2 burned valves in 12 months! what is going on?

[nikolajs]

My 1997 850 N/A just burned a valve on cyl #1 (no compression) at ~210,000 miles.

Same thing happened last year on my 1998 v70 T5 (170,000 miles). (One bad exhaust valve.)

Similar scenarios on both -- full throttle going up a big hill at highway speed (~80mph).

Both times it happened at altitude (last year at ~7000 feet, this year at ~5000 feet).

Neither time was there any indication that stem seals were bad (no smoke on startup, ever).

I use mid-grade fuel (88 octane but with up to 10-15% ethanol blended), high quality synthetic oil with diligent changing, Volvo OEM or Bosch iridium plugs. One car had OEM blue Volvo coolant, the other had regular prestone green- neither had been overheated.

What is going on???

Going to pull the head next week, (luckily?) i know the drill from last year...

[101]

Many theories exist. My personal theory is that burned valves are caused by some combination of leaking valve stem seals, deposit buildup on the valves themselves and use of lower octane fuel. Your failures with the engines under load going up a mountain mirror my experience.

What was your oil consumption like at the time of the failures? On my 98 GLT I was using around 1 quart per 1000 miles with no visible smoking. After my burned valve incident and rebuilding with new valve stem seals the oil consumption is now MUCH less. I also found significant deposit buildup on the exhaust valves and I was guilty of using 87 octane fuel.

I now only use 93 octane fuel and occasionally drive both cars "enthusiastically" to try to keep deposits to a minimum.

[erikv11]

It fits the age of the car but still a bummer, sorry to hear. 88 octane is pushing it in the NA, and definitely a mistake if you are saying you also run it in the T5.

Oh well, at least it will get new valve stem seals.

[j-dawg]

We have a data dump thread that would be fantastic to have your input on:
viewtopic.php?f=1&t=74239

Every data point helps. It's not exactly a statistically significant sample, but it'll probably get big enough to draw at least some conclusions from.

[tryingbe]

Similar scenarios on both -- full throttle going up a big hill at highway speed (~80mph).

Slow down is not an option?

[rspi]

I have seen a lot of burnt valves in my travels. My take is that most of the cars that experience burnt valves are cars that are driven passively. Over time the valve ends build up carbon, etc., minor leaking setting on the valve ends and carbonize when the engine is shut down. When the engine is NOT rev'ed over 3,000 rpm's often enough, this build up continues. So now the valves are not seating well. Valves cool when they are seated, heat is dissipated in the head when they are closed. When they are leaking, they got hotter and hotter. When you are going up hill, applying the gas, the valves get hotter and hotter until they meet their failure point. Bam! Burnt valve.

My car has leaking valve stem seals. The thing is, I make it a point to rev my engine above 3,500 rpm's a few times per week. No burnt valves in over 150,000 miles (over 335,000 total). The PO was a fire chief and used the car to chase fires.

[misha]

Low octane fuel & high revs.
I use bmb 95 premium in my non-turbo.

[jimmy57]

In recent years where burnt valves are not a given at 100K miles or less, the vehicles that are run at low revs and low throttle and then run at highway speeds, usually with some load, are the ones burning valves. The valves get face deposits and the lower engine speed does not cause the spring wrap that rotates valve for seat and face cleaning cleaning. Then you go run at higher load or uphills and the increased heat and flow burns the valve like a cutting torch was taken to valve. The older less hard valves would get V notch cracks. Octane can contribute as lower octane causes retarded timing and retarded timing increases exhaust gas temps since the burn completion is later and thus it is still hotter when exhaust valves opens. I have seen plenty of burnt valves on vehicles I know the owner to run high octane fuel in. Stem deposits and valve guide wear are other things contributing to or causing burnt valves. And there is seat or face wear that decreases lash and decreased lash lets valves stay on seat for cooling a briefer period. Wear on face of valve can go through the face hardening and result in lower temp needed for valve failure.