P0303 Misfiring cylinder
Asked by Isaiah Oct 26, 2023 at 01:49 PM about the 2007 GMC Yukon XL 2500 SLT 4WD
Question type: Maintenance & Repair
I was driving on the highway and didn't notice anything out of order until I
pulled off and stopped at the stop light. The engine light was off, but my
engine was idling rough. I pulled into my house and got out the scan tool to
reveal P0303. I started by pulling the spark plug wire and the connector was
loose and pulled out of the wire. As I was already going to replace the plug
wire I pulled the plug to inspect for issues. There wasn't any fuel on the plug,
but the plug wasn't clean so I installed a new plug and plug wire. I cleared the
code and started the car only for it to continue to idle rough. Is it possible the
injector is not supplying fuel?
9 Answers
It's possible since the plug is dry. I'd suggest a complete tune up, especially if you have over 100,000 miles on the vehicle. It's overdue if it hasn't been done. While you have the spark plug out compression test the cylinder. If you get a very low reading It's a valve problem. If you get a low reading that improves with a little bit of oil in the cylinder (wet compression test) it's the piston rings. But do the tune up first and go from there. Also, check the ignition coil for that cylinder. Hope that helps! Jim
Thank you, I'll check that.
Do a dry then wet compression test, if that shows good the proceed with a good tune up. I had a bad injector on my Yukon on number 5. So injector trouble is not uncommon. better check that coils output also.
OK, iI am not sure the next step. Compression when tested dry was around 130psi and wet was 155psi. I tested the coil and the light was flashing through the plug wire, and I replaced the plugs and nothing has changed. My next guess would be checking if the injector is getting signal and continuity. Any suggestions?
130, sounds like a tired engine, my old Yukon had over 150 dry with 175000 miles. I would have a leak down test done before sinking any more money into it. If the engine is tired nothing you do will fix it.
It has 250,000 so I am sure it is tired. I would love to get a little more out of it, and mostly just trying to figure out what is the culprit of the misfire. If it is something more than worth it I will need to move on, but if it is just replacing an injector then I can limp it along a little longer with soon to be 4 kids in college.
Ok, so you have spark and compassion. The only thing left is fuel. So yes, check the fuel injector. If it's getting a pulse signal, replace it. If not, check the wiring and the connector going to it. Jim
