Okay, maybe I'm missing it, but I cannot find what the gaps are supposed to be in the FSM for an 2005 4.0 liter. Please help me out!
What is the correct spark plug gap for a 2005 TJ 4.0?
- Thread starter Long Rider
- Start date
.035 for the older TJs with a distributor cap, .04 for those with the coil rail since it generates a bit hotter spark for each spark plug.
The factory specified spark plug gap for the 2005 TJ with 4.0 engine and the factory specified NGK ZFR5N spark plugs is 0.035" which can be found on the emissions decal under the hood.
Page 8I-3 of the Factory Service Manual lists the acceptable spark plug gap range for the NGK ZFR5N spark plugs in the 4.0 engine as 0.30-0.35".
Page 8I-3 of the Factory Service Manual lists the acceptable spark plug gap range for the NGK ZFR5N spark plugs in the 4.0 engine as 0.30-0.35".
Hmm.... Under the hood of my 05 it says .035. The ones I took out (original Mopar's) at 90K gapped at over .080 and it still ran just fine. It runs smoother with the correct (according to the plaque under the hood) gapped new plugs.
Head Lice
TJ Addict
So which is more likely incorrect, an underhood decal or the factory service manual? Does it really matter?
The reason for bigger .04 gap per the FSM in newer TJs with the coil pack is the hotter voltage at the plugs when compared to the older ignition coil system.
Head Lice
TJ Addict
So which is more likely to be the reference for the 'backyard-side of the road' mechanic?
Just informing folks as to what the factory printed on their under hood decal on my '04 . . .
How fortuitous for OP that the 2005 under hood emissions decal and the 2005 factory service manual both specify a spark plug gap of 0.035" for the 4.0 engine, particularly because OP drives a 2005 with a 4.0 engine.
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Note on edit: The 2006 under-hood emissions decal and 2006 FSM also specify a spark plug gap of 0.035".
Interestingly, Autolite's website specifies a gap of 0.034" for the popular APP985 in the 4.0 motor for the 2004-2006 model years while NGK's website specifies a gap of 0.036" for the factory installed ZFR5N for those years.
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Note on edit: The 2006 under-hood emissions decal and 2006 FSM also specify a spark plug gap of 0.035".
Interestingly, Autolite's website specifies a gap of 0.034" for the popular APP985 in the 4.0 motor for the 2004-2006 model years while NGK's website specifies a gap of 0.036" for the factory installed ZFR5N for those years.
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I'd say that the "hotter" ignition system can drive gaps larger than .040. When I recently swapped plugs on my new to me TJ the originals I took out were over .080 and it was running fine. It runs better now but there wasn't any noticeable issues with it before either.
The Fram/Autolite technical trainer I know, Jay Buckley, just replied back to a text I sent him last night seeking his insight. He taught it as the coil-on-plug design puts out significantly a higher voltage so the gap can be opened up more to produce a hotter spark which improves combustion which helps to lower emissions. That's why the recommended gap in the Factory Service Manual went from .035 to .040.
I don't doubt this. I wonder if I should pull the plugs I just put in and gap them up to .040. Maybe I'll do it after the first tank of gas at .035 and see if there is a difference in efficiency that is noticeable.
Thanks for the clarification.
I personally wouldn't spend any time pulling existing plugs and regapping them .005 wider. I'd only do that when installing new plugs.