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Discussion Starter #1
Can a coil whose windings (both primary & secondary) repeatedly pass a resistance check still be capable of flaking out, particularly when the engine is under load? This coil passes visual inspection as well.

Application: '91 Sable 3.8L

Regards... Mike
 

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Can a coil whose windings (both primary & secondary) repeatedly pass a resistance check still be capable of flaking out, particularly when the engine is under load? This coil passes visual inspection as well.

Application: '91 Sable 3.8L

Regards... Mike
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I'd like to know the answer to that as well. I have a 2002 Taurus with the 3.0L OHV. I am getting an intermittent miss and once had a pending code for a misfire on cylinder 3. The secondary resistance is out of spec acccording to my Haynes manual.
 

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Discussion Starter #3
jaba,

I think I would be inclined to replace your coil, given your symptoms and the fact that your secondary windings are outside of specs.

Regards... Mike
 

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Goathead: what is your original problem? The coil could still be weak under load, with higher rpm's. Is there a spark leak anywhere?

jaba: What secondary resistance is too high? At the coil, or plug wire?
 

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Discussion Starter #5
94clit Posted Today, 12:41 AM
" Goathead: what is your original problem? The coil could still be weak under load, with higher rpm's. Is there a spark leak anywhere?"

94clit,

The problem is what feels like random misfiring when the engine is under light lugging conditions/fairly low rpm. I refer to it as a "chuggle". At one point I thought it was TCC shudder, but the problem has gradually worsened and I can feel it when the TCC is not under lockup now as well.

I've been over the fuel system, EGR and ignition system with a fine tooth comb; everything checks out as being OK.

My timing light indicates a significant miss on cyl #3 when I rev the engine, but only slightly on other cylinders. I also get it when connected to the coil wire, which tells me the problem is at the coil or upstream from it. I might add that my light does this somewhat on my other car too (that one runs well!), but the fact that it's more prevalent on cyl #3 arouses my suspicion.

I have new wires, plugs, cap and rotor. I substituted a different #3 wire and coil wire; it didn't help. I tried a different distributor and ignition module; no difference.

Early on, I substituted a spare coil (which also passes resistance and visual examination), which didn't help. I suppose it's possible that both coils are weak and exhibited the exact symptoms, but it seems unlikely to me.

The problem doesn't seem to reveal itself (or at least, not very much) until the engine is warmed up. Perhaps I should try temporarily relocating the coil away from the heat of the engine to see if any difference is noted.

I do get an occasional DTC indicating a lean condition on bank #1 (where #3 cyl is located, by the way). I swapped the O2 sensors side to side to see if the DTC moves, but I'm still waiting for a code to reappear. The O2 sensors pass the propane torch test, although I suspect that is a very crude way of testing them. They have only about 15K miles on them.

It's probably some stupid simple thing that I've overlooked... I've been guilty of doing so in the past.

Regards... Mike
 

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The coil resistance will change with temp. First put it in the freezer for 8 hours then test, then room temp for 8 hours, then in the oven at 200F for 1 hr. This is how I found my V8 coils were bad.
 

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Discussion Starter #7
SHOZ,

Thanks for the tip. I tried it with both coils (although I didn't have time tonight to freeze them for a full 8 hrs).

At 15 deg F the resistance in both was slightly below specs. However, at room temp and at 200 deg F they were spot on.

What about the radio noise suppression capacitor? Do these ever fail? Are they capable of shunting too much energy to ground, thereby leaving insufficient voltage for ignition?

Regards... Mike
 

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Goathead: what is your original problem? The coil could still be weak under load, with higher rpm's. Is there a spark leak anywhere?

jaba: What secondary resistance is too high? At the coil, or plug wire?
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The secondary resistnance is out of spec at the coil. Haynes says it should be 13600 ohms across each coil and I measured 11800 ohms. No range is given so I don't know if that's acceptable or not. I notice the miss starts when the engine is warm and the RPM's are low (in overdrive) and the engine is under a little load.
I have replaced the wires and plugs with new ones. They should be fine.
 

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Discussion Starter #9
goathead said:

"I swapped the O2 sensors from side to side to see if the DTC moves, but I'm still waiting for a code to reappear."

Ahhh... and now it has happened: Today my CEL came on; the O2 sensor lean condition has now moved to bank #2, leading me to believe that I have a faulty sensor. Perhaps it became somewhat fouled when I was burning antifreeze a while back.

It is my understanding that a bad O2 sensor can cause crummy fuel economy (I have that). However, I'm not so sure that it could cause a condition that feels like a miss, especially since it triggers the CEL only infrequently. I've replaced the sensor... we'll see.

Regards... Mike
 
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