94 LeSabre 3800 Series I misfiring

[Billski]

Thought I'd put this out there for folks to learn from. 94 LeSabre 3800 Series 1 had intermittent misfire at 104k miles. Pulled plugs - original plugs! - found #4 had carbon tracking/fouled out. Replaced plugs, wires, O2 sensor, PCV valve, all vacuum lines - all this stuff was original (we just bought it from original deceased owner's son). Misfire continued constantly. Progressive loss of power and really poor fuel mileage. Started searching forums, found bits and pieces of info, but nothing that tied it all together. Hoping others can learn from my experience.

Pulled coils, all primary resistance checked ok (0.2-0.7 ohms), and two coils ok on secondary resistance (6.0k-11.0k) but 1-4 coil (on the fouled plug) had infinite resistance on secondary circuit. I pulled the ignition control module (ICM) out from under the coils and took it to AutoZone to have them check it on the Wells tester and it failed. Replaced both the ICM and the 1-4 coil and problem solved. Starts right up, runs smooth, total power, no sign of a misfire at all.

Lessions learned - if plug fouls on a 3800 Series 1, it takes out the coil. Once the coil's done it takes out the ICM. Don't replace the ICM without testing resistance on both primary and secondary circuits on coils or you'll kill the ICM due to the bad coil.

[Tech II]

I would have checked the output of the ICM in the vehicle, instead of taking it out to be tested.....might have just been the coil....

[Billski]

I neglected to mention it had occasional backfire coming off idle, seemed like dwell was off, so I suspected ICM. I didn't know how to check it with just a multimeter while in the vehicle, and I knew they had the right tester. I like the old days... if you had a dwell meter, a timing light and a multimeter you could diagnose anything!

[Tech II]

What you can do is check spark output with a spark tester....chances are 1/4 won't have spark....so swap 1/4 coil with another coil......if 1/4 now has spark bad ICM....if it still doesn't, bad coil....

CAUTION!!!!!!!!!!!!!

You are close to the battery when testing spark......I temporarily cover it to prevent hydrogen gas from exploding near a spark from the spark tester......