What causes a valve to "burn"?

[osman]

I run only synthetic Mobile One in all my cars and Shell Premium (Never use corner-store gas!) My understanding (especially with the higher compression of turbos) is that lower octane gas is actually more flammable meaning under the increased pressure it is much more likely to cause preignition which is the most common reason for burned valves. For some reason the "summer blend" of 87 is more likely to cause this than the "winter blend". I am not one of those people that insist on OEM everything and usually pull all my own parts used, but petrol and oil are something I never skimp on.

[rspi]

Well, I have run the numbers on 87 vs 91+ and in my 960, I was getting 15% better mileage with the 91+. So I was actually saving money with the 91+. If it takes more "gas (87)" to get the same power as the 91+, they you would use more in power demanding situations where the motor is dialing back timing to avoid knocking, i.e. climbing hills, accelerating with a loaded car, etc.

A recent observation tells me that cars that have been sitting and then put on the road experience more burnt valve issues that those that are driven daily. My guess is that car with leaking valve stem seals that sat for a while with oil dripping off the valves had that oil harden. When the car was put back on the road I believe the valves were not seating tight and lead to a burnt valve soon.

I think I read that the valves rotate while the car is running, especially when rev'd over 3,000 rpm's. When I was burning 1 quart every 1,500 miles I was about to pull the cover and replace those valve stem seals, then I switched from 10W-30 to 10W-40 and the consumption stopped. Now I use 1 quart every 4,000 miles which is super. I no longer fear a burnt valve.

It would be interested in knowing how many burnt valve vehicles experience burnt valves with recent low miles vs recent high miles. Take 10 cars with a burnt valve and figure out how many miles that were driven in the previous 6 months prior to the valve burning???

[rspi]

Just got off the phone with a guy with a burnt valve. Car sat for 2 years, started driving it this summer and a month or so ago it started missing. Pulled the plug, in with a compression gauge and "0" compression.

[E Showell]

My valve burned notwithstanding years of 15,000 plus miles/year of operation. Mine went due to a failed front oxygen sensor.

[lesII]

I would like to update my experience with what I believe to be as close to the final answer as possible.

After replacing one vehicle and repairing the second only to replace the engine 25,000 miles later some things became very obvious.

1. Engine oil is critical, I had the shop put mobile 1 in the used engine (2.4T) upon installation. I changed the oil and filter 5,000 miles later and the canister was filled half way up the filter with some kind of carbon grit. The next oil change 5,000 miles later had only a residual amount of this in the filter. I changed my 850 (2.4 200,000 miles) to mobile 1 and after the first 5,000 miles the filter was completely clogged up--meaning I drained virtually zero oil from it. The amount of baked junk left behind by conventional oil is amazing.
2. Fuel octane, no matter what you hear, the owners manual was written for a reason. While the knock sensors may quite the engine, the heat produced on a grade with regular gas and the transmission shifted down is terrific, the temp gauge will not show it but the internal parts will feel it.

3. If you travel in mountainous area like I do, let up on the speed. Again the owners manual on the 850 says to use gear location 3 for long grades, doing this when the transmission kicks down at 65 mph up a long highway grade drops the rpm several hundred.

The threshold for the v70 with the 2.4t has bee 30,000 miles. I probably have a year to go before we pass that point. If this engine fails after all my attempts to treat it properly, I guess that despite liking the Volvo's it will be time to "fish elsewhere"--BMW? Porsche Cayenne?

[mecheng]

I would like to update my experience with what I believe to be as close to the final answer as possible.

After replacing one vehicle and repairing the second only to replace the engine 25,000 miles later some things became very obvious.

1. Engine oil is critical, I had the shop put mobile 1 in the used engine (2.4T) upon installation. I changed the oil and filter 5,000 miles later and the canister was filled half way up the filter with some kind of carbon grit. The next oil change 5,000 miles later had only a residual amount of this in the filter. I changed my 850 (2.4 200,000 miles) to mobile 1 and after the first 5,000 miles the filter was completely clogged up--meaning I drained virtually zero oil from it. The amount of baked junk left behind by conventional oil is amazing.
2. Fuel octane, no matter what you hear, the owners manual was written for a reason. While the knock sensors may quite the engine, the heat produced on a grade with regular gas and the transmission shifted down is terrific, the temp gauge will not show it but the internal parts will feel it.

3. If you travel in mountainous area like I do, let up on the speed. Again the owners manual on the 850 says to use gear location 3 for long grades, doing this when the transmission kicks down at 65 mph up a long highway grade drops the rpm several hundred.

The threshold for the v70 with the 2.4t has bee 30,000 miles. I probably have a year to go before we pass that point. If this engine fails after all my attempts to treat it properly, I guess that despite liking the Volvo's it will be time to "fish elsewhere"--BMW? Porsche Cayenne?

I don't understand what you wrote in 2)
In your opinion, what is the route cause? What gas did you use? Were you burning oil?

[E Showell]

I am guessing it should be "quiet" as opposed to "quite" in item 2.

[dosbricks]

I'm reading this whole thread and feeling a bit alarmed at having put so much money into parts for the S70 this past year to try to ensure the cooling and timing systems were fully renewed, plus the front suspension, steering, CV boots, etc. Now I learn that a burnt valve might be in the offing just any time at 220k when my plan was to make these cars reliable to 300k or more.

What I take away from the posts here is that there are many variables as far as driving habits, maintenance, quality of oil and fuel--but leaking valve stem seals can sink your boat anytime. Not what I had hoped for in expected reliability. Makes me wonder if I should acquire a salvage engine and set it up in my shop to start overhauling for a spare.

[cn90]

I have a thought and wonder what people say...

A main cause of burnt valves is the crud building up on the valve seat area, therefore not allowing the valve to cool properly. So, what if you do the following with high-mileage engine:

1. Remove all 5 spark plugs.

2. Bring one cylinder to TDC at a time: basically put a long chopstick or any small stick inside the spark plug hole, then turn engine by hand until the cylinder is at TDC.

3. Make an adapter hose to fit the spark plug hole and pour some Seafoam into the cylinder, and let it sit there for a good 30 min. or so in order to dissolve any gunk. Basically soaking the entire top space of the cylinder with Seafoam.

4. Repeat the process for the other 4 cylinders.

5. Dry out the cylinders, then change engine oil.

What do you guys think: good or bad idea?

[mecheng]

@cn90 You need the seafoam on the exhaust valve seat so you need to make sure the exhaust valve is open during the exhaust stroke, it could be closed at tdc during power stroke. I guess you need to reference the cam or crank to determine. To ensure you don't bend a rod, crank engine with plugs removed, after cleaning period. I think during cleaning period plug the hole with rag or plug so solvent does its work and not dissolve. Not bad idea.

@dosbricks. I agree. Nobody has determined a root cause; just guesses and theory. You would think with enough failures people would post some background. Sometimes it's non enthusiasts and then they dump the car to a diy guy, so not many have a clear picture. I run 89 in winter and 91 in the summer. Use Lucas cleaner every 6months or so, switched to Volvo blue coolant. And give it thr Italian tune up; how can't you with a turbo lol. If it burns a valve then it's not my fault and I won't buy another Volvo for this reason