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Topic Review (Newest First)
02-21-2011 11:32 AM
Jerry Bransford Rich after a few moments thought thought about that ignition coil circuit, I quickly realized I was wrong and you were absolutely right on the polarity to the spark plugs not changing. I read that the polarity alternated on a DIS crcuit years agp but that isn't true on the 4.0 engine. Nice catch!
02-21-2011 10:30 AM
rrich I used to think that too - but not so.

The coils are double ended - when the magnetic field collapses around the windings both ends get hot - one positive, the other negative (on a standard ignition the bottom end of the Secondary is tied back to the the primary - like an auto transformer or flyback, which it is.) The coil secondary ends are connected to the plugs - on the companion cylinders.

The coils fire on each TDC stroke for those cylinders - Secondary current goes from one end, through a plug, through the block to the other backwards (reversed polarity,) then back to the coil.

The cylinder under compression is harder to fire - more required voltage needed. The waste spark is relatively easy to fire since it's almost a vacuum at that time.

IF the primary was to alternate polarity, then they each would get alternate charges like seems logical. But since there's only one current source and one polarity primary signal from the PCM they don't alternate.

I'd been told they fired both ways (and assumed,) plus and minus, I was expecting to see it, but since I have a dual trace lab scope now I can see it. It's still hard to see, too difficult to snap a picture of it - I've tried - my cameras just aren't fast enough to get anything but a blur.

The double plats you like are supposed to resist the electrode wear on both center and ground electrodes. It would be interesting to see some single plat plugs that have lots of miles on them - they'll most likely show 3 worn bad and 3 that lived. I haven't been able to see some - yet.

I developed a dislike for plats over the years - seemed like every time I used them or got involved with poor performance and misfires they had plats. I could see them from across the room on a scope - they always looked fouled! (I just tossed them in the trash - too bad - if I'd saved them for the platinum we could be having this conversation on a beach in Hawaii . We tossed out lots of them. )

Now that was years ago - with conventional Kettering type ignition systems, points or electronic, even CD's.

Times have changed - "my jury" is still out . It's possible the Jeep ignition has a faster rise time, or more current in the secondary - I have yet to look at that.
02-21-2011 09:42 AM
Jerry Bransford Rich, all six plugs on the DIS 4.0L engine get fired both directions equally so there will be even electrode wear if it is a conventional non-platinum plug. They fire one direction for the compression stroke and in the other direction for the waste spark/exhaust stroke. If the plugs are double-tipped platinum or iridium, you won't see any visible gap erosion at all for their first 100K miles.
02-21-2011 09:34 AM
rrich Since plugs are not designed just for one specific application they can be most any reasonable gap right out of the box. You may get lucky (then go to Vegas,) maybe not.

They may be right on for your Jeep, but they may even be gapped for a Yugo, Chevy, VW or ? if they are used in them too.

ALWAYS CHECK AND RESET IF NEEDED.

The general rule of thumb is - a large gap gives a better idle (more spark surface area.)

A smaller gap gives better top end (the ignition starts running out of "steam" at higher R's, and the cylinder turbulence is greater - trying to blow out the spark.) A longer spark has a more difficult time reliably jumping at speed = misfires.

If the ignition has been enhanced with more available voltage AND more energy, the stock gap can be increased somewhat.

Best to stick with what's printed on the underhood label - the makers of the engine and ignition knew what it's designed to use. (Gap, make, number etc.)

The coil rail type systems are even more critical on gap. 3 of the plugs have to fire with reverse polarity all the time - that makes them harder to fire. Normally a spark prefers to jump from a hot electrode (center) to a cold one (ground electrode) - the center electrode gets a negative voltage.

With the Jeep DIS 3 of the plugs get a positive voltage (the coils are double ended.) Each plug fires twice as often too, once on the power stroke, once on the backside called the waste spark.

You can tell which plugs are which, positive and negative polarity - 3 of the old plugs will show more wear on the ground electrode - those are the ones with the reversed polarity.

If you modify an ignition scope to work on the Jeep DIS to see what's going on in the ignition you can see it all. You can also see it with a lab scope.
02-21-2011 06:45 AM
StinkyMud
Quote:
Originally Posted by cork1958 View Post
I ALWAYS at least give the gap a quick check. I know they are SUPPOSED to be pre-gapped, but after getting all bounced around in shipping and what not, who knows what they might be at when you actually install them.

Otherwise, IMO,
You're only doing HALF the job!! I know if I took my vehicle to a shop and had them install plugs and found out they didn't check the gap, it would going right back to them!
Agreed!!!!!! My dilemma as what the correct gap should be, hood states .050 for OEM plug but Champion site and Napa list my plug as .040 for my 2.4. I went with what the hood stated for the OEM plug.
02-19-2011 10:44 PM
cork1958 I ALWAYS at least give the gap a quick check. I know they are SUPPOSED to be pre-gapped, but after getting all bounced around in shipping and what not, who knows what they might be at when you actually install them.

Otherwise, IMO,
You're only doing HALF the job!! I know if I took my vehicle to a shop and had them install plugs and found out they didn't check the gap, it would going right back to them!

Quote:
Originally Posted by pslamp32 View Post
but I'm trying to be a better wrencher thanks to you guys!
Nice try on that!!
02-19-2011 09:42 PM
pslamp32
Quote:
Originally Posted by Jerry Bransford View Post
The gap is supposed to be .035 and most spark plugs now come pre-gapped to what the application they are made for says they need to be gapped at. If the engine is running ok, I wouldn't bother checking them as odds are very good that they were already gapped to the required .035 gap.
You sir are amazing. Wish you lived near me...
02-19-2011 09:31 PM
H20workr I usually dont check the gap,espeacially since they have the protector on them when packaged as new.
02-19-2011 04:45 PM
Jerry Bransford The gap is supposed to be .035 and most spark plugs now come pre-gapped to what the application they are made for says they need to be gapped at. If the engine is running ok, I wouldn't bother checking them as odds are very good that they were already gapped to the required .035 gap.
02-19-2011 04:31 PM
pslamp32
Spark plug gap question...

So I cleaned my throttle body and changed my plugs today. Went off for a drive and everything seems great. I just realized that I didn't check the gap in my new plugs. Do I need to pull them all out again or just ride the wave since everything seems hunky-dory? To be honest I've never checked my gaps when changing plugs before but I'm trying to be a better wrencher thanks to you guys!

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