How to test coil pack....rough idle on 92

[redly1]

I'm looking at a 92 lumina that is in decent shape, and the price and miles are great. I went over to look at it, but couldn't drive it (had my kids with me). It had been sitting all day, and started up and ran fine.
This was at about 3pm

at about 7:30 PM, I returned to test drive. Just prior to me, another person had test driven the car...so it was warmed up.

this time when I started it, it had to turn over for quite a few seconds, and then there was a rough idle. I looked at the guy like "WTF happened"
Anyway, I drove it. At lower speeds, there was a bit of stumbling hesitation, but as soon as you got on a main road (30+ MPH, I guess), it ran great..and on the interstate, plenty of power when you hit the accelerator. Same hesitation when I returned the car and let it idle in the driveway.

One other thing was the exhaust tone. it sounded like one or two cylinders were misfiring (healthy tone for 4 or 5 beats, then one or two less than healthy tones)

After scouring this forum, my guess is a faulty coil pack.

BUT, since it's not my car, I can easily just take the coil pack off and run it to autozone. I am somewhat mechanically inclined, and I do own a multimeter.

Anyone know if there is a way to test the coils with a multimeter to determine if there is a problem????

I assume I will have to try this both cold and hot.

[redly1]

oh, and it has new plugs and wires recently

[Hyperbusa_al]

Which motor does it have? If it's the 3.1 liter V6, the coil packs (3) are on the front of the engine under the exhaust manifold. You should take em off and check em one at a time. Mine were within specs, a little weak but the problem was the Ignition Control Moduale, not the coils. If you read the posts, you will see it could be a number of things. I'm surprised jeffcoslacker has not replied to your email, he has a lot of excellent ideas on how to narrow down the problem. Read (study)his posts, you will probably find one or several that addresses rough running older Luminas.

[jeffcoslacker]

Since each pack fires two cylinders, when they miss it is real noticeable. I actually don't know the "by the book" procedure for testing these packs, but a coil's a coil, so you'd be looking for any open (infinite) readings across the primary and secondary windings obviously, but I don't know what spec for resistance is. (I don't have a manual for it)

And since temp seems to be an issue, you'd want to heat it while observing the reading, a hairdryer might work good for that, because it sounds like the test might not show you anything cold.

I've seen a few cases where a specific coilpack was breaking up, causing those two cylinders to cut out, but after replacing it the misfire continued. The problem then was in the ignition module, under the coils, and the internal circuit to that particular coil was failing.

I have sometimes been able to use an inductive timing light to "see" the ignition misfire and identify the cylinders affected, then swap that coilpack with one of the others and see if the misfire follows that coil to those cylinders. Then you know it's bad.

Thanks for the comments, Al! Yer gonna give me a big head.

[redly1]

what I suspect is that one of the coils has indeed opened up, ever so slightly that under low load, alot of energy is being lost in the arc (across the gap in the coil). At heavier load (highway speeds, heavy acceleration) the drop across the arc is less noticable.

Just a theory

Oh, and it's a 3.1L