: Single Cylinder Misfire - Ignition Coil Info



sirmqc
01-27-11, 12:35 PM
A few weeks back my 98 Catera suddenly started running rough, sluggish when accelerating. I wanted to post what I found in diagnoses in case its useful to someone else.

The OBDII code indicated Cylinder #1 misfire. There were no other misfires.

To make the first part of the story short... I measured the resistance of #1 wire to be ~12K-Ohms. Service manual says somethign like 7-9k. So after I orderd and got the new wires, found that the new #1 wire measured even higher at 14K-Ohms. Then did what I should have done in the first place - I tested the original #1 wire on a different ignition coil post - got spark, which confirmed the wire is not the problem.

One tip at this point - if you have the intake manifold off (fuel lines disconnected), its helpful to pull the fuse for the fuel pump before cranking to check for spark - unless you like the smell of gas in your garage :banghead:

So, after further reading (also should have done in the first place) - here's what I found about the ignition system:
- There are three coils, each drives 2 spark plugs
- Both plugs on a given coil fire at the same time, every time. So (if I'm remembering correctly) #1 and #5 share the same coil. When its time for #1 to fire, both #1 & #5 fire. When its time for #5 to fire, both #1 & #5 fire.

I tried different combinations of wires no different ignition pack ports to confirm that I always get spark on #5, but never on #1. So since only #1 was not getting spark, and knowing how the system works, the question was whether there was a problem with the connection in the ignition pack between the coil and #1 wire port, or whether the computer was failing to tell the pair to fire when it was time for #1. So I connected all the wires to spare spark plugs, matched up the pairs, and grounded them all to the engine. If it was the ignition pack, I would see #5 spark just as often as the other two pairs. If it was the computer failing to signal spark for #1, I would see #5 spark 1/2 as often as the other two pairs. I could see the spark chasing across the pairs, with #5 just as frequent.

I'm a big RockAuto fan, but the ignition pack from them was $270. I found an off brand (new) on ebay for $70. Its been running fine for 4 days now. I was able to replace the ignition pack without much trouble without removing the wiper motor. I'm returning the plug wires to RockAuto (which has an amazingly easy return process by the way).

hgqguy
01-27-11, 05:23 PM
Wow.. thats a lotta thought.. Im not so educated, so I never would have thought all that..

My catera has had misfire issues. the last mech replaced the coil for left side 1,3,5? its a 2000 catera.

Im still getting misfires. Its actually in the garage getting a diog as I type. maybe the other ignition coil is bad? all the plugs are new as of august...

BTW i get a fuel smell real bad when I start the car when its under 45* outside, and the car sat overnight... stuck FI? or bad egr?

andy thoughts?