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Discussion Starter · #1 · (Edited)
Hey all. I suspect a 'weak' spark is happening on my 3.5L, causing a somewhat rough idle and a noticeable groan / vibration between 1300 and 1500rpm. It doesn't matter if I'm moving, or if stationary (in gear, feet on gas and brake), the noise / feeling is there. I've checked compression and we are 170-175psi across the board. Next thought was the hissing sound coming from the tube via the valve cover to intake manifold. Research here stated the hose would need to be replaced to stop that. That of course equals a full new intake manifold as there's no good way to remove the cover w/o either destroying or cutting a hole in it (didn't want to do either). In any event, while that was not fun to replace and did get rid of the hissing, the sound / vibration remained.

So that brings me to the coils. In testing them, cylinders 1, 2, 3, and 5 show 12k ohms. Cyl 4 shows 11.94k ohms. I am unable to find what a good resistance is and from those I can, the range highly varies. On top of this, the plugs were recently replaced. Only the #4 plug shows a little tarnish on the white insulator. The other 4 remain white.

So, would a slightly weak coil (assuming 12k ohms is standard) cause the 'rough' idle and noise / vibration at rpm? There is no OBDII codes or Check Engine light on.

Note: have pics if one would like to see various things.

Edit: There is no significant loss in power. Only notice a 'slight' loss (on hills I traverse daily) and a decrease in fuel milage.
 

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Not a big enough difference in the "weak" coil to make a difference. It's only off by about .5%. Electrically speaking it's 12K.
 

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The 2004 GM Service Manual has no reference to coil resistance. Your best bet for troubleshooting this is to find a scan tool that capable of reading live data and monitor the fuel trims, misfires, MAP, MAF, Temp, etc, while the engine is operating, either while standing still or driving. If a coil is defective, it probably will cause a misfire, which in turn will result in a 'check engine' light.

You have in excess of 270,000 mi on the engine. Have the O2 sensors been replaced? Have you checked the condition of the motor mounts?
 

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Discussion Starter · #4 ·
Not a big enough difference in the "weak" coil to make a difference. It's only off by about .5%. Electrically speaking it's 12K.
Yeah, that's kinda what I expected as well. While it looks weak compared to the others, it's really not weak at all.

Have the O2 sensors been replaced? Have you checked the condition of the motor mounts?
Both O2 sensors were replaced 2 months ago, after reading a few threads here. Motor mounts look ok, no visual degradation that I can see. Outside of the coil, it would have been my next thought. Replacing those. Without an engine lift, I assume lifting / supporting the engine temporarily via the oil pan w/ wood and a jack is ok?

reading live data and monitor the fuel trims, misfires, MAP, MAF, Temp, etc, while the engine is operating
For this, my scan tool can read live data. Is there anything specific I should be looking for?
 

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With the scan tool you would be looking to see what the difference is between the readings when the problem occurs vs when it doesn't.

I don't think supporting the engine from under the oil pan would cause a problem if a wide board was used and only enough pressure was applied to loosen the bolts. I recently had to remove the transmission in my truck to change a rear main seal and I just used a bottle jack and a 2x4 to support the engine. Mine is a 5.3, but shouldn't be much difference.
 

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Discussion Starter · #6 ·
So, both motor mounts and transmission mount replaced. Needless to say, the noise / vibration is gone! Per the pics, the mounts were indeed shot. The location of the rubber degradation made it hard to notice by eye visual. While the passenger side was no real problem, the driver side was a real bear. Unbolted the ground cable, as well as the knock sensor, just to get some room above. Took quite the finagling to get it out and new one back in. In any event, all is well now! In time, I may tackle the ball joints, as they are still stock. We shall see what the future holds.
 

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