Coil pack resistance and misfires

[Need For Steve]

Ok, so after a busted radiator caused the Buick to overheat a couple weeks ago, i replaced top end gaskets this past week. Ever since the overheating, the car had been running very rough. Since i got it back together with the new gaskets yesterday, it's now running fine in any throttle position other than IDLE and very low overdrive speeds (45-50 mph light throttle in OD). IDLE exhibits a barely noticeable roughness, and a slight "missing" sound from the exhaust, almost as if it were mildly cammed and "loping". Light throttle at 45-50mph in overdrive produces a quite pronounced, almost violent bucking that frankly makes me afraid i'm gonna break a motor mount or something if i allow the car to cruise at low overdrive speeds. A tad bit of throttle clears this up. Highway speeds above 50 mph and acceleration are smooth.

It's my understanding per TECHINFO that a bad coil can cause this, so i pulled the coils and tested them. Here's my findings, listed from top coil to bottom coil if you're standing in front of the car, looking down at the engine:

top: Primary 1.5, Secondary 5.5
middle: Primary 1.4, Secondary 5.65
bottom: Primary 1.4, Secondary 5.5

it's immediately apparent to me that Primary resistance on all three coils is atleast .5 ohm out of tolerance according to the TECHINFO article. could this be causing my rough idle/low speed chugging problems, even though they're all very similar in resistance?
If this is not causing the issue, what else could cause these symptoms? IAC, TPS?

It wouldn't surprise me if a couple of those 18 year-old sensors were baked when the car overheated...

[Jrs3800]

You could have a bad coil, Hard to say for certain...

Do you have a spark tester?


as well if you do get coils, try to find a 97+ supercharged 3800 and pull the coils, if they have Blue tabs on the bottom they are the ones you want...

[Need For Steve]

Is it normal for a set of coils to all be that far outta spec? Not a set-in-stone kinda thing?

I'll pick up a spark tester and check things out.

I'll keep an eye out for a set of L67 coils, but these cars don't pop up in parts yards around here often at all

Oh, TPS showed .48V to 4.3V opening the throttle at the TB. Dunno if a bad TPS could cause this, but figured I'd check while I had the meter out. How's these numbers look?

[Need For Steve]

ok, got myself a spark tester, a Lisle in-line with the bulb in it. what exactly should i be looking for?

[primetime2]

my 93 bonneville sounds like it is running exactly like yours! I might just get a set of coils and start there i'll keep you posted on my findings

[Jrs3800]

ok, got myself a spark tester, a Lisle in-line with the bulb in it. what exactly should i be looking for?

You want to look for a strong spark... Also you want to listen to to the misfires while looking at the spark tester... If the spark looks weak it most likely is, if it drops out and then starts again then you may have a bad coil...

Just test them and see what you have...

[Need For Steve]

weak spark on 2 wires and got shocked several times handling the wires. new ones will be procured today...

[Need For Steve]

i tested for spark at the coils, and all seemed uniform. i tested the spark at the ends of the plug wires, and a couple looked weak so i replaced them.

brand new Autolite wires are on the car, slight improvement in idle quality but it still sounds like it's down a cylinder. i tried pulling the wires with the engine running, and there was NO change with the plug wire on or off #3 plug. plenty of spark between the wire and the plug, but no change in idle when removing the wire from #3 plug.

*EDIT*
tried swapping #3 and #5 plugs. still no change in idle when removing #3 plug wire.

this is very puzzling to me. #3 plug is getting spark, and #3 plug fires fine when moved to another cylinder. i did notice that the spark on #1 cylinder seemed noticeably stronger than #3 and #5...weak ICM perhaps? i don't know how to test that...

[Jrs3800]

While its running, unplug the #3 injector and see if this makes a difference... If there is no change at all then possibly a bad fuel injector... Also when you did the top end did you get all the injector pig tails into the right place? I made that mistake once...lol

Just a couple more thoughts... Gotta be simple..

[Need For Steve]

ha, yea i got all the injectors connected correctly...took plenty of pictures before i dove in

i just went and played with the injector pigtails. the engine responds negatively when any of the 6 are unplugged, so it seems the injectors are, for the most part, functioning. i'll go out again in a few minutes and try it again, just to make sure my ears weren't fooling me. i'm still intrigued by how unplugging the #3 wire has little to naught effect on idle or revs...

Here's how it's running right now. you can hear the idle as well as holding steady RPM; you can hear the missing when i try to hold engine speed above idle.


sorry for all the questions, but this is the first car i've had to exhibit these symptoms. this is a new learning experience for me

[tpulliam79]

Have you looked at the icm to see if its dirty or corroded? Also a broken spring on the rocker of the misfiring cylinder could cause it, had same pproblem with wifes explorer and turned out to be a broken spring under the rocker, just have to pull off the valve cover gasket to check.

[Need For Steve]

Have you looked at the icm to see if its dirty or corroded? Also a broken spring on the rocker of the misfiring cylinder could cause it, had same pproblem with wifes explorer and turned out to be a broken spring under the rocker, just have to pull off the valve cover gasket to check.

I don't think any of the ICM tabs were corroded, but i'll check again. I had the springs off a couple weeks ago while it was apart to replace the valve seals, and they seemed to be in fine condition...they are 18 years old though, so anything's possible.

interesting tidbit i noticed tonight; i went out and played around with the injectors earlier today on recommendation from jrs3800, and the miss has gone from steady to intermittent. it doesn't sound or feel like it's down a cylinder anymore, the idle is still rough and so is low speed OD, but it's not like it's missing every time that cylinder comes 'round...more like every other time. wonder if i jiggled something when i was messing with the injectors?