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Over the weekend, I changed one spark plug on my 2001 Ford Explorer Sport. It was running fine but it had been about 30,000 miles and I am about to sell it and wanted to have everything running good for whoever buys it.

I only had time to change one spark plug - it took forever to find my Gap tool and socket. Unfortunately, my truck now runs like crap. I'm pretty sure that it's missing now.

I've taken out the plug, cleaned some grease of the tip the connects to the plug wire and put it back but it still runs the same. I ran it in the dark and there are no shorts in the plug wire.

How do I troubleshoot the issue? What else could I have messed up?

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    It happens rarely but sometimes a new plug is bad. Can you reinstall the original and see if it improves. – mikes Jun 3 '15 at 21:31
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One of 3 things:

  • Spark Plug Wire isn't fully connected
  • Gap on plug is not actually correct
  • Plug wire got damaged

I would check the connection. Sometimes spark plug wires can be hard to get in all the way and they don't always make a snapping sound when they go in properly. If that doesn't help, check the gap on the plug. What kind of gap meter are you using? Make sure you're using it correctly. If none of that works, I'm betting you damaged the wire from pulling on it. Also, make sure you didn't accidentally pull other wires. You can measure the resistance on the wire from end to end with a volt meter. The rule of thumb is 1000 ohms/inch of wire length and 30,000 ohms max.

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It turned out that the issue was that I had disconnected the battery while working on it.

The computer had apparently reset and I hadn't driven it long enough for it to reset/configure/whatevs.

Once I drove it for half an hour straight, the engine started running normally again.

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