Exhaust Valve

[Mark Millbauer]

Jim,

An ongoing exaust leak at the manifold to head can cause a burnt exhaust valve. Do you recall hearing a slight tapping or popping sound in the manifold area when the engine was running? How does the manifold gasket and mating are look?

[romantic comedy]

An air leak can cause the engine to run lean, which means it runs hot. This can cause a burned valve.

Never heard of an exhaust leak causing a burned valve. How does that work?

[Mark Millbauer]

I'll give this a shot but this is old school stuff that I learned in college auto shop a looong time ago.
The theory as I understood it was that an exhaust leak at the manifold can significantly decrease the exhaust back pressure of the affected cylinder. Most A4's, particularly if they are pushing exhaust through a water-lift system do have some back pressure. With less back pressure on the cylinder, the cylinder can then be allowed to take in a larger more oxygen and fuel enriched volume of fuel. That charge would then burn at a higher temperature. Any carbon in the exhaust valve area would therefore burn as well and as that hot air blew by the valve face, the face would slowly, over time, burn and disintegrate. Once there is and area on the valve face or seat that degrades enough to let just the slightest bit of exhaust gas by, the erosion advances faster and faster. There may also be a slight chance that some of exhaust gasses move back and forth a bit, again due to back pressure and the design of the exhaust system, and it's possible then that a bit of air could be sucked in through an exhaust leak at the manifold. More air means more oxygen which means more heat.
Any of the other previously described theory's are also likely causes as would be timing issues.
We don't see too many burned valves anymore due to the myriad changes in engine and fuel system management. We used to see a lot more burned valves--almost always exhaust--on older technology cars. Our A4's are older technology.

I could also be totally off the mark.

[romantic comedy]

Mark, it is coming back to me now. I remember that back pressure stuff.

[jpian0923]

I did some work/investigating/cleanup tonight. The culprit is likely "Carbon"!
On the block between the piston and valve was carbon build up that I actually thought was part of the block casting.

[jpian0923]

Found this video.

[YOUTUBE]7GEmuQa3dPY[/YOUTUBE]

[jpian0923]

After chipping the carbon off. The valves weren't able to seat due to carbon buildup.

[jpian0923]

While cleaning out the water passages in the head and block I decided to remove the water jacket side plate and this is what I found...Sea grass. There is so much other junk inside the waterjacket though. The grass is unusual but there is so much other stuff in there...mostly rusted waterjacket/block stuff. I'm wondering if this junk could have contributed to the burnt valve. I was running 140 temp but for only about 10 minutes to get to sailing grounds and 10 minutes to get back to the slip. You can see I have two of the Water jacket side plate repair kits (2 or so years and they are still holding up well). I recommend them.

[hanleyclifford]

Maybe a raw water strainer is in your future?

[jpian0923]

Hanley,
Agree. Thought my hull cleaner would have notified me though. Might be time for a new hull cleaner.

[sastanley]

Jim, I'd also recommend converting the remaining holes in the side plate to studs...even if they don't need the repair studs yet.. Moyer has a cool kit for that too. Maybe Ken will sell you half a kit..

[jpian0923]

I'm considering buying a valve seat cutter. Does anyone know the specs I would need to get the right cutter for the A4? My overhaul manual is on the boat and I'm not sure if it includes all the specs I would need.
Also, any recommendations (by personal experience) for where to get one?

[hanleyclifford]

You should contact this company. I have bought tools from them and they know their business. http://www.goodson.com/ But if you are thinking of doing what I think you are thinking of doing please remember that to cut those seats you must have good valve guides the installation of which on board could be challenging.

[jpian0923]

Yes I'm thinking of doing what you think I'm thinking of doing.
I have not yet measured the valve guides but I think you think my guides are worn beyond specs, and you may be correct, I'm not convinced they are yet.
But I say that based on only one thing. I de-oiled the stems and guides on a few and upon trying to reinsert the valves into the guides I found it difficult to get them back in. It was a tight fit with no play. They were the right valves for each guide too.
I can post some video of this to see what the consensus is.

[hanleyclifford]

The only way to know for sure is to measure valve stem OD and guide ID with the appropriate tools and do the math. I cannot see the videos but I do study the pics carefully. Sounds like you have the situation in hand.