Strange Spark Plug issue

[ralmo94]

My 94 has 3.42's, 265/70's almost seem too much in oD . Fine empty, but it really runs a lot better at about 2k rpm.

You could consider a smaller tire? Cheaper, or keep it out of OD

[1project2many]

The only difference I know of is that the "C" in the recommended plugs stands for commercial. That tells me nothing. I can visually see that the electrode is larger on the commercial plug but most people swear by them on these trucks.

Any insight is welcomed.

The only difference is that the electrode is larger. Each time a spark occurs, metal is transferred from the electrode. A smaller electrode wears out faster. GM put a ton of work into making sure TBI trucks were as reliable as possible, including "longer lasting" commercial plugs.

I think that changing the plugs fried my ignition coil.

Typically new plugs, properly gapped, require less voltage to create a spark.

The pertronix was fine I guess but I'd like it to last longer than five years like this one did.

That's not a bad timeframe. It's very, very hard to find quality parts these days. I'd expect a used GM original coil to outlast most new Chinese made parts.

I'm starting to think I understand what's going on. I suspect that the larger electrodes on the commercial grade plugs increase resistance just enough to destroy coils that may already have enough mileage on them to finish them off.

Larger electrodes do not significantly alter resistance across the plug. Plug gap significantly alters resistance. If the gap is the same it would not make sense to blame plugs without actually finding a problem in the plug.

My main goal has always been to get the best fuel mileage possible so I will likely try the 3.42's.

Sometimes, keeping the rpm low increases load to the point where you lose economy. I've played the same game you're playing now, and the truth is that you can achieve better fuel mileage running a numerically high gear such as 3.73 or 4.10 and tall, skinny tires than you can find with wide tires and numerically low gears. During cruise, a wider tire generally has higher wind resistance and requires more energy to sustain speed. During acceleration a wider tire will generally have greater mass and will require more energy to accelerate. How does the work and money for new tires compare to the cost and effort required for a gear change?

[PlayingWithTBI]

I think that changing the plugs fried my ignition coil. You have any suggestions for a new one? The pertronix was fine I guess but I'd like it to last longer than five years like this one did.

I fried a Pertronix Flame Thrower and a Spectra Premium ICM in ~2000 miles. As @Tayto said they're junk. If I can find it, I'll run ACDelco GM Original Equipment, more so than their other lower tier stuff. A lot of times, if your coil goes out, your ICM is probably toast too. Yeah, they're somewhat expensive - you get what you paid for.

https://www.amazon.com/dp/B08NMVWW8H...roduct_details

https://www.amazon.com/ACDelco-D1984...7-93deae8f9840

[Purv-Dawg]

My 94 has 3.42's, 265/70's almost seem too much in oD . Fine empty, but it really runs a lot better at about 2k rpm.

You could consider a smaller tire? Cheaper, or keep it out of OD

I appreciate your info. I may go to a smaller tire and I don't mind dropping out of overdrive on uneven roads.

[Purv-Dawg]

The only difference is that the electrode is larger. Each time a spark occurs, metal is transferred from the electrode. A smaller electrode wears out faster. GM put a ton of work into making sure TBI trucks were as reliable as possible, including "longer lasting" commercial plugs.



Typically new plugs, properly gapped, require less voltage to create a spark.


That's not a bad timeframe. It's very, very hard to find quality parts these days. I'd expect a used GM original coil to outlast most new Chinese made parts.


Larger electrodes do not significantly alter resistance across the plug. Plug gap significantly alters resistance. If the gap is the same it would not make sense to blame plugs without actually finding a problem in the plug.


Sometimes, keeping the rpm low increases load to the point where you lose economy. I've played the same game you're playing now, and the truth is that you can achieve better fuel mileage running a numerically high gear such as 3.73 or 4.10 and tall, skinny tires than you can find with wide tires and numerically low gears. During cruise, a wider tire generally has higher wind resistance and requires more energy to sustain speed. During acceleration a wider tire will generally have greater mass and will require more energy to accelerate. How does the work and money for new tires compare to the cost and effort required for a gear change?

The r44TS plugs I was running before the change were gapped @ .045. I installed the CR43TS plugs with the stock .035 gap. I remember that these trucks had issues with a lot of electrical noise.

I have noticed that my average fuel mileage improves when taken out of overdrive on uneven roads. The tires I'm using now are actually slightly more narrow than the stock @ 225. Your point is exactly why I don't understand the reason why so many people want a wider tire on a jeep whenever it not only decreases mileage it decreases traction. Sure, it looks cooler but it's impractical.

[Purv-Dawg]

I fried a Pertronix Flame Thrower and a Spectra Premium ICM in ~2000 miles. As @Tayto said they're junk. If I can find it, I'll run ACDelco GM Original Equipment, more so than their other lower tier stuff. A lot of times, if your coil goes out, your ICM is probably toast too. Yeah, they're somewhat expensive - you get what you paid for.

https://www.amazon.com/dp/B08NMVWW8H...roduct_details

https://www.amazon.com/ACDelco-D1984...7-93deae8f9840

I thank you for your input. I know the pertronix modules get terrible reviews on summit racing. They have about six immediate failure reviews listed on them. I suspect the pertronix coil was recommended was because it's 50% cheaper and one of only two coils that boast 45k volts on summit racing website specifically for this truck. The other is the Davis unified and the reviews put the ratings almost identical. That's really not a truly reliable way to determine quality but without professional input it's all there really is available.

[ralmo94]

I have noticed that my average fuel mileage improves when taken out of overdrive on uneven roads. The tires I'm using now are actually slightly more narrow than the stock @ 225. Your point is exactly why I don't understand the reason why so many people want a wider tire on a jeep whenever it not only decreases mileage it decreases traction. Sure, it looks cooler but it's impractical.

I agree

I run 225/75 -16 on my 454 with 3.73's, I think more room in the wheel wells look better than them being over filled. You are right a narrow tire has less rolling resistance and more lbs per square inch = more road traction. Shorter sidewall less flex = better handling.

OD = more parasitic loss, I think you are better off with tall gears or tall tires and no OD. Exception is if you want to tow, gear for towing in 3rd, and empty cruising in od,

[tayto]

My squarebody with 30" tires, 700R4 and 3.42s OD is pretty much useless. going up hills suck too. I am wanting to goto 4.11s or 4.56s next year.

I would go with AC delco for plugs, coil and module. I think the Davis stuff would be fine for coil and module, but have only heard good things as i stated before.

[ralmo94]

Yeah nearest I can tell the best thing to do is tune cruise rpm,
2200-2400 @75-80mph seems to be a good target for aerodynamic cars,
2200 @ 70-75 fairly good for trucks, JMHO

[Fast355]

Yeah nearest I can tell the best thing to do is tune cruise rpm,
2200-2400 @75-80mph seems to be a good target for aerodynamic cars,
2200 @ 70-75 fairly good for trucks, JMHO

My van is a little under geared. Needs a 4.10 gear and has a 3.73. 30.47" tall tires and a 4L85E.

[LeMarky Dissod]

I've had to reduce the spark tables down to 12 degrees in the cruising speed range to get the ping down enough while climbing after dropping down out of overdrive.
The locked torque converter (with 2.73) only worsens the problem and I have added about 15 degrees of retard when it's locked.

I'm going to replace the rear gear within the next couple of weeks but I'm trying to decide on 3.42 or 3.73.
My main goal has always been to get the best MpG possible so I will likely try 3.42.
I just don't want to be pissed off at myself for being unable to either stop the ping or decreasing my fuel mileage to the point I don't want to drive it any distance.
Power is great but this engine has serious limitations without more modifications than I can afford.
These are the main reasons I've stuck with the stock heads, which most people consider garbage.
If I dropped a pile of cash on a set of summit racing heads it would still cap out at around 330 horses unless I change the intake, injectors, etc.
It would be far less expensive to swap to an LS and it would still get better MpG.

Purv-Dawg,
I daily drive an '02 Tahoe Z71 - L59, 4L60E, 3.73, 265/65R18 (31.56" tall) - all over NY, NJ, CT (and occasionally PA RI & MA).
If I could afford 4.10 for both front & rear axles, I'd do it immediately, without reservation or regret.
4.10 would make things easier on both the L59 & the 4L60E in terms of durability and longevity.
4.10 would actually let me spend even more time in lean cruise than 3.73 - basically whenever the TCC would be locked while in 3rd or 4th.
(Lean cruise, formerly known as highway mode, yeah?)

For whatever it's worth to you.

[Purv-Dawg]

How does the work and money for new tires compare to the cost and effort required for a gear change?

Believe in or not the cost will be about the same amount after a gear swap. The five 18 inch wheels and tires were just under $350 and they had less than 5k miles. The hub centric wheel adapters were $120 and summit racing has a set of gears for another $122. The quote for new tires on the original wheels was over $600 for four

[Purv-Dawg]

Interesting perspective. You've given me something to think about. I appreciate that!

[LeMarky Dissod]

Sometimes, keeping the RpM (too) low increases load to the point where you lose MpG.
... truth is that you can achieve better MpG running a numerically high gear such as 3.73 or 4.10 ....

Reminds me of my ol '94 Caprice wagon, came with 2.56.
Without a downhill or a tailwind, 4th gear was utterly USELESS. That wagon spent so much more time in 1st or 2nd or 3rd than in 4th.

... and tall skinny tires, than with wide tires and numerically low gears.
During cruise, a wider tire generally has higher wind resistance and requires more energy to sustain speed.

In that same wagon, 235/75R15 always got better MpG than 255/60R15, 255/65R15, or 255/70R15.

How does the work and money for new tires compare to the cost and effort required for a gear change?

In NYC, going from 2.56 to 3.42 a few years ago cost me $750, including a limited slip diff.
Without a doubt, best $800 I ever spent on that wagon. 4th gear finally became useful.
$750 in tires would never be able to do that.

Believe in or not the cost will be about the same amount after a gear swap.
The five 18 inch wheels and tires were just under $350 and they had less than 5k miles.
The hub centric wheel adapters were $120 and summit racing has a set of gears for another $122.
The quote for new tires on the original wheels was over $600 for four.

For a given tire load rating, going from a taller tire to a shorter tire - I.E. 31.6" tall to 30.4" tall - is not going to change the effective axle ratio all that much.
1750RpM, 2.73, 4th gear, 31.6" tall tires = 86.08MpH
1750RpM, 2.73, 4th gear, 30.4" tall tires = 82.82MpH

Not enough gear for a Caprice wagon, nevermind heavier trucks with a worse drag coefficient, AND more drag area.

Going from 2.73 to 4.10 is FAR more dramatic.
1750RpM, 4.10 4th gear, 31.6" tall tires = 57.32MpH
1750RpM, 4.10 4th gear, 30.4" tall tires = 55.15MpH

To be redundant, I'd prefer to stay in 3rd or 4th with the TCC locked for as long as reasonably possible - even without lean cruise.

[Fast355]

I agree

I run 225/75 -16 on my 454 with 3.73's, I think more room in the wheel wells look better than them being over filled. You are right a narrow tire has less rolling resistance and more lbs per square inch = more road traction. Shorter sidewall less flex = better handling.

OD = more parasitic loss, I think you are better off with tall gears or tall tires and no OD. Exception is if you want to tow, gear for towing in 3rd, and empty cruising in od,

With a 4L80E/4L85E I gear to tow in overdrive. Easier to get rolling from a stop having 4 useable gear ratios with the deeper gears available to get you rolling. 700r4 and 4L60Es can barely hold up moving a light 305 TBI powered car around much less a truck towing a trailer.