Musche

After my last NASA race my car developed was running rough on the cool down lap and would idle as if missing. I originally thought it was a bad plug or something. When I pulled the plugs #3 was black and the others were nice and brown. Put new plugs in and no change and while cold I could hear the swishing in the exhaust when turning it over. Thought I bent a valve. When I took off the cam box the straight edge revealed all valves back in their seat but still no compression. Off came the head and the pic attached tells the story. My question is what caused this failure of the exhaust valve? It looks as though the degradation started from the top side of the valve and worked its way to the edge. If you look closely at the pic there is a spot on the left where another chunk is starting to go. All the other valves are good. Could this be a result of the crappy ethanol gas we are forced to live with? Any expert opinions are appreciated.

xsboost90

could be an injector not flowing on that cylinder- pull the rail, drop each injector in a measuring cup and crank to see if one cylinder is putting out less. Time to get an EGT gauge...

67King

You burning a lot of oil? I'd guess valve guides. Oil wll seep down. It darn sure looks like your exhaust valve has been cooking some serious oil. Anyway, it will build carbon desposits, which are nice heat sinks. Since they hold the heat in, they'll eventually melt the valve, and exhaust gas will blow through.

That is what happened on my father-in-law's track car. His valve wasn't that bad, though, it only had one spot. But, I would still bet that is what is happening on yours.

Side note - I notice in your sig you've got an SVO. Nice. I'm a turboford junky, myself. Actually still have a lot of leftover stuff from parting out my XR.

2bridges

NICE Burnt valve - I would say that cylinder went way lean, better figure it out before you put it back together.

Here is a a google formal definition

Why Valves Burn
There are several reasons why valves burn. One is normal wear. As an engine accumulates miles, the constant pounding and thermal erosion wears away the metal on the face of the valve and seat. The exhaust valve sheds most of its heat through the seat, so when the face and seat become worn and the area of contact is reduced, the valve starts to run hot. Eventually the buildup of heat weakens the metal and pieces of it start to break or flake away. Once this happens, it forms a hot spot that accelerates the process all the more. The valve begins to leak and compression drops. The result is a weak or dead cylinder and a noticeable drop in engine power, smoothness and performance.

A bad exhaust valve will also increase exhaust emissions significantly because it allows unburned fuel to leak into the exhaust. High hydrocarbon (HC) emissions, therefore, may also be an indicator of a burned valve.

An exhaust valve can also burn if the valve lash closes up for some reason (improper lash adjustment, cam or lifter wear, a bent push rod, worn rocker arm or cam follower, etc.). The lack of lash (clearance) in the valvetrain prevents the valve from closing fully, which causes it to leak compression and overheat.

Valve burning can also be caused by any condition that makes the engine run hot or elevates combustion temperatures. This includes cooling problems, abnormal combustion like detonation or preignition, loss of exhaust gas recirculation (EGR), retarded ignition timing or lean fuel mixtures.

Chris White

With our cars the usual cause is the valve being held off the seat by a chunk of carbon when the engine is hot. The combination of reduced cooling (no valve seat contact) and hot gasses passing by the valve head at very high velocity /pressure / temperature will act just like an acetylene torch.

For those of you that have played with torches you know that until the metal gets to a certain temp then it won’t cut – and when it gets to the right temp it cuts like warm butter –same thing with valves!

Typically you will only see this on a heat soaked engine.

geartech

Chris's reply is the probably what happened. Something caused the valve to not fully seat allowing the hot exh gasses to cut thu the valve. The same thing can happen with a blown head gskt sometimes. The hot high pressure gasses can escape thu the defect in the gasket and start cutting a grove in the block. If your tracking the car a exh temp gauge is going to be critical to keep an eye on what you motor is doing.

Musche

Thanks guys for the info.

Yes, I would use about 1/2 quart over the course of the weekend (2+ hours of hard driving)

Head gasket looked good and no problems there.

I did have turbo failure in May. The nut on the cold side of the impeller worked loose and allowed the impeller shaft to fall back into the downpipe. This in turn left a vacuum into the oil system and sucked oil into the intake and I was basically a bug fogger until I got the thing shut off. Could have been a small piece of metal caught on the seat from the cold side impeller although there was no visible damage to the impeller other than some bent vanes. Never looked at the inside of the housing to see if any metal was scraped off when it let loose. I did have a good bit of oil in the cylinders after it sat for a while. I tried to get as much out as I could by turning over the engine with the plugs out and the DME relay pulled. I'm hoping this could have been the major cause of failure. I have done EGT tests before with an IR handheld and they were all pretty close. Of couse that was before this happened.

OntarioTurbo

I just went through the exact same thing in May. Same cylinder. I agree with Chris's explanation. Don't forget, our cars are force fed and the increased presurized air flow can really make those carbon embers glow. Similar to blowing on hot coals in a fire pit. They give off much more heat. Eventually, as Chris said, it gets hot enough to cut the valve away. Amazing really. I have since had all my injectors flow matched so hopefully that will help.

My question is why always # 3? I have seen at least 2-3 more besides mine on here and all # 3 exhaust. Any ideas?

67King

#4 in the one I referenced. The rear cylinders run the hottest, so it isn't surprising to see the rear ones.

Chris Prack

I had the same thing happen at VIR a few years back but on #2. In my case the combination of worn guides and weak coil springs added up to the burned valve.

reno808

I am no engine expert but the shiiiiit is f*cked up