Compression test and spark plugs disagree. WTF mate?
 

This weekend I'm helping a buddy put a '99 engine in his '92. Because he's only got 99k on his 1.6 motor and it runs nice and smooth, I was considering purchasing that motor that is coming out of the car to put in my garage, awaiting the eventual day that my current 1.6 gives up from massive cardiac boost-induced arrest.

So today I did a compression check on his motor to make sure I want it. Here's where it gets strange;

Compression test results are great. 185-190-180-190.

BUT, every one of his plugs has oil all the frick over them. Doesn't look like the oil is coming from the valve cover and getting through the threads - the top of the plug looks relatively clean, but the threads and the tip are wet.

My understanding was that oil on the hot side of the plug means the rings are wearing out - letting oil into the combustion chamber. If the rings are bad/going bad, the compression check will show low numbers - because the rings aren't sealing. But his compression is great. So I'm confused.

Can anyone shed some light on what is going on in this motor?

Here's a pic of one of the plugs. They all look the same:

https://www.miataturbo.net/attachmen...ine=1350097346
-Ryan

That is a lot of oil.

Did you pull plugs immediately after compression test? Maybe he filled cylinders with a bunch of oil?

I say put in new plugs, drive the car around a while, then pull the new plugs and look see.

That was how the plugs looked when I pulled them to do the compression test.

Bad valve seals. The seals leak, and after sitting, all the oil drains down the valves. Does it smoke on start, but clear up after a minute or two? That's the indication that the seals are bad. It is a fairly common issue, and after 100k is pretty much a given.

agree^

valves themselves seal fine but they also open and let oil in.

I bet it smokes at light load and idle too.

You're not going to believe me, but on a smokage scale, 0 being it has never puffed the tiniest puff of smoke and 10 being that the car looks like it is constantly trying to evade imaginary baddies with some sort of james bond smoke screen contraption, it's a 0.5 .

-Ryan

Adding up, this is not.

[/yoda]

https://www.miataturbo.net/attachmen...ine=1350097346

So, a couple of questions:

It looks like the porcelain insulator around the center electrode is pretty clean, relatively speaking. And dry.

It also looks like there's oil on the hexagonal wrenching-surface, as well as the band just below that (above the washer, where it says BKR7E)


True and true?


I'm really voting for valve cover seal here. Oil accumulated in the bottom of the well, and as you unscrewed and removed the plug, that oil transferred itself to the threads. It didn't come into contact with the center insulator or electrode because they didn't touch the threaded portion of the head on the way out.


I mean, just look at the amount of oil on the ground beneath the plug, and on the threads. In order for that much oil to have survived on the plug, in liquid form, while the engine was running, coming from inside the chamber, two things would need to be true:

1: You'd be burning a quart of oil every half-mile, and
2: The threaded portion of the plug would need to protrude about a half-inch into the combustion chamber.


Oil is funny stuff when it is leaking out of weird places. I had oil showing up at the very back of the valve cover which was coming from the seal between the oil pump and the block where I forgot to re-install that one bolt after I removed the A/C compressor.

I agre with joe, but i know nothing at all about valve seals.

Joe's right (but he already knew that). Valve cover gasket.

No worries here.

Oil consumption during running (whether from the rings or from intake valve guide seals) would be reflected on the insulator and firing tip.

Your oil is on the threads (which are engaging the metal in the head during running) and on the metal hex that is outside of the engine (where the oil pools if your valve cover gasket is bad).

Pro tip

Follow super wussbucket torque specs when you replace gasket

Not the valve seals themselves (the seals around the individual valve stems, which press onto the valve guides), but rather the rubber seal (aka gasket) between the valve COVER and the head. Probably more proper to call it the head cover or the cam cover than the valve cover. (Why do we use that term, anyway?)

This piece:

https://www.miataturbo.net/attachmen...ine=1350151050


This is leaking, and allowing oil to drip down into the bottom of the spark plug wells on the "outside" of the engine, which pooled around the hex area of the plugs (the lowest exposed portion.) When the plugs were unscrewed from the head, this oil coated the threads on the way out.



In this composite cutaway view which I just whipped up, oil pooled in the area marked in red:

https://www.miataturbo.net/attachmen...1&d=1350151809

Yeah, Joe's explanation makes much more sense, especially in light of the fact that it's not smoking.

We use the term "valve cover" for senior citizens who occassionally drive around in a 1966 Plymouth Sport Fury with a Super Commando V-8 (i.e., MOPAR Big Block). Such senior citizens (like me) are easily confused due to an advanced stage of dementia brought on by raising daughters. Even the valve cover bolts on something as manly as a Super Commando requires wus torque.

oooh ooh my turn:

and the oil looks like it's hanging out on the threads and not the hex and cylindrical smooth surfaces because of surface tension???

Adhesion and Cohesion, so, yes.

please do a leakdown test.

HI FIVE

(brohesion)

Get that Autolite shit out of here.

When I'm doing a quick-n-dirty sketch, I don't care what brand the spark plug is. I only care that it's picture perfectly upright, is against a solid-color background that I can easily remove with the wand tool, and that it comes up near the top of a Google Images search for "spark plug."



https://www.miataturbo.net/attachmen...ine=1350489778

its also precious metal, and you know no one here would ever spend more than $2 a plug.