I just had my daughter's 97 Explorer 4.0 SOHV tuned up (plugs, wires, fuel filter) before she heads off to college. It was driving great, but with 60+ thousand miles, I thought better to tune it up now than have a problem later. (It has already had the tensioners and intake manifold o-rings replaced).
Driving it home, I noticed a very slight by persistent hesitation and surge when holding the throttle steady. It accelerates, decelerates, and idles fine and seems to run great, just this very slight bucking at any cruise speed. After taking it back to the mechanic, he suspected a vacuum leak and said he'd take care of it. He later told me he couldn't find a vacuum leak, but that it was "throwing" an O2 sensor code on the right side O2 sensor. He said he reset the computer, it came back up fine with no codes, he test drove it, and the problem was solved. As I drove it home, exact same problem.
Today I took it to a Ford shop and had them inspect the tune up my mechanic had done, specifically the wires, plugs and fuel filter. They told me the 1 and 2 cylinder plugs were not seated all the way because they had been cross-threaded into the aluminum heads. Their only solution was to pull the engine and take the heads to a machine shop ($1500 estimate).
I'll let you know how the discussion goes with my local mechanic after we talk tomorrow. I'm not optimistic he will own up to anything.
My question is this: has anyone experienced this and have any ideas how to fix it short of the $1500 route? Is it possible to install longer plugs in the "new threads" to get the spark closer to where it belongs (looks to be about 1/4" from being fully seated?) Just live with it (it is a minor annoyance, doesn't seem to be leaking compression, but how long will that last?) Any other ideas? I am extremely frustrated and appreciate any advice. Thanks,
RL

 

Ahh, the fun of aluminum heads...most mechanics never realize they are aluminum until they cross-thread a plug. I thnink you are between a rock and a hard place on this one. Aluminum cross-threads very easily and I bet your mechanic did not chase the threads before putting the new plugs in and I bet has also ran them in with the socket and not by hand. The machine shop option is the best bet because that way you don't get any metal down into the cylinders. Longer plugs will not solve your problem because the "new" threads are really just the old threads smashed down. It will just compound the problem if you try to put another plug in there. You could take a chance on one of those plug thread repair kits at Autozone. They come with a tap to enlarge the hole and a Helicoil insert which will taper the bigger threads down to the proper size. The problem with this is that you may get metal down in the cylinder, which could score the cylinders and cause oil leakage and compression problems. If your plugs are level or pointed slightly downward, this is the route I would go. But if the tips are pointed up, then the metal is going to go into your cylinders and the only way to get it out is to remove the heads...

 

I've fixed crossthreaded plugs on other cars but never had to on an Explorer. The helicoil kit does work. Rotate the engine by hand to bring the piston up to the plug hole. Drill and tap it. Use a long skinny flexible magnet to pull the shavings off the piston head. Put in the helicoil adapter and your good to go. Of course this is all assuming you can get a straight shot at the plug hole. I use anit-seize on plugs when putting them it. This helps them start easier, and more importantly makes them easier to get out next time you change them.

Good luck!!

 

I don't know if that will work since you have aluminum heads. Can't use a magnet to get the shavings out. Still run the risk of getting shavings down inside the cylinders and scoring them, causing an oil burner and low compression, not to mention eventually wrecking the piston rings...