how to tell if a spark plug is not working?

[JatZilla]

I was able to change only 2 spark plugs on my 98 explorer so i had a local mechanic change the other 4. After 2 days, check engine light came on and the engine started running rough.

how can i diagnose this? how can i determine if one - or more - of the plugs isn't working? side issue...i can't believe how hard a couple of the plugs are, to reach.

TIA

 

[ajr2000]

What I used to do on lawnmowers was take the plug out connected to the wire and just ground the plug to the engine, but outside of the engine so you can see it. Just be careful that you don't screw up the gap.

 

[budwich]

if its not working.... typically when you pull the plug, it will be wet meaning that the gas is not being burnt. You can check the wiring / plug as suggested. If you have an induction timing light, clip the light on the "suspect lead", if it triggers, that will tell that the spark "signal" was at least sent.... and should have made it to the plug assuming there aren't breaks in the wire, poor connnect at the cap snap on, or cracks in the plug.

Ultimately, get the codes read, they will help you eliminate a lot of stuff quickly.

 

[JatZilla]

What I used to do on lawnmowers was take the plug out connected to the wire and just ground the plug to the engine, but outside of the engine so you can see it. Just be careful that you don't screw up the gap.

just so i get this right...touch the outside of the plug to some part of the engine, then crank the starter to see if it sparks?

 

[DCExplorer]

Another trick is to simply pull each plugwire one at time. I you pull off a plugwire and the truck runs worse, chances are the plug is firing. However, if you can find a plugwire where the truck runs exactly the same whether it is attached or not, then that cylinder is likely the one giving you a problem.

 

[MrShorty]

After 2 days, check engine light came on and the engine started running rough.

1st thing I would do would be to pull the codes from the computer. EEC-V can identify misfires, often can pinpoint which cylinder(s), and otherwise give you a good place to start.