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 Post subject: Misfiring, p0305 could use some advice
PostPosted: Tue Jun 11, 2019 5:49 pm 
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Ok, so the story starts back in December. On a road trip from Massachusetts to Virginia, my 06 Xterra with 210k miles started running rough and misfiring. It was throwing a p0300 and p0301 codes. being away from home and without tools, Into the shop it went. Turns out the passenger side downstream catalytic converter was clogged and was blocking up the exhaust flow causing problems. So that was replaced, and off we went.

Fast forward to about a month ago, now with 220k miles. I started noticing another misfisfire, and a p0305 code, but this time no p0300. Same side as last time. So remembering what happened last time, I figured maybe the upstream cat is clogged now too. So I went ahead and ordered the remaining 3 that I hadn't replaced yet figuring with the milage, and being original, they're probably not in great shape. and set out to get it at least running so I could get to work.

So I unbolted the previously installed downstream cat and started it up. Still misfiring. So I figure the front is probably clogged now. So I went to break the honeycomb out for now just so it will run until I have the time to replace it, to find there is NO honeycomb left whatsoever! So it's definitely not an exhaust problem...

So I replaced a spark plug and a coil because I happened to have a couple spare ones handy. No luck. It sucks too, because to get to that side of the motor you need to basically remove the whole plastic Intake plenum. All wires and cables seem to be connected.

Before I spend any more money on this thing, I'm going to try a compression test and hope for the best. Any other advice?

Thanks in advance!

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 Post subject: Re: Misfiring, p0305 could use some advice
PostPosted: Tue Jun 11, 2019 8:54 pm 
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Location: e. long, mass
The fact that there is no honeycomb in the primary, and there are also no catalyst codes makes me think it could be the O2 sensors. Not sure if it would be the upstream or downstream ones. Downstream would be my uneducated guess.

It’s also not a good sign that the honeycomb is missing. Generally it gets sucked into the cylinders during the EGR process. A compression test may be a good idea.

It seems improbable that 10k miles would completely gut a CAT unless something else is significantly wrong. Like dumping raw fuel into the exhaust.

Any chance the ‘mechanic’ just busted out the honeycomb and sent you on your way? Could he have covered his tracks by reprogramming the ECU, removing the CEL from triggering? Seems like I’m cooking up a pretty solid conspiracy theory. Maybe someone with more automotive experience could correct me.

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 Post subject: Re: Misfiring, p0305 could use some advice
PostPosted: Wed Jun 12, 2019 11:23 am 
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murphy wrote:
The fact that there is no honeycomb in the primary, and there are also no catalyst codes makes me think it could be the O2 sensors. Not sure if it would be the upstream or downstream ones. Downstream would be my uneducated guess.

It’s also not a good sign that the honeycomb is missing. Generally it gets sucked into the cylinders during the EGR process. A compression test may be a good idea.

It seems improbable that 10k miles would completely gut a CAT unless something else is significantly wrong. Like dumping raw fuel into the exhaust.

Any chance the ‘mechanic’ just busted out the honeycomb and sent you on your way? Could he have covered his tracks by reprogramming the ECU, removing the CEL from triggering? Seems like I’m cooking up a pretty solid conspiracy theory. Maybe someone with more automotive experience could correct me.
The one that's missing the honeycomb is the upstream cat. The one that was replaced was the downstream. It's possible ye broke out the upstream catalyst, but it's also possible that's what clogged the downstream one in the first place after 200k miles. It is getting g a catalyst efficiency code as well, but I know about that already which prompted me to get cats and do the rest of them.

In this case I don't think the rough running is part of the cats, but I'm still plugging along.

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 Post subject: Re: Misfiring, p0305 could use some advice
PostPosted: Wed Jun 12, 2019 9:53 pm 
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Usually it’s the upstream cat that causes the problems. It breaks down and clogs the downstream and get sucked into the engine. I’d for sure do the compression test. Change the plugs, and change the down stream as well.

Not for nothing, I would ask starting thinking of looking at something new. This is going to be expensive, and it may not fix it. That money may be better served on a newer vehicle more reliable vehicle.


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