Spark plug gap

J

JMoring

New member
So my sons 97 Wrangler TJ 2.5L started running rough a couple of days ago. While looking under the hood with the engine running, we noticed an intermittent arc coming through the spark plug wire on cylinder 3. I was thinking bad wires. After letting the engine cool, I wanted to check the plugs to make sure they were gapped right. Not by a long shot. All four plugs were gapped three times as wide as factory specs. We set the gap at .035, factory specs, and the engine runs smooth. Now we have a new problem or concern. The engine runs smooth but now it has that that old engine sound. Not clicking or tapping but something like it. Any thoughts?


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A wider gap on the spark plug is probably a BETTER idea if it is a high energy ignition system and all the components are in good shape.

Narrowing the gap back to 35 reduces the stress on the ignition wires (which reduces the tendency to arc-over in places you don't want it to) but is not really solving the problem as it reduces the amount of spark energy delivered to the fuel-air mixture in the cylinders at the time of Ignition.

I would recommend replacing all of the ignition wires including the coil wire with high performance silicon solidcores. If it is an HEI system, open up the Gap to maybe 50 to 65. I run Platinum or Iridium multi-electrode style plugs too. Also check your cap and rotor to make sure they're in good shape and make sure everything is clean of contaminants so you don't get arc-over in damp environments...

Finally, I'm guessing that the sound your hearing is actually arc-over occurring at one or more external points within the ignition system...? It can sound like a ticking or clicking sound. Checking the engine at night time or in a darkened garage looking for the flash of the arc-over can be helpful... :) RR

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RestorationRides said:
A wider gap on the spark plug is probably a BETTER idea if it is a high energy ignition system and all the components are in good shape.

Narrowing the gap back to 35 reduces the stress on the ignition wires (which reduces the tendency to arc-over in places you don't want it to) but is not really solving the problem as it reduces the amount of spark energy delivered to the fuel-air mixture in the cylinders at the time of Ignition.

I would recommend replacing all of the ignition wires including the coil wire with high performance silicon solidcores. If it is an HEI system, open up the Gap to maybe 50 to 65. I run Platinum or Iridium multi-electrode style plugs too. Also check your cap and rotor to make sure they're in good shape and make sure everything is clean of contaminants so you don't get arc-over in damp environments...

Finally, I'm guessing that the sound your hearing is actually arc-over occurring at one or more external points within the ignition system...? It can sound like a ticking or clicking sound. Checking the engine at night time or in a darkened garage looking for the flash of the arc-over can be helpful... :) RR

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Click to expand...

I do not agree.


Even with a high energy ignition there is a gap range that is useable. Too little gap and you might not ignite the gas or have a weak spark that doesn't last long enough. Poor gas Milage, sluggish no power. Too much gap and the spark may not be arcing to the plug or across the gap or not arcing at all.

In reducing the gap you are more likely arcing to the plug. However if there was external arcing in the wires it may still be there.

The only benefit I'm aware of is going through plugs to sequence the gap. You would want the gap to face the valve openings and center of the chamber. The benefit is minimal on these engines but is a practice used in racing.

If it were my jeep, I'd replace the wires and plugs. And be sure to use the correct plug either champion 3412 or RC10PEC or autolite AP985. Gapped to 0.035 +/-0.001. They are usually correct right out of the box. Do not attempt to gap iridium as you will damage the tip. The iridium are more expensive and are less likely to grow a gap, but are overkill and may not perform without high energy ignition. Also be sure the plugs are threaded and tightened properly. The old engine noise may be continued arcing or loose plugs... Without heating it's hard to determine.


With that large of a gap you may have damaged the coil and other ignition components.

Regards,
JPNinPA

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Last edited:

I respect your opinion Jeepn but I'm going to stick to what I said before. In an HEI system such as a late-model factory system or maybe an MSD or something like that, they can jump a very large gap and it won't do any damage to the unit. They're built robustly and ballasted to avoid that sort of issue. The real issue here is the necessity to ionize the fuel air charge sufficiently to allow that spark to occur. That takes power not just volts. Power is volts X amps. So everything in the system has to be designed and implemented to be able to deliver more than enough power to effectively ionize any foreseeable air fuel charge situation that an engine is going to experience during normal operation. 45, 55 and even 65 thousands are not unreasonable IMHO given high quality Equipment. The larger gap makes the ignition system generate a larger charge energy (power) before the spark occurs . That's where the extra power creates more ionization saturation within the fuel-air charge around the spark plug to create a more optimal condition within that cylinder at the moment of spark flashover.

BTW, the Mustang GT run gaps in the 45 to 55 range. That's the factory spec. I have run up to 55 to 65 thousands in everything from my street race , built 8,500rpm bug-eye Healy to my 1st generation blown Hemi in my vintage hot boat with excellent results. Instantaneous starts, crisp throttle response and excellent fuel mileage. Yes, it is going to be a lot harder on the ignition system at those large gaps but that's why HEI systems were developed and adopted.
That being said, everything else must be in top condition to deliver that sort of performance...;-) RR

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We could agree to disagree.
This is a jeep not a mustang or race car.
And the OP is trying to fix issues not peak and tune a racer. Going to a baseline is best and doesn't introduce more variables. You initial suggestion of 50-65 is too large to start at. 0.035 then up in 0.005 increments at most.


JPNinPA
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Last edited:

0.035 is correct. My bad. I'll fix..

Post a pic of your distributor and coil.


Regards,
JPNinPA

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Sorry man, I forgot the distributor pics. New Champion 3412 platinum plugs properly gapped and pencil sized hole in the header welded up. Fuel injectors cleaned, and man this Jeep runs great now. Thanks guys for all the awesome help.


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Glad its running better. if this is a HEI distributor, I totally agree with opening up the plug gap anywhere from 40-60, many go 70. My plugs are gapped at .45 with a HEI. The HEI is a high voltage dist, and to benefit from that higher voltage the gap must be opened up to create a larger hotter spark. If your ignition system is stock, leave it at the factory spec .35
 
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