Still trying to diagnose an erratic positive fuel trims. Next step will be making sure that the lean condition i have is caused by less fuel delivered or weak spark not a vacuum leak. I pulled spark plugs today and took a picture. I would appreciate any insights on the status of all four cylinders.

Thank you!

image 1

  • That looks like oil deposits to me, but it could be due to fuel additives too. Have you run a compression check? It could be overheating as well, which would be consistent with an overly lean mixture. Can you take pictures from a better angle, showing more of the top of the plugs?
    – GdD
    Commented Dec 15, 2020 at 19:36

1 Answer 1


The end with the spark gap is primary importance . What are you burning ? I have not seen that much crud since the leaded gasoline days. This much fouling would be typical of leaded fuel , not a worst case. At that time the deposits were very complex ; some combination of lead oxy sulfate with bromine. No significant difference between cylinders. You might try platinum plugs , although I never used platinum with leaded gasoline. Another thing that has changed along with lead removal is significant reduction of sulfur. So, I have no clue other than it looks like leaded gas fouling.

  • Hi blacksmith37. I bought the car 4 months ago and not sure what was the original owner using. Now i use Chevron only. Thanks!
    – Flamenco77
    Commented Dec 15, 2020 at 19:59
  • New plugs has to help.. In the lead days you needed to change plugs at 15 to 20,000 miles. Lube oils can have zinc, barium ,calcium , maybe other stuff but it would need to be burning oil to foul plugs. I have changed 2 sets of platinum plugs in cars with 100,000 miles and they looked so good I hated to throw them away. Before I retired it would have been interesting to find out what the deposits were ( off the books). Commented Dec 15, 2020 at 21:43

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