Slight Miss If Lugging Motor - ZR-1 Net Registry Forums

randy ransome · 10-01-2019

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 · 10-02-2019

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 · 10-02-2019

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 · 10-02-2019

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 · 10-02-2019

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 · 10-08-2019

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 · 10-08-2019

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.

10-01-2019	#1
randy ransome Join Date: Mar 2006 Location: Olive Branch,MS Posts: 291	Slight Miss If Lugging Motor 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?

10-02-2019	#2
lfalzarano Join Date: Feb 2011 Location: Clayton, North Carolina Posts: 1,133	Re: Slight Miss If Lugging Motor 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. Sent from my iPhone using ZR-1 Net Registry __________________ Lou[COLOR="Red"][B][SIZE="4"][/SIZE][/B][/COLOR]

10-02-2019	#3
Demps Join Date: Jan 2006 Location: Niceville Posts: 998	Re: Slight Miss If Lugging Motor 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

10-02-2019	#4
A26B Join Date: Dec 2003 Location: Arcadia,OK Posts: 3,394	Re: Slight Miss If Lugging Motor 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. __________________ Jerry Downey JERRYS LT5 GASKETS & PARTS http://www.jerrysgaskets.com 1994 *ZR-1, Black/Black, Lingenfelter Aerobody, 416cu in, 3.91 gears, coil-over susp, Brembo brakes, etc. 2016 Black-Red, 3LT-Z51 Auto 8-speed. Last edited by A26B; 10-02-2019 at 11:14 AM.*

10-02-2019	#5
randy ransome Join Date: Mar 2006 Location: Olive Branch,MS Posts: 291	Re: Slight Miss If Lugging Motor 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

10-08-2019	#6
rkreigh Join Date: Jan 2004 Location: Alex VA Posts: 1,087	Re: Slight Miss If Lugging Motor 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. __________________ 95 390 LPE ZR1 (505 rwhp) LSV = Lingenfelter Super Vette Twin Turbo 2003 Z06 (800 RWHP)