85euro928

First let me apologize for the quality of the pictures, these were taken with my phone, I don't have the pics I took with my camera available right now.

This picture of the piston top is typical of all 8 pistons, does this look like normal carbon buildup or could the clean areas be due to oil being washed passed the rings or something else?


Before you answer a little background:
- I did not do a leak down test or compression test before tearing the engine down, however I did do a compression test a couple years back and all cylinders were good.
- I've only use Mobile 20w-50 and change the oil before the driving season and before putting the car away for the winter.
- The car has 70k ish miles on it.
- I have receipts from the women who owned the car before me for 8 new exhaust valves and other miscellaneous head work due to a TBF so the previous mechanic MAY have cleaned the pistons while the engine was apart and this is just new carbon buildup in this pattern (doubt it).* BTW, that was about 6 years and about 15k miles ago.
- I run Techron in the gas once or twice a driving season.
- I do not burn excessive amounts of oil but I was told once by someone I passed that I blew slight oil droplets on his windshield.
- The car does exhaust more vapor then I'd like when idling in cold weather but the head gaskets look in relatively good condition and I wasn't loosing coolant.
- Lastly, with the exception of one small scratch, which looks new, the cylinder walls are in excellent shape.

Any insight or adviced would be appreciated.

SeanR

Looks fairly normal to me, if a bit cleaner than normal, what's with that ding in the piston at the top?

85euro928

Not sure but it looks new. I'm thinking that I may have gotten something in there during disassembly and buggered it up.

mark kibort

looks like any 928 piston pulled from a running engine of more than a year or two.
the clean area might be a result of a slight gasket leak (water most likely), but probably not. usually a head gasket leak pressurizes the water system and pushes it out the overflow.

85euro928

Good to know. Thanks.

GregBBRD

Sorry....don't agree....at all.

Not even a little bit of agreement.

No combustion chambers should have that kind of oil deposit....especially on the exhaust valve. Look at that intake valve....it looks like you just plucked it out of drum of oil.

When oil goes past the rings, the areas where the oil gets on the pistons will not allow carbon to form. This is the primary way all engine builders detect when rings have seated....carbon will build up uniformly, without any "washout" areas.

That "washed out area", after 15,000 miles, or even after 1500 miles, is not a good thing.

You need to keep disassembling.

Imo000

Looks like there is a ridge at the top of the cylinders? Is it there or only an illusion? If it's there, how bad is it?

85euro928

There's no ridge at the top of the cylinders. In fact even the carbon build on the tops of the barrels is minimal (compared to other photos I've seen here) and is much softer than I would have expected. I could wipe most of it away with a clean rag if I scrubbed hard enough.

85euro928

Greg,
Any idea of what would have caused the rings to wear like this?

Edit: A quick search points to lean conditions being a major cause of ring wear and in my case this is probably true as I've been fighting a lean condition since I've had the car. FWIW it turns out whoever removed the seconday alarm system used an under-sized wire to feed the fuel pump, and did a good job of hiding it.

BRB-83-911SC

Not intending to hi-jack, but thought I would share pictures from when I pulled the heads to address a head gasket breach on #5. This is an 84 Euro with just under 40k miles, never opened before.

I observed uneven carbon build-up on #1 and #7, accompanied by ash build-up. I ended up cleaning everything up and putting new head gaskets on. In hindsight, maybe I should have gone deeper? The car runs great, although I do get occasional whiffs of burning oil on overrun, such as when rolling to a stop light (no cats, no visual smoke, no excessive oil consumption).

I do think my car runs a bit lean (verified with LM2 and sensor up the tailpipe), and has what I call a "random misfire" just off idle with no load/light load. Other than that it runs smooth and strong to redline.

Your pictures do show much more "oily" than mine, so not sure if there is any relevance. I am very interested in what you find, as well as veteran opinions on what is normal appearance, and what abnormal appearances point to.

Did you show visible signs of smoke? High oil consumption?

mark kibort

your right ... I didn't look at the pictures very close. the one cylinder head area the valves are dry and the other they are wet... that's oil contamination. probably rings or head gasket.
the more I look at the clean area, that's probably a water leak in the gasket too.

85euro928

Brian,
I didn't have what I would call high oil consumption or visible smoke (see my original post) but now that I opened this can of worms I'm glad I did. I have the girdle off and will pull the pistons this weekend. I'll post pics of what I find.

mark kibort

why take the girdle off? the mains are probably fine. just pop the pistons out and see what you see.
re-ring it and put it back together if everything checks out.

GregBBRD

How many miles are on it?

Generally, lean running gets things hot and generally helps to seat rings. Rich running is hell on the ring seal.

That being said, it is possible to run these higher compression engines so lean that the pistons/rings get damaged.

From my visual inspection of your picture (tough to diagnosis things from a picture of one piston and a couple of combustion chambers), I ruled out a "water problem". Water generally "steam cleans" things....instantly....and leaves only "hard carbon...not soft carbon. I do not think you could have that wet, soft carbon shown on the intake and exhaust valve with the presence of water.

GregBBRD

Once water enters into the equation, it is tough to tell much from looking at the deposits. The water backfeeds into the intake system and "hits" every cylinder. It's like a steam cleaner. We use water "injection", occasionally, to try and knock loose carbon. Won't touch the hard carbon, but cleans off the looser carbon very quickly.

The carbon deposits at the edges of the piston are always (softer" than those in the middle. No matter how good piston rings seal, there will always be some oil present at the edges of the piston. This "retards" the formation of hard carbon. Water steams this layer off, generally quite easily.

There are exceptions to this particular "carbon/oil/buildup" discussion.

The GTS engines, which generally have copious amounts of oil in the combustion chamber, form a super hard carbon layer that can grow to almost a millimeter thick...all the way across the entire piston. This layer is super hard and extremely difficult to remove. It significantly raises the compression ratio and is one of the reasons that these engines "ping" so badly". (In combination with the reduced octane of the fuel, from the oil dilution.)