TheSamba.com :: View topic - Piston Diagnosis Help -Photos-

Wed Jul 30, 2008 4:16 pm

I rebuilding a 1.6L Turbo Diesel and am in the process of cleaning the carbon crud off my pistons. This indentation on the piston surface has me worried because its so close to the combustion "dish?". Should this be a cause for alarm or should I just clean them and run them? I tried to give a couple different angles to get a clear picture of the area in question (6 - O'Clock on the piston face)

Thu Jul 31, 2008 10:29 am

If you don't get replies here, try the Vanagon email list. They have some diesel experts that will probably be able to give you some insight.

http://gerry.vanagon.com/info/subscribe.html

Thu Jul 31, 2008 11:17 am

Looks to me like a burned out piston. I would not reuse it.

Thu Jul 31, 2008 12:06 pm

Looks toasted to me too.

Thu Jul 31, 2008 1:27 pm

It's kinda like tooth decay. It ain't gonna get better. If it were a for sale car, you could clean it to minimize hot spots and ship it. But, I would tend not to want to use it. Also the compression ratio will change in that hole.

Did you check your rods for straightness (based on the valve hit?)

Doesn't the guy in LA have a good short block you can buy?????

What's a set of next repair size pistons cost?

For that matter, why not go for the gusto and go to a 1.9????

Come to think about it, I sort of recall you can bore a 1.6 to a 1.8 using a piston that I forget what it comes from.......old age creeping in...

Thu Jul 31, 2008 4:13 pm

I guess what you're asking about is the pitting at six'o'clock, not the apparent inprint of a valve head on the left side. I'm guessing that's directly below the prechamber. I suppose it's due to poor fuel atomisation and consequent burning at high temperature on the piston surface. Perhaps you should get the injectors tested and cleaned, see if there's some inconsistency in spray or fuel jetting on that cylinder's injector. Is that pitting only apparent on that single piston or on any others as well?

Thu Jul 31, 2008 4:26 pm

That not bad at all!! The pitting is common on the 1.6 turbo diesel. Its caused from heat under boost for long periods of time. (like up the pass)

The fix
'INTERCOOLER' and rap the intake with the thermal rap.( like for exhaust headers)

Wile your down that far you should get a set of Total-seal ring and a tdi grind cam will bring new life in the mid-range.

Cut off the muffler and run straight pipe from the turbo out. This will help lower the exhaust temp.

I try to load pics of a 20psi melt down

Thu Jul 31, 2008 4:59 pm

So is it just because the crowns get red hot so when any fuel droplets hit it they burn and remove material? It must be due to the fuel introduction to be so localised. Maybe it's so hot some of the fuel spray heats to a plasma and erodes the crown there. Theories?

I don't doubt the pistons are still usable, the damage seems minor, I'm just wanting to understand why they show that pattern.

Thu Jul 31, 2008 5:31 pm

I'll take a stab at it. Its called sulfidation. This occurs and is a form of corrosion when sulfur in either the fuel or the air get over the sulfidation temp. This is a bit hard to calculate on a diesel, as the compression is quite high, so the pressure changes the temperature that it will occur. It shows up as pits as the corrosion kind o boils out small bits o metal. It happens a lot on turbine engines, especially in urbane areas like LA. Higher temp turbines get this a lot. They put on high cost, plasma sprayed, coating to prevent the sulfides from eating away the metal.

Thu Jul 31, 2008 6:39 pm

r39o wrote: It's kinda like tooth decay. It ain't gonna get better. If it were a for sale car, you could clean it to minimize hot spots and ship it. But, I would tend not to want to use it. Also the compression ratio will change in that hole.

Actually it has been shown that teeth DO rejuvenate and reverse decay when there is not sugars or acids present.

As others have mentioned that piston is extremely normal and not really of concern. It's kinda funny, Captainpartytime posted on several other forums and the diesel specific ones all said, "No problem". I have always assumed it was poor spray patterns from old injectors, but I can't really denounce Dogpilot's chemistry lesson.

Andrew

Thu Jul 31, 2008 8:00 pm

All great info guys! From answers I received on other vw diesel forums the consensus seems to be poor spray patterns from the injectors. I will be rebuilding the injectors and pop testing them shortly. I also now have enough confidence to re-use these pistons, honestly, it's only a 70 hp motor :o

Dogpilot wrote: It happens a lot on turbine engines, especially in urbane areas like LA. Higher temp turbines get this a lot. They put on high cost, plasma sprayed, coating to prevent the sulfides from eating away the metal.

At first I thought you were referring to the sulfur in the diesel fuel, but it seems you mean the gaseous sulfur from the SO2 in the air? Interesting... :?

Thu Jul 31, 2008 8:24 pm

Here is a little blurb from our turbine shop:
What is sulfidation?

Sulfidation is a particular kind of corrosion which occurs to both nickel and cobalt based alloys. It is caused by sulfur which is found in ALL petroleum products including jet fuel. Since there is no such thing as sulfur FREE jet fuel, sulfidation is a persistent problem which requires our constant attention. Salt, air pollution, agricultural chemicals, and lead act as catalysts in the sulfidation reaction. As catalysts these four materials encourage the sulfidation reaction to progress more rapidly. Removal of these four catalysts, the use of protective coatings and metal alloys which are less susceptible to sulfidation are the only methods available to battle sulfidation.

Fri Aug 01, 2008 10:32 am

Hmmm....very interesting. I'm going to dig a little deeper into the sulfidation reaction. I'm now wondering what kind of alloy these diesel pistons are made of. James, my wife will be thanking you for giving me one more temporary obsession!

Fri Aug 01, 2008 10:54 am

Andrew A. Libby wrote:
As others have mentioned that piston is extremely normal and not really of concern. It's kinda funny, Captainpartytime posted on several other forums and the diesel specific ones all said, "No problem".
Andrew

I may just not know about diesels, but I played around with enough machinery.
The surface of a piston is supposed to be nice and machined not chewed up like that.
Are you just throwing it back together so you can drive it, or are you spending some money on a rebuild to last a while?
I mean if you got a new set a diesel pistons and one is nicked like that, the people that you bought them from wouldn't take it back cause its ok?
I've never rebuilt a diesel so I could be wrong.

Fri Aug 01, 2008 11:40 am

Im not sure about the sulfur.? Its possible!

What Iv seen is on the 1.6 tds the Prechamber is made of steel and is also were a lot of the pressure exerted on the piston comes from. Its a small area. The exploding gases/diesel shoots out of the opening and the combo of heat and pressure melts and removes small bits of alum.

The noise the older diesels make, rod knocking, is a result of that.The newer diesels direct inject and work with out a prechamber and run quieter.They also have the combustion take place over the hole piston.

Iv rebuild injectors , pumps , and have had the same problems. A set of total seal rings , inter cooler , and a good head rebuild is highly recommended!!

jcl

Fri Aug 01, 2008 1:38 pm

mightyart wrote: Andrew A. Libby wrote:
As others have mentioned that piston is extremely normal and not really of concern. It's kinda funny, Captainpartytime posted on several other forums and the diesel specific ones all said, "No problem".
Andrew

I may just not know about diesels, but I played around with enough machinery.
The surface of a piston is supposed to be nice and machined not chewed up like that.
Are you just throwing it back together so you can drive it, or are you spending some money on a rebuild to last a while?
I mean if you got a new set a diesel pistons and one is nicked like that, the people that you bought them from wouldn't take it back cause its ok?
I've never rebuilt a diesel so I could be wrong.

If it was nicked like that new, I'm sure a piston manufacturer would take it back. But, that isn't really the issue. I'm quite sure that piston will be functioning just fine long after the engine bores are worn waaayyy out of spec. That slight (and normal for the IDI diesels) erosion will not shorten the life of his rebuild any at all.

Andrew