post #16 of Old 10-12-2009, 01:15 PM
1996BlackXJ
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1996 XJ Cherokee 
 
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I run good ole copper cores. 202,000 miles and going strong.


-Max Black 1996 XJ Build Thread: My Maine-iac XJ part deux
4.5" lift of assorted brands, 33x10.5 KM2s on D-Windows, JCR rear bumper, Nates Slimline w/Champion 8k winch, custom half doors, roof rack, and tire carrier, Maine Woods dents and pin striping, and various other cool things.
www.maine4x4.org Maine Jeep Club Member #1 Firefighter Jeep Club Member Black Jeep Society Member #163Survivalist Jeep Club Member #14
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post #17 of Old 10-12-2009, 09:26 PM
BrunoS
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Quote:
Originally Posted by colt933 View Post
The RN14YC plugs that I bought in response to this thread - to smooth out my rough hot idle - do not fit my '01 XJ. They require the large size socket and I cannot physically install them in the XJ as the large size socket will not slip down inside the recess in the motor.

Is this particular to the late model XJs?
I'm glad that the autolite 3924 are working good for those who have gone to them, and work well for overall well running 4.0 motors that have a hot idle "miss" due to reasons i've run in the thread.

as for the RN14YC, they are spec for 4.2 and early 4.0 amc/renix motors.

tmk, the 3924 autolites will fit all the heads, and am not too shure about the champion diameter size of socket needed.

the 3924 are the old school skinny socket, the champion truck "fatties" use a bigger socket. (same thread pitch and thread total length, of course)

are you shure you have the right thread size and length? compare both the RN14 and the 3924 autolites to see what i'm talking about.

Here's another tip: IF the autolites 3924 are running well but when you pull them they are super clean....go down another heat range step. in the autolites, it's the last two numbers of the 4 digit plugs that as you go down one number, it's one step "colder" in how the plug runs. that is, it moves heat quicker from the tip of the plug to the "colder" cylinder head.

in that case, you could go to 3923, but this will depend on how much air in/ exhaust out upgrades you've done, such as a high flow cat/ muffler / cold air intake/ bored throttle body where this can be done.

Newly Painted! Emerald Green '96 XJ 4x4 4.0 HO 5-spd
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post #18 of Old 10-12-2009, 11:25 PM
Carves
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Wellllll ..... Im confused ....

RC12LYC is whats on the engine bay stickers of all the ones I play with.

OE now specifies NGK in the later models and since I always preferred NGK over Champion Ive been running ZFR5F plugs for a while now anyway.

Seeing all the excitement on this forum about the AutoLites .... Im thinking of giving them a go on the next service.

Interestingly, the recommended AutoLite plug is the 985 ..... no mention of the 3924 except for the V8's and V6's ?????

According to a supplier the 985 is a copper electrode with a projected tip - whereas the 3924 doesnt have a projected tip ....

Am I missing something ... regarding using the 3924 / 3923 in the 4.0l ?

End result I am looking for - is the correct length/type plug with a lower heat range to trial.

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post #19 of Old 10-13-2009, 07:28 PM
BrunoS
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yes, champ "12" series are on the engine bay stickers.

yes, the 985 autolite are the "long reach" copper core specd' to cross reference the champion OEM 12 series.

However....on a well tuned, good running/no oil burning 4.0 of the HO variety...that has a mysterious fully warmed up what i call "hot idle miss" that can NOT be traced down

... going to the old school autolite 3924 series (copper core, long reach...but has shorter poreclain and the tip runs cooler than the 985 with the looooong porcelain insulator) has resulted in smooth idle.

long ago...the 3924 style old-school plugs were spec in the 4.2 and older 4.0 amc motors and do run well.

twould seem that the 12 series plugs run really hot in the 4.0 and start to miss at idle once the engine is fully warmed up.

Newly Painted! Emerald Green '96 XJ 4x4 4.0 HO 5-spd
3" Lift + 31" Tires / D35 TracLok / NP231
Drilled out Stock Airbox / Gauge Cluster Upgrade
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post #20 of Old 03-18-2015, 09:45 PM
austinaubinoe
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Hey I know this is a very old thread, but great info here! I would recommend running a Champion double platinum #7034 on a 4.0L with the coil rail ignition. The problem is when they messed with the engine in 1999 to meet emissions it was detuned. To retain HP they added the better intake and a hotter spark. Im sure this is simplifying it a alot, but basically if you put a 2000-2001 short block in an older cherokee you have a less powerful motor.

I had an issue 2 years ago at school where my jeep kept stalling out. I replaced the air idler control valve and the plugs. The plugs were champion coppers with less then 5k miles on them and they were worn out already. Its been running great since with the double platinums. Cant say for sure if that was part of the stalling issue.

So cant say for sure if all of this is true, that is just what I was told once and it worked well for me. When I get another XJ with a distributor I will be trying those autolite 3924s, CHEAP!
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post #21 of Old 03-19-2015, 08:14 AM
1stockjeep
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Quote:
Originally Posted by BrunoS View Post
well, truth be told... i never noticed a difference. perhaps they will be rebranded.

just look for a truck plug in autolite 3924 equivalent , ask for what they have for that, that shows up in a champion truck plug.

why? 'cause the autolite 3924 is the original design style plug for the 4.2 and 4.0 motor...old school...before they spec'd the autolite 895 "hotter" tip style plug. problem is the 4412 champs, and RN12YC plugs and similiar are too hot a plug for a well tuned 4.0, and you get a hot idle "miss" that the 4.0 motors are famous for...caused by the incorrect plug being listed on the tables for the motors.

as for that, the older champ RN14YC plugs were spec and solved that problem (they are "colder"), so those and autolite 3924 are the same. go check them out at the shop counter!
Never heard of the hot miss but I'm gonna pull the plugs and check the number cause I have rough warm idle
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