Misfire in one cylinder

Hey everybody, 99 3.4L owner here

I recently started having a problem with my engine where it seems like one of the cylinders is misfiring. However, it only seems to misfire at idle and low RPMs (1000-1800). But when I get on the freeway and get up speed and RPMs, it seems to go away.

I did have a check engine light that I took to AutoZone to scan for. Got a couple different codes. I got a P0440, P0446, P0420, and a P0303 codes. Because of the P0303 code, I knew that it was cylinder 3 that was misfiring. Just to double check, I also took it to an O'reilly and got the same codes. Immediately I knew it was one or more of a few different things that could be causing the misfire. I first thought that it was a spark issue, so I pulled my cylinder 3 coil pack, as well as the spark plug. Plug looked fine at first glance. I suspected maybe something was wrong with the pack, so I swapped it with cylinder 5, did the same with the plug. Nothing changed, and got the same code again.

Then I thought it was something with the fuel. I ended up taking out all 6 injectors, replacing all seals and o-rings, and cleaned them all out with carb cleaner. Put everything back together, making sure the injectors were properly seated into the rail and the engine. Still nothing changed, check engine light went away though.

I ended up in a pinch for work and drove it for a day or 2 with no check engine light, but still the misfire. I spent most of my time on the freeway, which as I mentioned before there seems to be no misfire at. Coming home one day, just as I was pulling into my exit the whole engine seemed to completely start misfiring, or at least one or more cylinders were, and the check engine light started flashing.

The next day, I called a buddy and told him what was going on. He suggested maybe it was a vacuum leak. He told me to run the engine, and spray carb cleaner into every vacuum line connection and the fuel injectors. I sprayed just about every connection I could see and still no change.

Read another forum that suggested maybe it was a fuel pressure issue. Right off the bat I knew that my fuel filter hadn't been changed, so I disconnected it and blew some cleaner in there. I got my hopes up when I saw tons of dirt and nasty coming out. I didn't want to sink any money into a new filter, so I blew another cleaner into the filter with compressed air and out came another ton of nasty. Reconnected the filter, started up and still no change!

Finally, my last test was to check to see if my timing was right. I pulled the upper timing cover off, lined up my marks and all looked good.

I also cleaned all my sensors, my IAC valve, replaced the gasket on it, cleaned out my throttle body and intake manifold completely, and installed new spark plug wires. All doing nothing. I'm completely stumped.

My next guess would be a compression test to see if I have low compression in cylinder 3, but it doesn't make any sense if there would be low compression if I can drive on the freeway just fine. All in all, I've tested fuel, spark, air, and timing. Compression is the last piece of the puzzle to check for, but almost seems like a waste of time.

If anyone has any other idea or something to try that I haven't already mentioned, I would truly be baffled. Thanks!

 

Wow you have done a lot in chasing this down. I would have sent the injectors to a shop to be tested and cleaned. I don't think carb cleaner does anything. So you checked the coils but what about the ignition wires? In my experience the only way to rule them out is to swap one at a time with a new wire. On a side note why not properly take care of your vehicle? If the fuel filter is as dirty as you've described it, you really should have replaced the fuel filter.

 

Did you swap injectors? You said you removed them but did you verify that cylinder 3 didn't get the same injector back?

What brand of spark plugs? Anything other than OEM can cause issues.

You may want to do a coolant test for exhaust gasses. I know you said you pulled a plug and it looks ok, but maybe still a head gasket issue.

Did you verify the engine wiring harness is perfect? I've seen a few people on here chase down an odd misfire as a wire that has been nicked and is grounding on the intake or block. The plastic wiring harness protector on the 3.4 is notorious for disintegration. People remove it and leave wires sitting directly on the engine.