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Author Topic: Inconsistent spark plugs  (Read 68 times)

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HunterB

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Inconsistent spark plugs
« on: November 14, 2017, 01:46:06 »

Sorry for the noob question but I’ve never seen this before and wonder what you guys think.
So last weekend I pulled the the spark plugs out of the Gran Fury so I could fog the cylinders before putting it away for the winter this is what I found.
Some of them look black as if they might be burning oil on that cylinder, and others like number 4 and especially 6 look bone white? Also some of them are darker on one side of the electrode than the other (most drastic on #2) I’ve never seen that before so I don’t know what that means if anything? All gaps where still at .35 where I set them 6 months ago. Here’s some pics. 
First one is all 8 in order second is number 6. I have more if needed.
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lsm360

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Re: Inconsistent spark plugs
« Reply #1 on: November 14, 2017, 11:58:20 »

It isn't unusual to have some inconsistency in the plugs in a high mileage engine. I'm guessing it's high mileage? Might have some seepage of the valve seats (that would be my first guess). Do you get any smoke at cold start ups, or any other time, and if so what color?

If concerned, start with compression test. Otherwise, I'd just clean up the plugs, make sure the carb is clean and well tuned, and gas up with a company that has a good additive, like Chevron/Shell/Mobil www.toptiergas.com
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'89 Diplomat S code AHB now with FI 410 C.I.D. smallblock

lsm360

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Re: Inconsistent spark plugs
« Reply #2 on: November 14, 2017, 11:59:51 »

Btw, I'm digging the Mopar heater hose
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'89 Diplomat S code AHB now with FI 410 C.I.D. smallblock

HunterB

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Re: Inconsistent spark plugs
« Reply #3 on: November 14, 2017, 23:00:27 »

It’s got about 97k on it. I’ve never noticed any smoke on start up, except when it’s cold outside (40 or below) which just about every car does.
Something else that’s odd, I did a compression test when I first replaced the plugs back in June these were my numbers.
#1 160
#2 162
#3 155
#4 155
#5 158
#6 157
#7 159
#8 160
When saw what the plugs looked like I did it again here’s the new numbers in the order I tested them.
160
164
168
150
149
145
145
140
I thought it was strange that some were so much lower, so I retested my highest one, cylinder #5 and it came up with 150? Only thing I can think of is my battery was getting pretty weak by the end of the test and wasn’t turn the motor over nearly as fast?  Im not too worried about it I just want to make sure this engine is good shape before I spend money an intake,carb,and maybe a cam and heads.

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DobaMark

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Re: Inconsistent spark plugs
« Reply #4 on: November 16, 2017, 10:18:14 »

Did you do a wet and dry comparison? If the reading jump up wet (after a cap full of oil is put down the cylinder) then the rings are weak.
I'd do the test again with a strong battery. Even better than a compression test is a leak down test.

To me though, looking at the plugs, they don't look oil fouled. They look more rich fouled. Some of that will be common as the fuel isn't perfectly distributed to all cylinders, but I'm wondering if the affected plus weren't firing all the time (bad wire, cap, rotor) and that's why the deposit are there.

Honestly, unless the engine hs had work done in the past, slapping a cam into or new heads onto a 30 year old engine without refreshing it will always be a gamble no matter how well it is running now.
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HunterB

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Re: Inconsistent spark plugs
« Reply #5 on: November 16, 2017, 12:23:29 »

Compression tests were both done dry with the engine stone cold. Running rich would make sense since I haven’t touched the carb and I average 12 to 12.5 mpg on 91 octane. I replaced the distributor cap, rotor, and plug wires at the same time as spark plugs but I am still running the original coil and I’ve never check to see how strong the spark is.

  Yeah I know doing any major work to such an old engine may not be the greatest idea, and I don’t have a problem with freshening up the bottom end, I just don’t want to pull engine out if I don’t have to.
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MarkM

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Re: Inconsistent spark plugs
« Reply #6 on: November 16, 2017, 14:39:28 »

I agree with Mark, you should probably start with building a new short block over the winter rather than upgrading the top end.  I know replacing an engine seems like a big task, but it isn't that bad, especially if you do your homework first and keep it small block.  Myself and other guys on the forum here have done this swap dozens of times and can help you every step of the way.
« Last Edit: November 16, 2017, 14:39:57 by MarkM »
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HunterB

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Re: Inconsistent spark plugs
« Reply #7 on: November 16, 2017, 23:50:14 »

That has me thinking. Because I still have my old Dakota with its 318 magnum, that’s a drop in replacement right? It’s a high mileage motor (210k) but it still runs great 170 psi of compression on every cylinder (except #7 that one had 150) and 25 lbs of oil pressure at warm idle. I can’t be sure about this, but I think there was work done to it before I got it, because it pulls waaaay harder than any other magnum I’ve ever driven. In fact the only reason I haven’t gotten rid of that truck is I just couldn’t sent it to the junkyard with that motor in it.
I know that sounds dumb but that engine is special to me since it was in my first truck and I got it to way to much trouble with that thing!
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HunterB

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Re: Inconsistent spark plugs
« Reply #8 on: November 16, 2017, 23:56:16 »

Rebuilding it would be a good project for the winter. Problem is it’s still in that truck, and I don’t have a cherry picker, not to mention the fact that it’s miserable to work on cars this time of year in Wisconsin when you don’t have a garage.
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