Slight Miss If Lugging Motor

[randy ransome]

I've got a miss if the car is lugging in 5th or 6th. If I gear down it will pull like a freight train. I'm thinking plug wire? bad plug? coil break down?

[lfalzarano]

At what RPM are you trying to throttle up in 5th or 6th. Most cars even with 4 speeds lug if you mash the pedal at low RPM’s.


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[Demps]

Randy,
My money would be on ignition...by your description. I would put traditional plugs (not platinum nor iridium) & gap them at .035. See what that does.

Ted

[A26B]

My money is on weak spark. I have personally experienced weak spark on a single cylinder that traced back to a bad coil & seen the same on other ZR-1's. The easiest way to determine if a weak spark exists is to put a spare plug in one wire at a time, start the engine & jump the spark to ground. A good spark will be blue and jump about 1/2". If its orange, that's a weak spark. If you have a weak spark, then you can track it down to a bad wire or coil, but don't assume it hs to be a wire if only one cylinder has weak spark.

With stock cams, the LT5 should accelerate smoothly in 6th gear from 40mph. Gearing down simply masks the weak spark.

[randy ransome]

Thanks everyone. Jerry, I'll check each wire with a good plug like you advised. My car usually will accelerate from 40mph in 6th gear without bucking or lugging. That's how I first noticed something was wrong.

Thanks Again

[rkreigh]

randy, most of the stock plug gaps are big and the engine can misfire under high overdrive loads at light throttle


too lean, and the weak spark won't light it off and you get some "bucking" and rubber band feeling from misfire


run some fresh plugs gapped between .032 and .035 and some good gas and see if it stops. Weak old coils or plug wires, same thing. Use a known good plug and pull one wire at a time and watch the spark event with the engine running and the test plug grounded


sometimes a weak coil will show as a pair that don't quite cut it.


Under load that pair can misfire on its turn in the firing order


Not really sure how to best test coil packs. Capers had a tool that we used in the turbo buick days (same waste fire plugs) but I haven't seen it around for LT5s and other than going with good ones, the visual seems to be a reasonable test.

[randy ransome]

Thanks Ron, I tried the New plug in each wire and the spark looked fine. I bought some AC 41-602 plugs and gapped them at .35. put them in and it ran better than it has in a while. No missing when accelerating, in sixth gear from 40 mph.

It had AC 5 plugs in it. The plugs had over 20,000 miles on them. I didn't realize, until researching it here, that these LT5's usually like new plugs before they get 20 thousand miles on them. From what I read 10-15,000 miles is about all you get from a new set of plugs. If that's true Why?

Thanks to All for your help.