was due. No problems with the truck, idles and starts fine, gets up and goes when you step on it, just figured I'd change 'em while I was in the mood.

This time instead of the cheap Champions I went with AC Delco plugs.

So I pull the old ones out noting that they are toasty brown (perfect!), toss 'em away, check the gap on the Delcos (adjusting as needed), screw them down and snug them up, put the wires back on, and fire it up.

Starts right up. Take it around the block. Misfires!

Misfires under load, like if I coast in 2nd to like 10-15 mph and step on it then I get a lot of misfires. At normal speed and free-rev it seemed fine.
Get it home, pull the plugs out, double check gap, put back in, double check tightness, put wires back on. Now it misfires at idle randomly and at random times when I'm driving around.

It's getting late so I park it for the night, and today I got new wires for it (and I opted for the more expensive ones). I figured when the engine shakes/vibrates a bunch and a wire isn't making good contact I get an intermittent spark.
Hook them all up one by one so I don't mess up the firing order, start it up, seems ok.
Take it around the block, seems good.
Take it on the local 45mph streets and it seems fine. Oh wait, no, it just misfired again...


Seems like if I keep the gears, speed, and RPM all in the correct range it's fine, but if I just stomp on it to speed up I can hear and feel it missing. Not every time, though, sometimes it does and sometimes it doesn't.
I can't make it happen. It just decides on it's own.
I can drive for 20 minutes without a misfire, then I'll take a turn, go under a bridge, the sun will come out, then I'll get a whole bunch in a row...



Not sure where to go from this, especially since it didn't have any issues before changing the plugs and wires.

Is it possible I got a bad plug? Will part stores even exchange spark-plugs (especially after they've been used)?


I'm waiting for the truck to cool down right now as I just got back from work some 20+ miles away on the expressway.
I'll pull the plugs and wires again in a few hours and have a look at everything. Triple check gaps, re-grease the electrode ends on the wires, make sure they seat fine...




Anything I should look for?

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Unread 06-21-2013, 07:07 PM   #2
M35A2
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I put ac plugs in mine fir the tune up after I got my truck, used them in all my gm trucks. Turns out Cherokees don't like 'em. Put champions back in and see how it runs.
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Unread 06-21-2013, 07:22 PM   #3
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After many, many, spark plug threads, most experienced 4.0L owners agree that the basic Champion spark plugs give the best performance, smoothest running, and longest service life. The 4.0L has definite likes and dislikes when it comes to spark plugs.
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Unread 06-21-2013, 07:37 PM   #4
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if you bought them at any major parts store then the plugs will have a warranty. take them back and get champions, if that does not remedy the problem then you sir have another problem.
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Unread 06-27-2013, 10:52 AM   #5
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Sorry I did not keep up with a response.


Interesting to know about the 4.0s liking champions.

I did get the Delco plugs to work better after I pulled them all out, put them back in, tightened them up, then used a whole tube of dielectric grease on the wires.
Not perfect but better.


I got the plugs at O'Rielys, not sure that they'd take em back but champions are cheap so I'm not concerned with that.


Thanks for the help!
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Unread 06-27-2013, 04:18 PM   #6
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What's the cap and rotor look like? Due to be changed every 30,000 miles.
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Unread 06-29-2013, 09:44 PM   #7
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Quote:
Originally Posted by cruiser54 View Post
What's teh cap and rotor look like? Due to be changed every 30,000 miles.
Looks good.

No excessive wear or discoloration on metallics, plastic is not stressed or cracked.



I'm sure it's factory original with 170K+ miles on it, but it's fine.
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Unread 06-30-2013, 07:17 AM   #8
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Quote:
Originally Posted by RxWatcher View Post
Looks good.

No excessive wear or discoloration on metallics, plastic is not stressed or cracked.



I'm sure it's factory original with 170K+ miles on it, but it's fine.
I'm glad you're positive that parts that are due to be replaced every 30K miles are still fine after 170K miles. Good luck.
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Unread 07-01-2013, 04:53 AM   #9
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Quote:
Originally Posted by RxWatcher View Post
I'm sure it's factory original with 170K+ miles on it, but it's fine.
I hope you're kidding.

You often can't see cracks in these secondary ignition items. They have a finite lifespan and don't last forever; change them and complete your tuneup with an inexpensive fuel filter.
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Unread 07-01-2013, 07:11 AM   #10
Floreancig_XJ
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Yeah deffinetly the delco plugs

, get yourself a cap and rotor with brass terminals/contacts that with some champs and you already have new plugs should work just fine
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Unread 07-31-2013, 02:53 PM   #11
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Again, sorry for not keeping up.



Changed back to champions, runs like... well, like a champion





Yeah there's no way that cap/rotor is factory, lol.

But is it really due to be changed every 30k? I don't think I've ever known anybody that has changed one that hasn't exhibited signs of damage. Definitely not in 30k miles.
Didn't matter what make or mileage, either the distributor worked or it didn't and it got replaced or it didn't.



Anyways, thanks for the help, guys. Glad I got my truck back to a smooth runner.
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Unread 07-31-2013, 05:25 PM   #12
tjwalker
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Quote:
Originally Posted by RxWatcher View Post
Yeah there's no way that cap/rotor is factory, lol.But is it really due to be changed every 30k? I don't think I've ever known anybody that has changed one that hasn't exhibited signs of damage. Definitely not in 30k miles.
On the 4.0 XJ, I change the plugs (copper) every 30k and I change the wires, distributor cap and rotor every 60k.

Never had a problem with adhering to this maintenance interval.
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