2006 4.0L P0303 with rough idle

[Odd Job]

I have a 2006 4.0L 2x4 and I can't seem to figure out what to do next. My explorer has a rough idle, and from time to time shows a P0303 misfire #cylinder code. Above about 1500 RPM nothing is noticeable other than terrible fuel economy. It began with a slight hesitation at idle without codes. About two days later a P0303 showed up so I changed plugs, and wires and ran sea foam through both vacuum and in the fuel tank. Still had the code after erasing DTCs. Took it to the shop and they tested for spark and found there to be strong spark. They changed the #3 fuel injector. No change. They also checked cylinder compression to be 150psi (not sure if they compared it to other cylinders and not sure if they did a wet compression test) and checked for vacuum leaks with none found. The code went away but the rough idle still existed. I've changed the fuel filter, EGR valve, and cleaned the throttle body.

Of note I've also seen a P0174 Bank2 system too lean code. I swapped the left and right upstream 02 sensors and the code cleared however looking at long term fuel trim, bank 2 tends to be about 19-20 at idle. Above idle it gets as low as 3-5. Bank 1 at idle is showing <10.

I'm at a loss for what is causing the misfire. It seems to come and go but I've gone through what I know for ignition, vacuum or fuel system problems (by the way fuel pressure/vac is showing 40psi).

Has anyone seen anything like this or have any ideas?

 

[boominXplorer]

Spray carb or brake cleaner around the area where the intake manifold and head meet and see if it stumbles. Pretty sure you know which cyl #3 is but for everyone reading it's on the passenger side all the way in the back.

If that passes I would pull the valve cover and check the valve springs. It could break a coil off and cause a miss while running but show good compression.

 

[Odd Job]

I've sprayed carb cleaner all around the intake manifold, no changes to RPM. I'll take a look at the valves. I'm kinda hoping whatever is wrong will be obvious at that point. I'm really hoping it's not a valve or valve spring.

 

[SteveOReno]

I have a 2006 4.0L V6 4X4 with a random P0306 code and a rough idle. I have replaced: plugs, wires, coils, fuel injectors, crankshaft position sensor. The problem always goes away after I disconnect the battery, but returns after about an hour of driving. The only thing left is a bad computer, or maybe even a software bug. I am about to install a 200A relay on the battery feed so I can disconnect the battery every time I park it. I know it's hard to believe that disconnecting the battery fixes it for a while, but there it is!

 

[Odd Job]

Well, I have finally fixed it. Pulled the valve cover off and found the #3 intake rocker arm roller was pitted. Surprisingly the camshaft lobe was hardly scratched (thank god). I took some 2000 grit sand paper and polished the lobe to remove the scratches and replaced the rocker arm. I was fearing having to change the camshaft but once buttoned up and running it ran like new.

While I was in there I cleaned the intake manifold and intake runners. Also, a plug for sea foam...just prior to this I had run sea foam in the crank case. When I opened the calve cover I was shocked to see how it really made the valvetrain squeaky clean. I was expecting at least some sludge build up for a motor at 130k but it was like new.

Then after the rough idle was fixed, for the lean code I decided to just change both upstream o2 sensors with oem ones and it seems to have solved the issue. Both LTFTs are <5 at idle.

I also did the pcm reset and idle relearn process. After all that combined I'm now getting better gas miles than when I first bought it!