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P0303 cylinder #3 misfire

They are connected with plug wires but they are sealed units and those connections are not accessible, at least on this setup. Still, one open plug wire would kill 2 cylinders because of the way the system operates. For each pair of cylinders, one plug is always fired negatively, and the other positively. That is, one fires from the center eletrtode to the outer electrode, and the other fires from the outer to the center. It's a series circuit. Open a series circuit and no current flows.

Reply to
bllsht
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Thanks, that would eliminate it as his trouble at least.

Mike

bllsht wrote:

Reply to
Mike Romain

Just out of curiosity, what's your source for this? I know the packs fire two sets of plugs but I wasn't aware it fired pos/neg, and there's no mention of it in the FSM that I can find.

Reply to
Matt Macchiarolo

The FSM is a good source for repair info, but it doesn't go into all of the details.

The 4.0L uses a waste spark system. That is it uses one coil to fire a pair of spark plugs simultaneously. One on the compression stroke and the other being wasted on the exhaust stroke. One post of the coil fires with positive potential and the other negative.

A picture will make it easier. Scroll down to the DIS section for an illustration of what goes on.

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Reply to
bllsht

Hmm, neat. Thanks!

Reply to
Matt Macchiarolo

Hi all, here is an update on my 2001 TJ from a previous reply in this thread. After replacing the broken bridge and reassembling the rocker, a test drive showed some improvement but the Jeep threw the P0302 engine code again. After several days of digging into possibilities and a few calls to Chrysler, the guys finally dug out the problem. The cam had a bad lobe on one of the number two valves. I now have a pretty much rebuilt top end, new lifters, cam, rockers, timing chain and gears, etc.. They showed me the old cam and, while the Chrysler tech wouldn't admit a bad heat treat to begin with, he did say that he has seen a "few" but they are not common to have go bad. The TJ came home today, runs great, nice and quiet, smooth at idle and driving speeds. I'm just over $700 lighter in the wallet but I should be able to run this engine til I decide I need a newer Jeep. Just wanted to let the group know the update on my problem about my misfire. Les

Reply to
Les

Les,

Thanks for the follow up.

Carl

Reply to
Carl

I just talked to the dealer who said the heat shield had been put on the intake to prevent overheating injector #3. I guess it rules that one out. I'll try looking at the injector this weekend. I'll wait on checking the coil since it would most likely affect 2 cylinders, although I'll keep it in the back of my head to not rule it out completely.

Thanks every> Les,

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Reply to
01XJ

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