P0305 misfire

[snoyes]

Hey guys...TIA for any help and guidance. I'm a noob Jeep owner and a noob to working on his own vehicles. But, i got rid of new cars and got an 2003 Sierra and a 2005 TJ so my sons and I could learn to fix the easier issues that pop up.

Anyways I got the p0305 #5 cylinder misfire code today after the engine light came on. It was clearly misfiring when I pulled into a parking lot. Put it in neutral and it seemed to level out. Drove 30 minutes to my house and read that two common fixes are replacing the spark plugs and ignition coil pack. Cleared the code then replaced the plugs, gapped them to .036 which I believe i read was the correct gap. Code came right back. Next I swapped out the coil pack. Check engine light didn't come back on so I drove an hour and a half to a job I needed to complete. About 65 miles worth of driving. When I started to Jeep to head home. The check engine light came back on after about 5 minutes. It accelerates fine but, you can definitely tell there is a misfire. Before I just throw money at parts. I thought I'd check with this community first. If it was the down or upstream 02 sensor I would think that it would show misfires across multiple cylinders. But, again I'm a giant NOOB. Don't worry about being kind. I'm a conservative living in Washington and I joined the WA Air National Guard at 37. I don't have feelings any more.

 

[AndyG]

Look really close at wiring harness man- especially where it goes behind the valve cover.

I mean look close close, feel the wires if needed.

Also use our search feature on that code.

 

[macleanflood]

I'd get a screwdriver to your ear and listen to the injectors and make sure they're clicking.

I'd say get one of these and test for spark...but I've got a 97 with a distributor and spark plug wires. Someone ought to be able to come up with a method for verifying spark or testing your coil pack.

https://www.harborfreight.com/in-line-spark-checker-63590.html

Don't forget the factory service manual is in the resources section and it's got some great troubleshooting procedures and wiring diagrams.

-Mac

 

[ItchyDong]

This is a waste spark system which means if the coil was bad, it’d more than likely be missing on two cylinders.

That leaves either spark plug or an injector issue. Could be the injector isn’t firing, the spark plug is junk, or the injector is firing all the time caused by a short on the control wire.

A visual inspection is one way to test for the short, but honestly unplugging the injector, and installing a noid light that can be rented at your parts store is the easiest and best way. If it flashes, the circuit is good. If it stays lit or doesn’t flash at all, there’s a problem. It’s a 30 second test that tells everything you need to know.

Could be that the injector is firing but is clogged as well. This is tested by performing an injector balance test.

 

[JKP]

20 oz bottle of chevron techron fuel system cleaner once you check all the wiring on 5. Then take it out and drive it, over 3500 RPM to rotate the valves and blow out the carbon. On my 06, it's cyl 1 that gets gunked up and throws a code. My OEM injectors went over 200K before the coil resistance of the injectors started to get out of spec.

 

[snoyes]

Look really close at wiring harness man- especially where it goes behind the valve cover.

I mean look close close, feel the wires if needed.

Also use our search feature on that code.

I did a search for the code before making my post. It seemed like everyone had a different response and/or solution to resolving it. I thought I'd post my specific scenario. I'll inspect the wires tomorrow. Thanks for the suggestion.