How to find a bad coil

[bobsblue95]

I've had a somewhat intermittent miss at idle for a while. New plugs and wires etc... basically I've suspected it's a failing coil for a while, but can't afford to buy four new ones. Today I tracked it down by simply examining my spark plugs. (95 doesn't tell you which cylinder(s) is/are misfiring.)

Pull the front plugs and look at the tips. They should be a nice light tan color. Three of mine were ash-white, indicating a lean cylinder. One (#6) was almost black, indicating a rich cylinder. Can you guess which coil was failing? If you're so inclined, pull the back plugs to verify that the other plug on the same coil shows the same condition.
I replaced the coil and my idle is rock steady now.

What happens is the missing or weak spark causes the fuel not to burn well, resulting in rich exhaust. The PCM sees this and compensates by leaning it out, which affects the good cylinders. Reading your spark plugs is always a good place to start when diagnosing engine troubles.

Hope this helps someone.

[tipsymcstager]

this is a nice pic reference...............can't find the original site, so a link to to the regal forums will have to do!


www.gnttype.org/techarea/engine/plugs.html

[Randy T.]

Very good! Sometimes the basics are the best and often overlooked. I have not thought to check coils this way before now.

[scottydl]

Added to the Classic maintenance sticky!

[aurora4088]

Another way, although a little more scientific but easier than pulling plugs is to use a Volt-Meter with ohm reader. Switch to over resistance or ohm's symbol omega and measure to two inputs to the primary side of the circuit (the primary side is the input for the ignition control module located on the bottom side of the coil) reading should be somewhere around 50-900 ohm's. Then read the secondary side(the two post for the plug wires) reading should be high somewhere around 25k to 100k ohm's. Normally this is where the problems occur so a low reading or a very high reading will indicate a problem also no resistance or open connection will indicate a problem. But GM has a good design with these coils they are used on most engine with Waste-Spark type ignition. My 96' has over 280K still original coils

[pain21]

Or just start the engine and pull the plugs/wire out one by one just about an inch or two(not out of the well) and quickly put them back. If you pull it and it crackles/engine winds down a little, that coil-wire-plug is fine. If you pull it and nothing happens (no change in motor noise or crackle), that’s the coil.