SureTrack Community Fix: Wrong Plugs

Real Fixes from the SureTrackĀ® expert information resource are documented issues from actual shop repair orders and community discussions. Read on to see how SureTrack can help you correct issues that are not easily diagnosed using OEM information alone. SureTrack is currently available free with the purchase of a current Snap-onĀ® diagnostic software version.

Sometimes you’ll be chasing your tail trying to fix an issue. You’ll find things along the way that might be the cause but it turns out to be something simple. SureTrack community member DNunez had a GMC Yukon in the shop with random misfires. They found several things to fix but still had the misfires. They posed the question to the community, and with the help of fellow community member Halsauto, and others, they were able to resolve the issue quickly.

The following Real Fix summarizes the interaction between them in the community that led to the solution.

2018 GMC Yukon SLT 5.3L

Random misfires, replaced spark plugs.

Customer complains that check engine light is on.

Diagnostic Steps Performed/Parts Replaced
Driving with scanner shows all cylinders have random misfires. No cylinder has consistent misfires and no consistent pattern. Cylinder #5 has the most misfires. Noticed under heavy throttle no misfires, but on deceleration most cylinders have misfires. Oil change is about 4ooo miles overdue. Customer replaced spark plugs and wires. Check ground at left front of engine block, cleaned it. Pulled air box to check for cracks and found no problems. Throttle body had carbon build up and cleaned it. Smoke test intake manifold and found no leaks. Clean mass air flow sensor. Replaced #5 ignition coil, still having misfires on #5 but not as many. Cleared codes and test drove, not as many random misfires but still misfires on deceleration after heavy throttle. Watch scanner, fuel pressure seems good at fuel rail and sensor. O2 sensors seem to be working correctly. Catalytic converter does not seem to be plugged, no smell & lots of power.

Could there be carbon build up between intake manifold and head ports? Where should I go next?

Best Answer from Community Member Halsauto
What plugs did the customer put in? I’ve seen the wrong plugs cause random misfires.

Customer had used Autolite Iridium AI6043 spark plugs.

Replaced the Autolite spark plugs with the correct AC Delco spark plugs. Road tested vehicle with scanner attached and misfire is gone.

SureTrackĀ® is a powerful value-added feature available with select Snap-on diagnostics tools. It offers an exclusive combination of expert knowledge and detailed parts replacement records, all wrapped in an interactive community.

Want more? Click here for more information on using SureTrack.

Looking for diagnostic tools? Click here to request more information on Snap-on diagnostic tools.

New or Expired SureTrack users: Click here for authorization key instructions.

About the Author

Nick Taylor

Nick Taylor is the SureTrack Community Administrator and a Senior Applications Specialist at Mitchell 1 with over 25 years of experience with electronic repair data systems. Nick previously worked in the automotive dismantling and engine rebuilding industries.