When you think your engine is a bit sluggish, you need to know how to test your vehicle's spark plugs. Before you even plan on going to a mechanic to have your vehicle checked, you can do the spark plug testing yourself.
Disconnect the spark plug wire on your vehicle's engine while it is still running.
You will know that nothing is wrong with a spark plug if, upon disconnection, the engine starts running roughly or drops speed.
If the plug is bad there would be no change in the idle of the engine when that plug wire is removed.
You can test the ignition of the spark plug by disconnecting it from the wire. Put the spark plug wire's end close to a metal surface and you should hear a crackling noise or see a spark if the spark plug is in good working condition.
When you crank your engine, check if there are sparks on each of the wires of the spark plugs.
A spark on each of the wires means means the compression is good, while no sparks mean a wire is not working anymore.
Keep in mind that all of the spark plug connections need to be securely hooked up.
Give the spark plugs' link connections a wiggle then start the engine again – the problem may just be as simple as loose wires.
Make sure that the spark plug's ends are free from dirt, grease or oil deposits – those may be the culprits why the spark plugs are not working well.