Spark Plugs

[jsnevets]

OK, my car has 140k miles on it and I finally replaced the spark plugs yesterday with the AC-Delco 41-101s. Checking the gaps on the old plugs showed an odd pattern. The plugs from the front side were worn heavily (as might be expected) but the back side almost not at all. What would explain such a difference?

Front (left):
#1, 0.086
#3, 0.095
#5, 0.084

Back (right):
#2, 0.062
#4, 0.062
#6, 0.062

[richtazz]

That is rather strange. I've seen the opposite, which is usually because someone is too lazy to get to the rear bank and only changes the front 3.

Look at the ceramic insulator of the front 3 plugs and look at the color of the deposits on them. A light tan color is normal. If they are white (lean condition), orange or green (coolant contamination depending on what color coolant you're using) or black (rich condition). Let us know what you find, then we can make a better educated assessment for you.

[tblake]

How long have you owned the car for? Maybe someone had to replace the back head for whatever reason at one point in its life.

[jsnevets]

I bought the car new. As far as I know the plugs were never replaced. Here are pictures of the plugs.

Plug #1


Plug #2


Plug #3


Plug #4


Plug #5


Plug #6

[tblake]

Wow, they look pretty used up. You weren't kidding.... Look how worn out those ground electrodes looks on #2, 4, and 6. Very odd. Just to confirm, these are from your 01 GTP with the L67, right?

Stupid question I know but on the 3800's the back bank is 2,4,6 and on the 3100's the back bank is 1, 3, 5

[jsnevets]

Yes, this is from my 01 GT with the L67. The fronts are #1,3,5 which are the most worn out. The backs are #2,4,6 which are pretty good by comparison.

[tblake]

Very pecular, Are all the plugs the same part number?

Maybe you have excessive knock retard on the back 3 cylinders.

What brand plugs did you replace them with and what gap?

Maybe pull the new ones out in a couple months to see how they look.

[jsnevets]

Yes, all the originals are the same and are marked: NGK PTR4B-15 12565030.

I replaced them with AC Delco 41-101s from the dealer gapped to 0.060.

Well if I'm going to inspect the new ones in a few months it's a good thing I used dielectric grease because those old wires were a total PITA to get off.

So, what might cause one side to have more knock retard than the other side?

[BNaylor]

Originally Posted by jsnevets

Yes, all the originals are the same and are marked: NGK PTR4B-15 12565030.

The NGK PTR4B-15 double platinum plugs are excellent and were delivered with GTPs only with the L67 engine. The others got the AC Delco double platinums.

BTW - The double platinums are only good for no more than 100K miles so this may be much to do about nothing. You were way overdue anyways.

Before anyone says anything bad about NGK spark plugs the AC Delco 41-101 Iridium plugs are made by NGK for GM. .....

[tblake]

Originally Posted by jsnevets

So, what might cause one side to have more knock retard than the other side?

The back bank is always going to run hotter than the front. Coupled with the fact that they are stock plugs might explain why they are so worn out.

[jsnevets]

Originally Posted by tblake

The back bank is always going to run hotter than the front. Coupled with the fact that they are stock plugs might explain why they are so worn out.

It was the front bank (1,3,5) that was more worn out.

Originally Posted by BNaylor

BTW - The double platinums are only good for no more than 100K miles so this may be much to do about nothing. You were way overdue anyways.

Yes. Very much so. I just wondered if something may not have been quite right for there to be so much difference in wear between the front and back set.

[tblake]

Laying in bet last night it hit me....

The plugs are stock with 100,000+ miles.

Its hard to explain, but GM wastespark system fires each plug on the intake and exhaust stroke. Due to the way the coils are set up, on the even cylinders, due to the law of current flow, the spark jumps from the center electrode to the ground electrode. On the odd cylinders it is the complete opposite. Coupled with the fact that each spark plug fires every single crankshaft revolution instead of every other, explains why on your odd cylinder spark plugs the center electrode is worn out and on your even cylinder plugs your ground electrode is worn out.

......Normal for the given milage. Make sence? Replace them, drive and be happy!!! LOL

[BNaylor]

Here is the book answer on the GM DIS "waste spark" system.

Each cylinder is paired by a single coil with its opposing cylinder in the firing order (1&4, 2&5, 3&6 on a V6). This means one cylinder on its compression stroke fires simultaneously with its opposing cylinder on the exhaust stroke.

Since the cylinder on the exhaust stroke requires very little of the available voltage to fire its plug due to the easy ionization of the hot exhaust gases, most of the available voltage is used to fire the cylinder which is on the compression stroke.

Reference: General Motors STC book #16020.05-3

[jsnevets]

Well I learned something new. Thanks! After searching the internet I came up with a few other tidbits.

1. The electrode initiating the spark will loose more material and erode faster compared to the one receiving the spark.

2. Double-tipped platinum and iridium are supposed to withstand this reverse polarity better. Obviously there are limits as seen by my plugs. Perhaps this is why I saw a patent which proposed reversing coil polarity every other cycle to maintain balanced plug wear.

3. Copper-core standard and single platinum plugs are not designed to withstand reverse polarity and will wear out prematurely.

4. One site even mentioned that plugs should not be moved from one cylinder to another because electrons in the plug become aligned with the particular polarity.

[tblake]

Thanks Bob for showing me up with the "book answer". LOL!

Friend of mine and I would joke at the shop and say how we could do a 5 minute tune up on the GM wast spark system by simply swapping the plug wires on consecutive coils. Thus making the spark jump the other direction, and causing the opposite electrode to burn out. (We never did it, but got a good laugh out of it).