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Discussion Starter #1
Hi All,

I've been getting a random misfire code on my Xterra (2006, 215K miles) the last couple of weeks. Shows up under load -- accelerating uphill is a quick way to evoke it. Seems to show up faster when the engine is warm, maybe?

To give you a little history: six months ago I replaced three of the ignition coils and all of the spark plugs, because I was having a misfire. In April I had a shop do a pretty complete engine overhaul -- new timing chain and tensioners, new oil gallery, new oil pump and oil sending unit. About a week and a half after that significant repair, the random misfire started showing up. Took it back to the shop... they couldn't find anything. No vacuum leaks, good spark As a precautionary measure, I had them replace the other three (still original) ignition coils. They took it to the dealer, who diagnosed the problem as a bad driver's side cat. I had both cats replaced two days ago. Picked up the car from the exhaust shop, and the code came back. Right now, it's only throwing the P300, not the P30X codes that would indicate a specific cylinder.

Any suggestions for what else might be going on?
 

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Misfires can kill (new) cats. I'd replace both cam sensors and the crank sensor asap. Hitachi (oem) cam sensors from Rockauto and the crank sensor from the dealer. Don't go aftermarket on these parts.

 

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Also, if they didn't do it when the parts were replaced, check the plugs again that were under the old coils.

I'd also check the connection on every coil, the connector clicks twice when it's fully put on iirc. It's easy to think it's fully connected when it's not.
 

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With all of the recent work you've recently had on it there's a decent chance one of he connectors has been left loose on something.

When I replaced my valve cover gaskets I first accidentally left loose the main harness for the coils on the drivers side of the engine (It only showed misfire code, no specific cylinder and it idled like a Harley, eventually figured that one out and still had a misfire. This one idled ok and drove mostly ok but would misfire really only under acceleration. If I let off the gas it would stop the misfire and holding a steady speed it would stop too.

That one I eventually tracked down to the fuel injector clip on cylinder #6. Visibly it looked like it was on but as I was going over the engine I pushed on all of the connectors and when I got to that one I felt it push in and click. That solved my problem and was easy to find so I'd start there just going over the engine and making sure everything is connected like it should be.
 

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Discussion Starter #7 (Edited)
Thanks for the good advice, folks. My mechanic tells me that the problem is that it's one of the rear cats that's causing the problem, and those were not addressed when I replaced the front ones. So, I've got a rear driver's side cat on order and will swap it out next week. At least that's relatively quick and easy work.

I don't know if the cam and crank sensors were replaced, and if so, if they were OEM. I'll check with the mechanic when I see him next week. I chatted with him today to confirm that his diagnostics last week had checked fuel pressure and compression, and he said yes, that both were good... so that rules out fuel pump and filter, and it also means any crud from a bad cat has not had a chance to damage the rings.

Finally, I'm not really noticing the misfire, other than the CEL indicating it. The truck runs pretty smoothly, starts smoothly, has power, and easing up on the throttle does not stop the CEL from flashing. In fact, I've gotten it cruising down a hill with barely any throttle. When I had the misfires back in December, those were real obvious -- the car would lurch and lose power. Whatever's going on now is relatively subtle.
 

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Discussion Starter #9
Not sure how Nissan diagnosed that, unless they did a visual inspection, but that was the problem. Replacing the rear driver side cat caused the misfire code to go away.
 
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