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?