How would you test a spark plug?

[Steve 82SC]

A couple of days ago I took my 82SC on some errands and noticed immediately that it was running doggy. It was a steady miss, only firing on 5 cylinders, regardless of my speed or rpm's. The car had been running fantastic and there were only 1400 miles on the Bosch copper plugs since the engine rebuild. I changed the plugs, thinking this would be the easiest and cheapest diagnostic route (NGK's) and it's running great again. All 6 plugs look good, brownish white, no oil, etc. Is there a test I can run on the old plugs with them out of the car to determine which one is bad?

Thanks,
Steve

[emcon5]

I have a spark plug test fixture in my garage. I throw all my used plugs in it, any that jump back out on their own, I reinstall. So far, none have jumped back out.

Once a week, I wheel it out to the curb, and a truck comes and empties it for me.

As cheap as spark plugs are, it probably isn't worth screwing with. If it happens again, there may be something else going on.

Tom

[911pcars]

It's difficult to simulate operating conditions off the car. It may fire out of the engine but may be marginal under compression pressure and heat. To find the faulty plug, you can substitute one at a time until you experience a misfire.

Are the plugs expensive versions to go through this trial and error process?

Sherwood

[TerryH]

The car is the best test jig. Pull one wire at a time until you see no difference in performance. If you pull the wire off a good plug the motor should barely chug on 4 cylinders.

At this point I would use Tom's test fixture.

[vash]

next time it misses, get under the car with a spray bottle. spray the headers, and you will find the non firing cylinder right away. it will be the one that doesnt steam the water up on contact. you dont have much time, because the running cylinders will heat the entire thing up rather quickly and hide the bad cylinder.

[Early_S_Man]

Tesla coil ...

[vash]

awww crap, now i have to go and google tesla coils?

[emcon5]

Originally posted by unfixed
awww crap, now i have to go and google tesla coils?

Nope, just go to the Fry's electronics in Fremont at lunchtime.

Impressive.

Tom