2003 V40 1.8: Possible bad piston ring

[scdk]

While changing spark plugs yesterday, I did a compression test, with the following results:
1: 190psi
2: 190psi
3: 190psi (a bit sluggish to get to 190, started on 170)
4: 150psi

I did the "pour a bit of motor oil into the cylinder"-test on number 4 and the pressure shot right up, indicating a bad piston ring.

This is what the old spark plugs look like (one through four):


Notice the debris on spark plug #4.

It looks like cylinder #4 experienced a higher heat than the other three: the picture does not show it, but the three first plugs have grease on the connector ends - I'm guessing dielectric grease - but none on #4. Did it burn off of #4?

Any ideas on what happened here? Any input would be great, thanks

[alschnertz]

How many miles/years are on those plugs?

Though 150 may sound low compared to the other readings, I'd be quite surprised to find that is an issue with your engine.
That's around 20% of the other readings. The threshold is normally about 15%, but that does not mean this is terminal.
What problem are you having?

[scdk]

I see I left out some important info

The car and engine has done 230.000 km/143.750 miles and looks like the spark plugs was last changed at 93.000 km/58.125 miles 8 years ago.

I have not experienced any problems as such with the daily operation of the engine. I frequently rev the engine to 5k RPM on overtaking and everything has been running pretty smooth.

I have experienced a couple of instances where the engine has a hard time starting, like there is not getting enough fuel. But I don't think that's related as it usually starts fine.

There was a bit of oil sludge in the engine when I bought the car at 220.000km, so I've been doing 5.000 km oil changes in the hope of getting some of that flushed out. Here I've noticed the oil being broken down more than I thought it should be at a 5k interval and smelling a bit more of gasoline than I'd like.

I'm thinking excess blow by would be a cause of this.

Maybe this is a non-issue?

[alschnertz]

I have experienced a couple of instances where the engine has a hard time starting, like there is not getting enough fuel...I'm thinking excess blow by would be a cause of this.

I would check the fuel pressure. It should be around 44 psi.
Also do a "glove test" to check for excessive crankcase pressure. A clogged PCV system would not be unusual with that mileage.

[scdk]

I've suspected fuel pressure with the start issues as well. But have yet to get a test tool.

The "blow by cause" was mostly to what I've seen with the motor oil. Believe it or not PCV is still fine

[alschnertz]

I wouldn't jump to any conclusions yet regarding contaminated oil. I know I wouldn't trust my nose to assess that there is gasoline in the oil.

My '01 was a little "sludgey" when I purchased it at 145,000 miles. I used an old school mechanic trick of adding 16 ounces of ATF to the crankcase for 500 miles before an oil change. ATF has a very high detergents. I like to think it cleaned things up ok, but I've never had the crankcase open.

I don't think your plugs looked that bad either.

[scdk]

I know I wouldn't trust my nose to assess that there is gasoline in the oil.

Ok, point taken

All in all I feel reassured. And I think I will try the ATF trick next time an oil change comes up. Last change, I had a good amount of sludge in the filter, which may also be because I do a lot longer trips and use the full RPM range more than what I suspect the previous owner had done. Maybe adding in some ATF will get some more dislodged.

It sounds like the difference in compression is nothing to worry about just yet. I'm of course still curious as to what could be the cause.

Thank you for taking the time to answer!