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Old 10-14-2011, 01:13 PM
cb8100
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Default Diagnosing Misfires on 2005 2.5L Altima

Hi, Everyone --

These forums have been a great resource for helping diagnose several problems with my 2005 Altima 2.5SL, but I'm absolutely stumped now.

The car has about 175k miles on it.

Long story inbound:

I've been having some problems on and off with misfires since May. First, the spark plugs went out. I replaced all the plugs and everything was fine for a couple of days. The check engine light came on and indicated the camshaft and crankshaft position sensors needed replacing. While I was waiting for the parts to come in, the car started misfiring on cylinders 3 and 4. I'd assumed this had to do with the CPS', so I waited for the sensors to come in, replaced them and the problem persisted. Next, I changed the coils. I ordered the coils from Amazon ($5 overnight shipping with Prime or wait 3 days for O'Reilly and Autozone to get stock). I swapped all the coils and everything was fine for another couple of days, then I started getting misfires on cylinder 4. I move the coil from 4 to 3 and the code follows the coil. I replace the coil (sent the old one back to Amazon) and everything's fine.

A couple of weeks later, another one of the Amazon coils craps out. A replacement and everything's fine for a month or so.

Since July, I've had days where I'll get misfires (usually humid days and on a warm start) for a couple of minutes and everything's fine.

This past Wednesday, the car was misfiring from a cold start. I hobble home and there's smoke coming from the hood. I pop the hood and there's oil coming out of the valve cover and hitting the manifold.

Today, I replaced the valve cover gasket and found oil on spark plugs two and three. After cleaning all the gunk off the engine block and valve cover, I replaced the gasket, cleaned the plugs and went for a test drive around the city. After about 10 minutes, I started getting misfires on two again. Given the oil I found on the two plugs, I stopped at O'Reilly and got four new plugs and swapped them all out. There was no oil on any of the plugs when I pulled them out this time.

Everything was fine for about half an hour, but now I'm getting misfires on cylinder 2 but only when idling or under 40mph. I'm going to start moving coils around again to see if the code follows the coil, but I started thinking about the fuel filter (yes, I know, I have to replace the entire pump).

Is it logical that the fuel pump/filter could be causing misfires? I'm really hoping it's just another coil from the Amazon batch that's crapped out, but I'm starting to think about backup plans. Also, is there anything else I should consider if it's not the coil (and obviously not the plugs)?

ETA: No oil coming out from the valve cover gasket, so at least I got something done right.
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