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Random misfire due to weak spark, WHAT'S WRONG????

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Old 09-27-2004, 05:16 PM   #1
Thread Starter
Join Date: Jun 2000
Location: Manassas, VA, USA
Posts: 180

Random misfire due to weak spark, any ideas?

OK I am really frusterated with this.

I have a non-computer controlled HEI distributor from a 70-something Chevy.

My problem is that it drops a spark every so often. I hooked up a spark tester and every spark is great, strong, blue-white, except randomly one won't spark at all.

That was with the tester gapped at 35, reduce the gap to 20 (not certain if the numbers on the tester equal spark plug gap or KV or what), and it fires every single time.

So this implies a WEAK spark not a MISSING spark, right? Or at least a spark that is going somewhere else (firing to distributor body or something).

What I wouldn't give for a clear distributor cap :-).

So what have I tried?

Swapped coil with a known good one.

Replaced module with a new one from GM.

Replaced cap and rotor.

Checked plug wire resistance (135 ohms) on the wire I had the spark tester plugged into.

I do NOT suspect the pickup coil because that would result in NO spark, right? As long as the gap is small enough, I never miss a spark.

I do NOT suspect the plugs because I used a spark tester.

I do NOT suspect the plug wires because the one I had the spark tester on was fine, and it is a random cylinder anyway.

I know the misfiring cylinder is sometimes on the even bank and sometimes the odd bank, because when you hear the miss in the exhaust note, you sometimes see it on the O2 gauge and sometimes not. My O2 sensor is in the downpipe from the odd cylinder bank.

I checked voltage at the power wire for the distributor, engine running, it reads 14.0 volts, versus 14.2 at the battery. Not a significant drop, I don't think. And it was nice and steady.

What could possibly be wrong?

Last edited by Eggplant Jeff; 09-27-2004 at 05:27 PM.
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