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

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Unread 10-12-2009, 09:26 PM   #17
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.
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Unread 10-12-2009, 11:25 PM   #18
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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Unread 10-13-2009, 07:28 PM   #19
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.
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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
Magnaflow Muffler / High-flo Cat / MSD Coil
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