Causes of bent valve?

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What can cause a bent exhaust valve? My Honda '98 CRV with auto transmission (85,000 km) bent an exhaust valve on its own. We determined there had not been any piston to valve contact, timing belt had not jumped a tooth, valve and spark timing were right on the money, and the engine had never been overheated. Valve lash was regularly and correctly adjusted every 25,000 km. After removing the cylinder head, the metal around the guide of the bad valve was ok and so the valve hadn't been off center on its seat before it failed. We're stumped.
 
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Which part of the valve bent? I'm wondering if the valve guide was starved for oil and the valve bound (stuck) in it. Was the valve burnt/overheated?
 
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 Originally Posted By: berniedd
What can cause a bent exhaust valve? My Honda '98 CRV with auto transmission (85,000 km) bent an exhaust valve on its own. We determined there had not been any piston to valve contact, timing belt had not jumped a tooth, valve and spark timing were right on the money, and the engine had never been overheated. Valve lash was regularly and correctly adjusted every 25,000 km. After removing the cylinder head, the metal around the guide of the bad valve was ok and so the valve hadn't been off center on its seat before it failed. We're stumped.
That's got me stumped too. Maybe the valve bound up in the guide? Maybe the valve itself was defective and time got the best of it, meaning it slowly bent. Usually the things you mentioned above would be the cause, but you're saying that all checked out. Sounds like the engine was well maintained too. [censored] happens. JMO
 
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Granted that there's no piston to valve crash (interference engine), are you sure that your exhaust valve is not bent but burnt? If not burnt and yet it's bent, then it could be failure in workmanship/metal. Q.
 

Kestas

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Not just metal interference, but something soft, like plastic, could have jammed on the valve, then disappeared.
 
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It's rare, but if a valve is red hot and a huge chunck of carbon loosens up and gets between the valve and the valve seat, it can bend a valve. Normally it's the job of the valve to break apart the carbon when it gets between the valve and the valve seat, but it doesn't always happen. Did any of the other cylinders have any carbon or other deposits or did any of the spark plugs have broken porcelain?
 
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Small valves can bend easily if there is a contaminant between the face and the seat. The mark left on a piston may not be visible - it is only an edge that hits, and the engine ran [maybe briefly] afterwards, masking it.
 

berniedd

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Thanks guys. The part that bent was the neck of the stem before it becomes the expanded part (dunno what it's really called). We looked at the valve face and seat and they weren't burned. It wasn't oil starvation either, it valve guide looked okay but I had it replaced anyway for insurance. Its sparkplug wasn't missing any pieces and we looked but couldn't find anything that the engine swallowed and got stuck in the valve seat. Carbon build up wasn't anything excessive either.The only thing left is bad valve metallurgy.
 
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 Originally Posted By: mechtech2
Small valves can bend easily if there is a contaminant between the face and the seat. The mark left on a piston may not be visible - it is only an edge that hits, and the engine ran [maybe briefly] afterwards, masking it.
I agree with this- along with the suggestion that somebody else had... that some foreign material like plastic or paper could have been involved- and has now been pushed out the exhaust. I deal with bent valves all the time- and on occasion, if the engine has been run for a while after the incident, any marks on the piston can be VERY difficult or [censored] near impossible to see. If it was my vehicle... depending on the condition of the cylinder head otherwise... I'd just replace the valve (and guide if necessary) and put it back together. Or go with a reman head- depending on circumstances.
 

Kestas

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I doubt metallurgy per se can be the cause. I don't see how a valve can spontaneously warp without stress, unless it's made from shape memory alloy.
 
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Burnt valves are common on the older CRVs if valve adjustments are not performed more often than recommended. These engines had problems with the valve clearances "tightening up" on their own.
 
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I've never seen a valve bend without being hit.Modern valve stems are very thin and easy to bend.There is too much denial - the cause has been mentioned in the answers.
 
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bent valves are due to valves hitting piston heads. why? wrong timing how? tooth not aligned during timing belt or chain installation. valves not fully closing when required to close when piston is at tdc
 
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That would be among the possibilities, spyghost. But generally when there's a timing problem, it will affect ALL of the cylinders. Hard to imagine how only one valve would be bent. I'm still betting on some odd source of foreign material. I'd look in the intake, check for any possible broken off sensor tips, any sort of vacuum hose that could've broken and allowed a piece to get sucked in, maybe look at the PCV system for loose/missing pieces, check over the intake pipe, air cleaner, MAF sensor (if equipped). Look for any minor little piece that could've got sucked down the intake.
 

berniedd

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Thanks for the input, guys. We all agree that piston to valve contact is a leading cause of bent valves. Almost always due to bad valve timing or the timing belt jumping a tooth or so. But it's not the case here. The valve and spark timing have been checked before we removed the cyl head, and they were spot on. We cleaned the carbon off all my piston tops till we saw shiny aluminum metal. Definitely we'd have seen the tell-tale gouges if any valve had hit a piston. It takes a lot of force to bend a valve this way, and that would leave tell-tale marks. Anyways, I've gotten her fixed and been back on the road the past 10 days.
 
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 Originally Posted By: bernieddT
It takes a lot of force to bend a valve this way, and that would leave tell-tale marks.
No,it takes very little force to bend a modern valve - you won't see any marks.In a belt breakage,sometimes only one valve in a 4 valve chamber will bend.
 
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