I got new spark plugs, same as the ones that came with the car, but in one of the cylinders, when I put in the new one, the cylinder missfired. So I put back the old one, and the cylinder worked. Got another new one, and cylinder missfires a lot also. Then I put back in the old one that had 120k miles on it, and it works.

What could be causing this?

Sounds like bad new spark plugs. Are you using NGKs or Ought-to-lights?
(Autolite)


What happens if you move the bad spark plug to a cylinder that works, and then install the other spark plug in the first hole? If the trouble moves around with the plug, you have found your problem. You will also know that the problem is not the injectors.

What's the brand of the new ones? I know you said they are the same but I believe only NGK work well with our cars. Possibly the gap is too large on the new ones?

Sorry can't remember the model number, but they are the ones mentioned in the manual... NGK something. They look identical to the ones they replaced, and they cost $10 each. I got some cheaper NGK before, but they were not good, so I got these.

Is it ok to reduce the gap on these spark plugs?

Ok, just checked, they are NGK PFR5G-11.

The gap should be per the FSM or owner's manual.

Check the terminal to ensure that it's threaded tightly.

$10/ea.? Sounds like you bought platinums or iridiums ... those can't/shouldn't be gapped.

My money is on the coil being the issue. Pull the stalk off the coil pack, look for corrosion where the spring contacts the coil. Clean it up is you can, but usually replacement is the best option.