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Old 07-12-2012, 10:17 PM   #1
williaty
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Default Valve and Piston Got Together for a Date. How the frack did this happen?

2007 2.5i. 80k miles on it. Automatic, so over-rev isn't possible. One of my friends had just floored the gas to get around someone on the freeway. At 80mph, the CEL came on and the car started running extremely roughly. Guy had it towed up to my place so we could work on it.

Testing on the first night showed 0psi compression and 100% leakdown on cylinder #4. Leakdown test definitely pressurized the exhaust but also had noise coming out of the other 3 spark plug holes. This made me think that he might have a valvetrain problem keeping the exhaust valves stuck open since there had been a previous report of an AVLS failure on NASIOC. Pulled the engine from the car and opened the valve cover. The rocker arms for the exhaust valves clicked up and down like they should and the valves appeared to move properly as you turned the engine over by hand. Re-doing the leakdown test at this point clarified that the air was coming out of the exhaust valves and nowhere else when #4 was checked. #2 had 8% leakdown. #1 was 6% and #3 was 4%. Knowing that it had to be a problem with the exhaust valves on #4, I pulled the head off.


With the head off, it was clear that one exhaust valve from #4 had made contact with the piston. The piston and valve each have a nice dent on them. Some portion of the valve bent (either head or stem) to the point that the valve no longer seals at all. Pouring solvent into the exhaust runner results in it running out the valve as fast as you pour it in. Tomorrow, the head is going to a head shop to be torn down and repaired.


My concern is that there's nothing clearly indicating how the valve came into contact with the piston. That doesn't just happen randomly, something obviously had to go wrong to cause it. However, I can't find anything that looks unusual other than the damaged piston and valve. Do you guys have any idea what may have happened or what to investigate so that I can assure my friend it's not going to happen again as soon as we rebuild this thing?
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Old 07-12-2012, 10:41 PM   #2
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I know this isn't really helpful per say, but I've got a rebuilt shortblock and a set of rebuilt heads for sale for fairly cheap (both are completely rebuilt and look factory fresh from the machine shop).
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Old 07-12-2012, 11:15 PM   #3
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Thinking about it in the shower, I wonder if that valve was sticking a little bit and just being a little slow to close. That would work ok at low RPM, but as he floored it and the tranny downshifted and let the engine speed rise, the time that the valve used to have to unstick and shut during disappeared. So the was kind enough to shut the valve
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Old 07-12-2012, 11:24 PM   #4
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Quote:
Originally Posted by williaty View Post
2007 2.5i. 80k miles on it. Automatic, so over-rev isn't possible.
Don't readily believe that you cannot over-rev an AT backed engine.
Quote:
Originally Posted by williaty View Post
My concern is that there's nothing clearly indicating how the valve came into contact with the piston. That doesn't just happen randomly, something obviously had to go wrong to cause it.
Valve float possibly happened. He may have over revved. Have the valve springs checked. Is there any signs of varnish build up on the valve stems? Then the valve may have stuck open, or a combination of valve float/sticking valve.
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Old 07-12-2012, 11:30 PM   #5
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^^ exactly, or I don't know what the chances are but maybe it stuck enough just once on a resonant vibration (once in a billion years, maybe. here it is!), or could there have been a small bit of contamination that is now in the oil filter or lodged elsewhere, that could have kept the valve open (a small bit broke off, buggered the valve's date with the rocker, then ran and hid)?

whatever. doesn't seem common (like life starting from scratch on a planet), would like to know exactly what happened, but it is an anomaly, I hope..
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Old 07-13-2012, 12:30 PM   #6
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I would investigate springs or something getting between the rocker/stem. How much clearance is there at the point where piston/valve are at the closest clearance during normal operation?
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Old 07-13-2012, 03:44 PM   #7
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Quote:
Originally Posted by RaceFaceXC View Post
I would investigate springs or something getting between the rocker/stem. How much clearance is there at the point where piston/valve are at the closest clearance during normal operation?
Last time I checked it, 4 thousandths. I've never checked this specific engine or even this year. Subaru dicks with that stuff without warning, so who knows.
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Old 07-13-2012, 07:35 PM   #8
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You never mentioned whether you checked to see if the engine (cams/crankshaft) was still in time. Sounds to me like you might have jumped time on bank 2 - possibly a tensioner issue since it happened during what sounds from your description like a downshift during a pass at highway speed.
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Old 07-13-2012, 08:03 PM   #9
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Just to clarify...was it both exhaust valves that were bent or only one?

It seems unlikely that only one valve would make contact if it was timing related. I would look at components of the individual valve and see if there is buildup on the valve stem/spring is messed up etc.
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Old 07-13-2012, 08:24 PM   #10
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Quote:
Originally Posted by Elbert Bass View Post
You never mentioned whether you checked to see if the engine (cams/crankshaft) was still in time. Sounds to me like you might have jumped time on bank 2 - possibly a tensioner issue since it happened during what sounds from your description like a downshift during a pass at highway speed.
I didn't mention it because I was a dumbass and forgot to check before I took it apart. However, all 3 other cylinders have no damage and good compression. That makes timing problems extremely unlikely.

Quote:
Originally Posted by geronimo81 View Post
Just to clarify...was it both exhaust valves that were bent or only one?
Just the one. That's what makes me think it probably got a little stuck and didn't close soon enough.
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Old 07-13-2012, 08:31 PM   #11
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Seen valve guides migrate/drop before.

Carbon, varnish.

Adjuster jam nut loosened up.

Unlikely candidates:

Rod bearing/rod wrist pin hole/piston wrist pin holes egging out allowing the piston to travel higher.

Stretched rod.

Rod bearing pooped the bed.

All can close the gap between the head/valves and the piston.
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Old 07-13-2012, 09:45 PM   #12
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I vote for just another 2012 omen.
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Old 07-14-2012, 10:48 AM   #13
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Quote:
Originally Posted by williaty View Post
I didn't mention it because I was a dumbass and forgot to check before I took it apart. However, all 3 other cylinders have no damage and good compression. That makes timing problems extremely unlikely.


Just the one. That's what makes me think it probably got a little stuck and didn't close soon enough.
My bad, in re-reading I see it's only a single valve so it can't be timing. Yup, flstffxe might be on it - look at the valve guide and the spring for that valve. In fact I had a 2008 SOHC a few weeks ago for head gaskets and one exhaust guide in #2 and one guide in #4 had started to drop. Just happened to get to the head gasket leak before the guides dropped too far.
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Old 07-15-2012, 07:26 PM   #14
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Quote:
Originally Posted by Elbert Bass View Post
My bad, in re-reading I see it's only a single valve so it can't be timing.
Yeah, even if other cylinders were spared somehow, you'd at least expect both exhaust valves to be bent.
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Old 07-15-2012, 11:55 PM   #15
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Quote:
Originally Posted by williaty View Post
I didn't mention it because I was a dumbass and forgot to check before I took it apart. However, all 3 other cylinders have no damage and good compression. That makes timing problems extremely unlikely.
No, the fact that a valve bent makes a timing problem extremely likely. And since you didn't check we can never no for sure so all this thread is going to get is a bunch of random guesses. Mine is that the belt skipped a couple teeth. How's the tensioner?

Doesn't everyone else set the timing marks where they're supposed to be and mark them before removing the belt?

Last edited by jamal; 07-16-2012 at 12:00 AM.
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Old 07-16-2012, 01:10 AM   #16
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Quote:
Originally Posted by jamal View Post
No, the fact that a valve bent makes a timing problem extremely likely.
I strongly disagree. Only one exhaust valve was bent. A timing error would have, at the very least, bent a pair of valves since they move up and down together. Additionally, a timing error would have been observable in the compression results for the 3 good cylinders.

Quote:
Doesn't everyone else set the timing marks where they're supposed to be and mark them before removing the belt?
Nope, never bother. If I suspect a timing error, I'll verify timing before dismantling the system. If I have no reason to suspect a timing error, why bother?
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Old 07-16-2012, 02:06 AM   #17
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It's good practice. You don't have pistons at the top of the bore and more valves than you need partway open when the cams spin and the valves spring shut. If you're re-using the belt it's also nice to put new marks on it.

And like I said, how are you ever going to know for sure?
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Old 07-16-2012, 04:23 AM   #18
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He already knows for sure based on the fact that only 1 valve is bent. Or do think it's more likely that there were 2 failures?
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Old 07-20-2012, 02:16 PM   #19
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Quote:
Originally Posted by williaty View Post
I strongly disagree. Only one exhaust valve was bent. A timing error would have, at the very least, bent a pair of valves since they move up and down together. Additionally, a timing error would have been observable in the compression results for the 3 good cylinders.


Nope, never bother. If I suspect a timing error, I'll verify timing before dismantling the system. If I have no reason to suspect a timing error, why bother?
I agree with part 1. It's a valvetrain failure. Not a timing issue.

On parts 2, these engines 90+% of the time they come to rest at TDC. In the rare instance that they don't, I handcrank them there just because. It's not required but I it sets me up to put the timing belt back on later...
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Old 07-20-2012, 02:20 PM   #20
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So, resolution to this is that 3 of the 4 exhaust valve guides had dropped. The valve in question was just the first one that moved far enough to get smacked by the piston.
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Old 07-21-2012, 05:14 PM   #21
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So the keepers were faulty?
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Old 07-21-2012, 06:25 PM   #22
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Sounds like the valve guides migrated/dropped in the head casting. They are pressed in, not uncommon for them to start to migrate with heat cycles if the guides were on the small side and the bore for them was on the high side of the tolerance creating a looser the desired fit.
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Old 07-21-2012, 07:50 PM   #23
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Quote:
Originally Posted by Matt Monson View Post
So the keepers were faulty?
Keepers are fine. Valve guides walked out of their holes in the head. One eventually got low enough that it either physically got in the way of the valve retracting or cocked at enough of an angle to bind up the valve stem.
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Old 07-21-2012, 07:56 PM   #24
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Interesting. Over the last couple of years I've heard from two other sources that dropped guides happens more often than people expect.
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Old 07-21-2012, 08:57 PM   #25
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This also has me curious of my own guides. Is there any way to measure the depth from the cam side of the head?
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