What causes a valve to "burn"?

[rspi]

My compression test instructions: http://www.atthetipwebs.com/technologyi ... n_test.htm

[charlyW]

The engine running lean will burn number 3 valve out especially in the turbo variants of 850's as will running really cheap grade fuels, thankfully here in the UK we only get 95, 98 and 99 RON fuels I understand in the USA you can buy a lower grade fuel which these engines don't like. Check all the vacuum lines on the engine and if it's really high miles time to rebuild it.

[gregandterra]

Just finished pulling the head on my 99 V70 XC, burnt/chipped valve on NR 2 cylinder. Lost power, running rough, checked compression (after coil, etc) and got 30 lbs. Did leak down and decided it was the valve. 6 hours later, head is on the bench. Still have to pull all other valves to see what I'm going to renew. Noted build up of burned oil on stem and base of valve. Wife has been going cheap and running regular. Car has 152000 miles, lot of build up on valves. Suspect combonation of age on stem seals, running 87 octane, and generally long life. Unfortunately broke a head bolt down the block bore, hoping to get that out without removing the block. Doing all the other things, flame trap, hoses, timing belt etc. Bought the car last year with 142000 miles, good runner, just catching up. Wife delivers parts, averages 300 miles a day. If all goes well with this repair, should be good. Pulled rear drive shaft 2 monthes ago, was averaging 28.5 miles a gallon (calculated at fill-ups, not trip computer), so want to keep this one going.

[gregandterra]

Update to my post 6/30/12 on burnt valve in 99 V70 XC:

Bought car in Dec 2011 with 142000 miles
4th owner, woman owned by by all prior owners (Carfax, scary huh!)
Appears regular maintenance performed (stickers on air filter box and cabin filter with mileage due dates, paint pen marks on cam gears and crank pulley, newer timing belt and tensioners, etc)
I have logged 10,000 miles since purchase with no problems except front left caliper hanging, and both parking brake cables rusted through (Michigan car, road salt.)
Dropped rear propeller shaft for better mileage, has new CV joints, must have been problem in past.
Wife primary driver, she has confessed to 87 octane as she has delivery business and puts on 200-300 miles a day since April.
Findings on valves; All exhaust valves (except burnt chipped one) have consistant 1/16 inch thick white deposits on valve stem, back and face.
The burnt valve had amber colored deposts, with stream running down stem from failed valve stem seal.
All intake valves like new, but oil leakage visible in intake runner side of head from valve stem seals. The bad exhaust valve also had approx 1/8 inch thick deposit of coke (carbon) on its top. Valve seats and valves in great shape once cleaned.
All stem seals are rock hard, not surprised. Have one shot at getting broken rod bolt out. New valve for head on order, lapped all other valves after cleaning. If things go well, should be up and running by weekend after the 4th of July.
Conclusion, not sure about 87 octane being problem, valve stem seals a real issue at 150000. To bad they don't make better ones, engine is impressive in the condition at this mileage, have seen American V-8's look this way at 75,000.
These valve deposits seem consistant with other engines I have overhauled with similar mileage.

[rspi]

So, you think the primary reason that most of these valves burn is because of leaking valve stems?

Also, did you use any instructions from online to remove the head? I'm going to remove the head on our S70 GLT tomorrow am.

[rspi]

Just seen a pic of a bottom of a head with a burnt exhaust valve.
https://www.matthewsvolvosite.com/forums ... =1&t=49775

The car has 175,000 and he has had it since 100,000 and has run 90+ octane and used Mobile 1 oil. I was shocked as to how much carbon build up was on the bottom of the head.

[jimmy57]

The valves on these engines do not rotate unless the engine speed is above 3500 RPM and even better above 4000.
The deposits on seat allow a leakage and the leakage is HOT and burns the valve like a torch. Some will look like a cutting torch was taken to the valve. If the valves are moving on seat a little it scrubs the deposits away.
My mother's 95 960 was always idling a little coarse and when I'd visit I'd go do the "Italian tune-up" by driving it at 4500-5000 rpm in a lower gear for 7-8 miles.
When I'd stop at the stop sign after this it would be very smooth. Volvo has a 16 year old service bulletin for start-stall-no compression/no start that has the tech drive the car like that after getting it running and installing new multi electrode or platinum faced electrode plugs.
The valve stem leakage is a contributor to the deposits.
Another thing that causes them to burn on hydraulic tappet turbo engines (through 1999 model year, 2000-> has solid tappets) is the exhaust valve stem getting hot if high throttle/load is placed on engine really soon after start. The stem gets hot and lengthens faster than the thick cold oil will let tappet contract to compensate for. Not widespread but someone who parks on the street and pulls a hill to leave their neighborhood 200 yards from their parking place could see this.
Octane isn't a contributor to carbon buildup but excessive dosing with fuel additives and oil consumption do contribute. Easy driving is a BIG contributor. There really is something to "blowing the carbon out of her".

[erikv11]

So, you think the primary reason that most of these valves burn is because of leaking valve stems?

People have been posting this for 2 years, including in the first half of this thread: yes, it's the valve stem seals.

Also, did you use any instructions from online to remove the head? I'm going to remove the head on our S70 GLT tomorrow am.

Everything you need for pulling the head: Volvo 5-cylinder: How to Replace the Head Gasket

[whoa]

Remember that you can replace the stem seals without pulling the head (if you have a free weekend and a thirst for pain):

https://www.matthewsvolvosite.com/forums ... =1&t=27982

I did this at 160K miles on an engine that was smoking and that seemed likely to fail the HC emissions test. The seals were hard and chalky. 35K miles later it's running great, with no smoke and low emissions.

[rspi]

I pulled the head today. I wanted to do it this morning but was surprised at the details in the thread. I backed way off of those instructions (he was doing about 5 jobs there) and wrote some "quick head job" instructions. Took me about 5 hours, maybe 4-1/2. I'll clean it up tomorrow and hopefully get the head back from the shop tomorrow. Maybe I'll put it all back together Wednesday or Thursday. I'll need a day or two to recover from the stress and strain of today.