When I first purchased this truck I inspected the 390's plugs and found #8 was black. I replaced just #8 and have driven about 400 miles. Distributor has a properly gapped pertronix ignitor. new coil, cap/rotor are good.
Then I replaced all the plugs on my 390, as shown below. The #8 plug on the left is the new one (w/400 miles) and the #8 on the right is the old one.
I'm not sure what to think of the new #8 plug. Is this normal? Prehaps not enough miles to tell?
I repeated a compression test on #8 hole and it is good. The plug wire measures the same resistance as the others. I get somewhat of a spark from the plug with it out of the hole. If I pull #8 wire off the cap while idling I can detect engine speed slightly decline, and get a decent blue arc, similar to any other cylinder.
I'm wondering if this plug still fires under other conditions, when cylinder pressure is higher.
I try not to just throw parts at stuff. The plug wires are of unknown age.


Close up of the #8 new, #8 old and #7

 

I would monitor #8 plug Justin but not worry to much about it. I think that original #8 plug is older than the rest. 1st off, it's more rusty than the rest. #3 may be close, but #8 looks totally different than the rest. And #8 is back there by the master cylinder. Harder to get to than the rest. The PO might have gotten lazy last time he changed plugs. The 400 mile one looks good. Blue spark and good compression makes me happy. Unknown age on plug wires? I'd change those. It is possible the spark is leaking out of that #8 wire every once in a while to ground.

 

And I'll add this. If it were me I'd replace the plug wires with Genuine Ford Motorcraft ones. Not because I have them in stock here, which I do, but they will be of higher quality. Will only be 7MM but the center core is more important than the insulation. I remember prolly 20 years ago I was installing a universal length Accel brand set. Just for SAG I cut in half the old Genuine Ford wire I was replacing to compare the center cores. The Ford wire's core was DOUBLE the diameter of the Accel's.
Part number E5PZ12259NR replaces D1AZ12259G,D6PZ12259A, and D6PZ12259AR for 71/72 Bumps with 360,390 (and listed for 300 I6 believe it or not)

 

Jeffaffa,

If I could ask a further question on the wires, is there a set with really nice silicone boots, angled properly, perhaps in a heavy-duty jacket?

I used some for a 350/460 dump truck, and they were great, except #1 was a bit short, and #8 was 5 inches too long.

 

When you say compression is good what does that mean to you?
Please supply us with the cylinders number of psi testing as in psi lbs. as this important info so we know actually what were working with.

As many people think 90-110 is good. A oily looking fouled plug many times has a higher rate out put since oily cylinder rings makes a better ring seal so it can be about 7-10 psi higher.

Rusty crusty stuff mostly comes from gasoline you use it's additives.

These fe engine come with straight wire boots most can be slightly bent like about 20/* degrees or so. A 90* degrees

A 90*degrees angel takes longer cables so may be get a universal set an cut to length maybe.

Orich

 

Compression test in #8 hole was #110 psi and held it for 40 min. Which seem ok given atmospheric pressure of 14.7psi and comp ratio of 8:1-ish
14.7 * 8 = 117.6. Similar number to #5 which I tested previously.
I don't think this hole has a oil issue, as the 'new' plug that has lived in it for the last 400 miles has no oil on it at all. But I'm no expert hence this thread.

 

Might have good compression rings, but the oil rings might be a bit weak and that is the black residue you see on the #8 plug. Or the valve stems seals might be cooked and that one is dripping oil down the valve?

 

Exactly why I said I'd monitor it but not worry too much.

 

Yepper. Every plug wire set I've seen for electronic ignition had the angled boots. And 8MM wires. As of 1975 the 360/390s got electronic ignition. With 8MM wires and matching Dizzy cap. The sockets on top of the cap were bigger I.D for the 8MM wire's bigger terminals and O.D. As of 1976 (last year for the FE) Ford had not come out with the 2 piece Dizzy cap yet. So I looked this stuff up for a 1976 360/390.
Wires...D6PZ12259AF,replaced by D6PZ12259AFR,replaced with E5PZ12259HR. Still available.
Cap.....D5AZ12106A (was blue),replaced by D7TZ12106A (from memory it's gray). Still available.
Rotor...D5AZ12200A ,replaced by D5PZ12200A. Still available.


If anybody is interested in buying the Original blue D5AZ12106A cap these places show them in stock.


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I received a PM from 1972-34ton asking me to post what I would do if I had a 360/390 with the two piece cap installed on it. This would also apply to anyone who put one of those HEI style Dizzys in their engine. As I believe Orich has.
1976 was the last year for the FE 360/390. And 1977 was the 1st year for the two piece cap. So Ford never made any plug wires for the two together. One would have to get a cut to fit universal set of plug wires. This way you can route them nice and have them look all uniform also. Me personally, I would buy one of these sets from FRPP (Ford Racing Performance Parts). AKA Ford Motorsport. AKA SVO. They are 9MM with the silicone angled boots. Last I heard they are made by MSD and come with terminals and boots for either one piece cap or two piece (which also fits HEI cap). Can handle whatever coil you throw at them. And each wire is marked the cylinder number and "Ford Racing". Years ago I put a set of these on a 77 Vette my brother had.
M12259C302.....Blue.
M12259M302.....Black.

 

I using wire for a 400/460 msd 8.5 all fit good with out cutting any to length but right out of the box. Also fit the HEI dizzy well.