CarTalk.com Blogs Car Info Our Show Deals Mechanics Files Vehicle Donation

Oil on spark plug threads, why? 2008 GMC Sierra 1500 WT

I pulled the plugs to change them: oil on the threads, otherwise they look fine. Truck also hesitates and misfires at times. Exhaust stinks like unburnt fuel, as though running super-rich. Easy to start. 32k on engine, 2008 GMC Sierra (truck is 4 years 10 months in my ownership, bought new). 3 years ago, it ran fine, getting close to 22 mpg highway Birmingham to Seattle. Now getting closer to 11 in town, maybe 16 on the highway. Oil changes every 3500 to 5000, manual suggests every 6k.

I have had oil on plug threads before, from spilling while filling, and one truck that had continuously leaky valve cover gaskets, but this appears dry on the outside, so I assume the oil is coming from inside the engine somewhere.

Please Tell Which Engine Is The Truck.

The oil on the plug threads may not be related to the misfire/hesitation/super-rich/stinky exhaust.

There could be two (possibly more) issues going on at one time and perhaps none of them is major problem.

The oil could be a red herring.
Although I’m not familiar with the engine options in these GMC trucks, many of today’s engines have spark plugs that sit in wells (some rather deep). Seals or actual o-rings keep oil from inside the valve covers from leaking into and pooling in the bottoms of these wells where the plug threads are located.

Is the oil residue on the plugs similar on all the plugs’ threads ?

Has the “check engine” light or any warnings ever illuminated ?

3 years and only 32,000 miles, are there any long priods of sitting dormant ?

CSA

I would be very concerned about oil on the plug threads as that can denote oil in the combustion chambers.

Since the truck has very low miles on it for the age you might consider checking the PCV valve and making sure that it’s not stuck.

If the PCV is ok then I would run a compression test and try to determine if there may be an issue with piston rings.
Both a dry and wet test should be done.