Oil on spark plug threads
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Joined: Feb 2010
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From: Vancouver, BC
Oil on spark plug threads
The truck: 2000 F250, 5.4L with 250,000km (156,000 miles).
A couple of weeks ago, I had a miss on the #5 cylinder. Pulled the COP, blew out the plug recess with air and got a fair bit of 'something' blasted out. Looked like oil. Plug was totally fouled (it had the miss for a couple of days), and the threads were oily.
Put a new plug in, and it ran fine. A couple of days ago, it's still running fine, but I pulled the plug to check the condition. The tip is clean, nice tan colour, but the threads are covered in oil. I'm reasonably sure it's oil, though the thermostat is right beside #5.
Any ideas where this is coming from? I'd guess a leaking valve cover seal, but that doesn't seem very common.
Thanks,
Todd
A couple of weeks ago, I had a miss on the #5 cylinder. Pulled the COP, blew out the plug recess with air and got a fair bit of 'something' blasted out. Looked like oil. Plug was totally fouled (it had the miss for a couple of days), and the threads were oily.
Put a new plug in, and it ran fine. A couple of days ago, it's still running fine, but I pulled the plug to check the condition. The tip is clean, nice tan colour, but the threads are covered in oil. I'm reasonably sure it's oil, though the thermostat is right beside #5.
Any ideas where this is coming from? I'd guess a leaking valve cover seal, but that doesn't seem very common.
Thanks,
Todd
Elder User
Joined: Jan 2010
Posts: 590
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since the oil is only on the threads, and not down in the cylinder (tip gives that away), I would venture to guess that the oil is coming down onto the plug. Going from this I would guess based off my experience with pushrod engines that the culprit is your valve cover gasket (this is more common than you think).
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