2003 Yukon, 5.3 Flex Fuel w/124K miles. SES light on, P0303 w/rough idle. Previous repairs and PM history:
Unfortunately, this did not help! :shocked:
Then with engine running, I used an old piece of quarter-round baseboard molding gently up against each injector. I could easily hear a difference with them plugged in, and then unplugged – injectors all working. Then I unplugged each coil input – the idle would get worse each time… this eliminated the coils as the problem. All this time, I never considered the spark plugs, since I had just changed them less than two years/30K miles ago and they ”provide 25% longer service life than original Bosch Platinums (100K promised)”
I thought there may possibly be a bad connection with the spark plug wires, so I removed each wire from both the coil and the plug to inspect. Found #6 would not remove from the plug end – corroded so bad it welded itself to the plug. Had to break the wire, then all the guts from wire were still attached to the plug. Had to use pliers to twist and remove as much debris just to get the spark plug socket to fit over the plug. Thought I was onto something, even though this was on #6, instead of #3. Cleaned up the plug with a Dremel, installed set of Bosch wires (which seem good quality)…and… no improvement…
Finally, I replaced two sparkplugs, #6 and #3, with AC Delco 41-110. Lo and behold, idle smoothed out, SES light went out by itself after driving ¼ mile. Turns out the problem was the spark plugs; they just took a dump after only 30K miles. Changing remainder of plugs to AC Delco’s today. Just wanted to share, hope this helps someone.
Edit: The take-away is that never rule anything out. I had ruled out that the problem could be my recently-replaced, fancy sparkplugs that I thought would last 100K+ miles, but they turned out to be the problem...
- Replaced intake manifold gaskets @ approx 85K miles in 2008
- Replaced OEM spark plugs @ 92K miles 2009 (just because I thought it was time, no problems noted). Installed Bosch Platinum Plus HR9LPX gapped to .060, as instructed in my Haynes manual
- Clean TB w/Deep Creep every 5K miles w/every oil change
- Clean MAF sensor w/CRC every 5K miles w/every oil change
- SeaFoam in gas tank Jan 2011
- LucasOil Deep Clean fuel system cleaner two weeks ago – no good
- Pulled & inspected #3 sparkplug. Buffed it up enough though not needing it (sandy colored). Checked gap.
- Swapped coil & wire from #3 to #5, still P0303 afterwards – no change
- Big blue spark on #3 COP using spare plug as indicator
- Using stethoscope, #3 injector clicks when connected, quits when unplugged – injector works
- Replaced the OEM fuel filter two days ago – no change
Unfortunately, this did not help! :shocked:
Then with engine running, I used an old piece of quarter-round baseboard molding gently up against each injector. I could easily hear a difference with them plugged in, and then unplugged – injectors all working. Then I unplugged each coil input – the idle would get worse each time… this eliminated the coils as the problem. All this time, I never considered the spark plugs, since I had just changed them less than two years/30K miles ago and they ”provide 25% longer service life than original Bosch Platinums (100K promised)”
I thought there may possibly be a bad connection with the spark plug wires, so I removed each wire from both the coil and the plug to inspect. Found #6 would not remove from the plug end – corroded so bad it welded itself to the plug. Had to break the wire, then all the guts from wire were still attached to the plug. Had to use pliers to twist and remove as much debris just to get the spark plug socket to fit over the plug. Thought I was onto something, even though this was on #6, instead of #3. Cleaned up the plug with a Dremel, installed set of Bosch wires (which seem good quality)…and… no improvement…
Finally, I replaced two sparkplugs, #6 and #3, with AC Delco 41-110. Lo and behold, idle smoothed out, SES light went out by itself after driving ¼ mile. Turns out the problem was the spark plugs; they just took a dump after only 30K miles. Changing remainder of plugs to AC Delco’s today. Just wanted to share, hope this helps someone.
Edit: The take-away is that never rule anything out. I had ruled out that the problem could be my recently-replaced, fancy sparkplugs that I thought would last 100K+ miles, but they turned out to be the problem...
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