Q: Had a spark plug blowout, but the threads are fine and intact, plug was able to screw right back into place. What can cause that?

asked by on May 05, 2016

So I was driving along and heard a loud pop, and stutter in my engine. I thought I had hit something, but I then noticed my car spitting and popping. When I ran a code scan I came up with cylinder 7 misfire, when I checked cylinder 7, the spark plug was out of the cylinder and the coil was broken where it gets a screw to be held on. The coil boot was torn, so it was about 1am and I was about 20 miles from home. I luckily had the tools in my car, I ended up putting the spark plug back in and it fit fine, the threads weren't hurt at all. I put the coil back in and hoped it would last until I got home. The car didn't spit or pop at all the rest of the ride. My question is, what causes a spark plug to pop without stripped thread? I have had problem with this cylinder before, last time there was oil in it and covered the spark plug. Should I be looking at something specific? Any thoughts or ideas would help. Thank you very much for taking the time to read about my woes. ;)

My car has 160000 miles.
My car has an automatic transmission.

This is a known problem with this engine and Ford/Mercury has a technical bulletin for installing an insert into the spark plug hole. You may not think that you don’t have a loss of threads in the hole, but you do. The heads only have about 4 threads for the spark plug hole. The head is made out of aluminum and will wear out if the spark plug is not tightened to exact specification the first time a plug was in the hole. When the plug moves it will wear the threads until the plug flies out. The next plug goes in fine and you might think that it’s held in securely but it is not and soon flies out.

The only fix is to put an insert into the hole or have the head replaced. The technical bulletin I mentioned for your vehicle is the one from 07-21-2 dated Oct. 2009. I’d recommend having a certified mechanic diagnose your particular issue, they will be able to inspect why oil got into the spark plug well in the first place and make any necessary repairs.

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