smtippin

For the record, gap your plugs to .035". The gapping tool is $1 so you have no excuses.

I just did an ignition tune up and my old plugs (cheap autolites) were gapped at .065" . What does plug gapping this far off do to our inline 6s? I couldn't believe it!

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tjwalker

I hope you replaced those old Autolite plugs, rather than gap them.

As plugs age with miles driven, the electrode starts to disintegrate with the kazillions of firing impulses imposed on it. That increases the gap.

I recommend a 30k replacement interval on regular copper plugs. Verify plug gap is at .035 before installing them; never trust the gap out of the box, even though it is often correct. Why take the chance that it isn't and the only way to know is with a gap tool.

adkodak

I recommend, as does most people I have talked to, using NGK plugs. I ran Bosch +4 for a long time and when I switched to NGK I noticed a massive improvement. Autolite are cheapo plugs. Why run cheap parts in a great vehicle?

smtippin

I can appreciate the comments so that this can be a great resource thread for anyone searching the term, but that doesn't address my original question. When I did the ignition tune up I replaced the plugs, wires, and distributor cap and rotor. I gapped all fresh plugs and double checked them before installing. My question is not about service interval or why the gap increases, but rather what does such a large gap do to our inline-6s (e.g., misfire, retard timing, etc.)?

Thanks for the comments.
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fishtaconc

Can/will lead to a misfire because the spark has to jump a longer distance, which cause a voltage drop therfore a weaker spark. This is true for all engines. there are some execptions like high(er) voltage coils, where your trying to get the spark further into the combustion chamber but its benefit is nominal.

adkodak

Sorry missed that original question. Fish is right it will jack with the engine timing.

Gorillaxj

causes a weak spark. lack of power and could lead to an incomplete burn which is a light misfire.

I have seen in books that champion plugs are good... i only run NGK and Denso plugs in all my cars. NGK and Denso have amazing quality control and make great products.

djb383

Was this a gap job done by the PO?......or had the plugs been in so long the gap had spread/worn to .065?

A wider gap exposes more spark/flame kernel to the fuel mixture in the combustion chamber BUT, you've got to have an ignition system that will consistently/effectively jump a .065 gap. Some ignition systems are designed for a larger gap, some are not. The XJ's ignition system is designed to consistently/effectively jump a .035 gap, little more.

smtippin

DJB383,
I have no idea what caused the gapping to be so large. My thought is that the PO just slapped plugs in without gapping them, but only becuase the old plugs were all .065" with little to no variation. Reading the plugs they didn't look too bad (a bit rich and old but nothing to suggest they were in long enough to cause major issues). I don't have any history on this vehicle and have only driven about 10k. This was the last step on a fairly major preventative maintenance list (except for brake change and fluid flush but I'm waiting to see if I can replace the dana35 without dumping money into it first). It runs better after the ignition tune up, which also included all new o2 sensors.

I was just wondering about gaping being off so far. Shoot, it was nearly 2x the gap spec. That is nuts.

s