I have seen this on 352s, 360s, 390s, and 428s. It is always the most oily, carboned, or otherwise fouled compared to the other 7 plugs. I am talking about the driver side rear most plug.

So far my only theory is that the engine angle forces the oil to the back of the engine which makes it more likely to leak out of the valve cover and down the threads of the rear spark plugs. But the number 4 plug is usually normal.

Are there any legit reasons for this?

 

Probably time to clean out the oil drainback holes in the heads- when they start to clog with sludge, oil stays in the cover, floods the stems, and excess oil goes into the chambers- if it's only partially clogged, probably only the last one staying flooded long enough to foul the plug

 

Can't happen.

 

Number 8 is the rear most plug, and running the coldest may be more susceptible to fouling.

Before I get jumped on, please be aware I'm speculating, but with some facts to back me up. And as far as it goes, a well tuned engine should show all plugs to be the same. To get a fouled plug from a cold cylinder sounds like the mixture would have to be quite far off to begin with.

But I do know when I had piston damage due to detonation, the front cylinders were worse (hotter cylinder heads near the front).

Anyway, coolant goes by the cylinders, drawing heat, until it gets to the rear. Then it goes up, and back through the heads to finally exit out and head for the radiator.

So although the front cylinders might be cooler than the rear cylinders, the rear of the cylinder head should be cooler than the front. And of course, the plugs are in the cylinder head.

This may answer what's different, and may cause the rear plug to look different, or even become carbon fouled, but it doesn't address the problem of an oily plug.

I'd suspect that the oily plug problem is co-incidence. It may be typical of any of the four corners and is likley caused by intake gasket problems. Not sure, but that's my best guess. In fact, a leaky intake gasket could be the cause of all your troubles. Since you've seen it on so many engines, maybe that #8 being the colder area allows the gasket to be more leak-prone.

 

I was thinking intake gasket or examine how you PCV is routed. If it feeds in a manner where oily residue from a worn engine is draining/leaking into the #8 that may be the answer.

good luck
Drew

 

Most of those sound like possible causes...I will start by checking the oil drainback holes for clogs since that is the easiest.

Thanks for all the suggestions

 

Brake booster?

 

Really, no one knows this one? It is because of the firing order and the physical location of #8. At the back of the motor and #8 fires right after #7 and for some reason to do with the design, it makes #8 foul more. Don't be pulling the motor to check the drains, that's bull. Do a compression test and replace the valve seals but if the valve guides are worn, the seal won't help.

 

Huh??? Since when do you have to pull the engine to check the drainback holes in the heads? They're right under the valve covers, and it only costs a set of VC gaskets and a half-hours work to check. The drainback clogging in an old FE or FT is a pretty well-known deal, usually fouling the back plugs and/or smoking. And it can happen with other engines as well, I fixed an old GF's '79 RallySport Camaro the same way- she had bought it new and never changed the oil (this was in '86) Valve covers were almost full of oil (more like tar at that point) when I pulled them off, and was smoking like a stove and killing the back two plugs on both sides in a day, as that little Chebbie doesn't have the gaps in the intake like an FE, and the drainbacks were completely clogged

 

Yeah and everyone I have ever seen with plugged drains blew oil out the breather, PCV or not. So if it is not blowing oil, it ain't the drains.

 

Since you apparently missed the question, here it is again-
"Huh??? Since when do you have to pull the engine to check the drainback holes in the heads? They're right under the valve covers"

There are no absolutes in diagnosis- it might have clogged holes, might not, but unless you have X-ray vision, only one way to tell- and as the OP posted, it's the easiest thing. I picked up a very clean '63 Galaxie 500 with a 390-4V for $200 a few years back, smoking like a locomotive and four dead plugs in the back, back drain holes plugged and no oil whatsoever out the breather- in fact the outside of the engine was very clean. Cleaned out the drains, cleaned the plugs and changed the oil a couple times real quick, no more smoke, no more fouled plugs. In fact I have never seen one with clogged holes blow oil out of a breather- once the oil backs up high enough in a clogged FE or FT, it just runs into the valley through the gaps in the intake and pushrod holes, no reason for it to come out the breathers unless they were old-style open-system added-to-the-valve-covers bolt-on breathers. I have an otherwise very nice set of original chrome 427 "baldie" covers that somebody ruined the value of by drilling them and adding some of those old breathers. That Chebbie was also very clean on the outside, with no oil leaking out- but of course the oil in it was about the consistency of Permatex 300

 

What you all missed is that it ain't the drains, so leave the motor alone.

 

I personally know of one 360 that ran great, didn't blow oil out anywhere, but smoked after running on the highway for a while.

When my brother owned the highboy in my sig, and the original 360 had over 120K miles on it, he figured it was time for a rebuild. Only after rebuilding another 360 and swapping motors did he pull the original one apart.

Valve seal pieces were in the drain holes, flooding the heads. The rest of the engine was perfect. It never blew oil out the breather.

30+ years later, he still shakes his head and kicks himself over that one

It's worth a check.

 

I ran into this same type of thing ~20 years ago. Got a 71 F100 from a freind who was convinced the engine was toast. Smoked badly most of the time, and had a dead miss at idle. Perfectly clean on the outside. I just needed a truck, and was very low on funds, as my son was only 3 days old (be 20 in ~ 6 weeks).
I put dual exhaust on to replace the nearly rotted off stock single, and only the pass side smoked, and then only if the truck wasn't parked in a nose-down position.
Popped off the right hand valve cover, spilling ~1/2 qt of oil, and rodded out the drain hole. No more smoke. Ever.
The miss was just #6 exhaust valve burned.
Got another ~60K miles out of that 360 before selling the truck with 175K on the original 360 and C6 still running like new.


So yeah, the drains can be blocked and show no outward signs beyond smoking and/or fouled plug/plugs. Particularly on an engine that has seen 20 years of use that amounts to only around 100K miles. Rather rare these days, but wasn't so rare not that long ago.........

 

So, you were able to easily and quickly clean out the drainbacks without pulling the engine? Yeah, me too- quite a few times
Clogged drainbacks and jumped timing chains have gotten me quite a few very good deals on vehicles over the years