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I recently had the pistons and rods in my Turbo 3.6 replaced with OEM parts. Cams, valves, valve springs etc were not changed as the workshop said those were fine. I then ran the engine in by gradually increasing the revs and all was well until a couple of weeks ago, when I revved up to about 5500 rpm in 3rd and let up the throttle (going a bit too fast). A vibration started coming from the engine, and it became pretty loud with loss of power, hesitation, etc. After pulling the engine apart, the workshop found that an exhaust valve hit the top of the piston in one of the cylinders. I'm not sure how this could have happened since I didn't redline or overrev the engine to cause valve float (this is what the workshop alleged that I did). Any suggestions on how this could have happened, and also what else should be checked/replaced (eg valve springs etc)? Not entirely confident of the workshop's ability to troubleshoot at this stage. Thanks.
Has the shop actually removed the cylinder head or borescope it to see the damage? Do you have any pictures?
And, do they think that the primary failure was engine over-speed vs. other issues such as oiling or mis-assembly, etc.
How many miles are on this engine and was this the "first" rebuild? Do you know if the valves have ever been cleaned, guides checked, valve stems measured specially on the exhaust side?
On a mountain downhill, of my drivers exceeded the engine speed limit in one of my diesel trucks causing valve float. In that case, the valve was bent, the top of the piston (iron flat top) was marked, but there were no signs of valve stem wear, oil starvation, discoloring and such.
there is lots of potenial reasons. starting with wrong clearance of valve / guide to lean fuel on one cylinder.
i had six oem exhaust valves failing after about 700miles in my 3.3. they all were shot. a head, piston, rod, etc. had to be replaced
if the company starts to discuss first you should find a specialist that can dig in. your wording leads to a company that takes your money and points on you now.
what a mess. and you can not be sure it is done right if they start over!
Simply going off the throttle at 5500 RPM does not lead to bent valves due to piston contact. So either your story or that of the workshop is not correct. Photos documenting the damage pattern would indeed be useful to have a guess of what could have happend.
I’m guessing there’s a problem with the rods and pistons being improperly spec’d, or the wrong head gasket was used. But since it’s just one valve, perhaps the connecting rod bolts stretched?
This is tough because how would the shop know that the valve springs are ‘fine?’ I don’t know of a way to figure out if a valve spring has failed. But 5500 RPMs shouldn’t be a problem either way.
The previous owner had rebuilt the engine at around 72k miles. Although it ran well when I bought the car, there was a slight rattling sound at idle. My workshop suggested that since it was running well, I can continue running the car for a while. This was a couple of years ago.
I finally decided to have the shop open up the engine at 77k miles as I had some fueling issues. The shop found that the previous rebuild was done with aftermarket pistons and rods, with a sizeable gap between the rod small end and the piston (seems like wrong pins used). Anyway, I decided to put everything back to stock and had the shop replaced the rods and pistons with OEM parts. The valves were cleaned, exhaust valve guides were replaced, and valve seat rings were reconditioned.
In terms of over-revving the engine, I'm pretty sure I did not let up the throttle at 5500rpm (it may have gone a bit higher but certainly did not touch 6000rpm). There was no mis-shift, as I wasn't shifting fast.
It sounds like they possibly messed up when installing the exhaust guide. Probably not set correctly. Sorry but all that work and such few miles and this is what the chambers look like. No cleaning of the heads when doing the guides. I would consider another shop.
I am curious what aftermarket pistons and parts were used prior that it ran with what you described.
When I get my heads back after having guides installed they look like this. Not sure what you paid but are you sure they did much of anything. Looks like the changed your valves but are you sure they changed the guides?
You also have a combination of the old style and new style heads studs. I would never do that and surprised they felt it was OK.
Looking at those pictures, I would not be able to tell someone that they oversped the engine! Your engine builder is speculating, but it is true that the first reaction when a valve hits a piston is to jump to the conclusion that you had valve float.
+ I would like to see your engine builder remove the head on that cylinder and look at the top of the valve, springs, retainer, rocker, etc. and look for any clues.
+ If this was my car, I would be looking at that particular valve spring, and then retainer.
+ I would ask about that particular cylinder. Did they replace the valve guide? If not, did they measure it? As in, is it possible that they shoved the exhaust valve back in and after another 5,000 miles it was wobbling around?
+ You said that your engine was noisey before the rebuild. Did they replace the tensioner?
Looking at those pictures this is what I see:
1) The piston top appears to match my '94 so that is good. But is it actually cocked to one side in the photo?
2) Although I have never disassembled an engine after 5,000 miles, there appears to be more soot and coking than I would have expected. It makes me think that the cylinder was running rich all the way up the spark plug hole threads.
3) Similar to Cobalt, I am disappointed to see mixed head studs on a total rebuild with new pistons, rods, etc. Not a classy move on an expensive car/project.
Finally, did the builder make any measurements and document anything originally? Specifically on heads.
In the past 5 years, I have re-sealed 3 engines in my own cars at my own house/shop: 930, my long time 965 #1, and my recently acquired 965 #2. Here are some pictures of 94 965 #2 with 25K miles.
7 out of 12 valves were BELOW spec and I replaced them. ditto Guides have to be measured before and after. Heads have to be cleaned properly as Cobalt said. All this crap from half of the engine has to be cleaned, measured, lubed, etc. Did they do this?
Looking at those pictures, I would not be able to tell someone that they oversped the engine! Your engine builder is speculating, but it is true that the first reaction when a valve hits a piston is to jump to the conclusion that you had valve float.
+ I would like to see your engine builder remove the head on that cylinder and look at the top of the valve, springs, retainer, rocker, etc. and look for any clues.
+ If this was my car, I would be looking at that particular valve spring, and then retainer.
+ I would ask about that particular cylinder. Did they replace the valve guide? If not, did they measure it? As in, is it possible that they shoved the exhaust valve back in and after another 5,000 miles it was wobbling around?
+ You said that your engine was noisey before the rebuild. Did they replace the tensioner?
Looking at those pictures this is what I see:
1) The piston top appears to match my '94 so that is good. But is it actually cocked to one side in the photo?
2) Although I have never disassembled an engine after 5,000 miles, there appears to be more soot and coking than I would have expected. It makes me think that the cylinder was running rich all the way up the spark plug hole threads.
3) Similar to Cobalt, I am disappointed to see mixed head studs on a total rebuild with new pistons, rods, etc. Not a classy move on an expensive car/project.
Finally, did the builder make any measurements and document anything originally? Specifically on heads.
In the past 5 years, I have re-sealed 3 engines in my own cars at my own house/shop: 930, my long time 965 #1, and my recently acquired 965 #2. Here are some pictures of 94 965 #2 with 25K miles.
7 out of 12 valves were BELOW spec and I replaced them.
ditto
Guides have to be measured before and after.
Heads have to be cleaned properly as Cobalt said.
All this crap from half of the engine has to be cleaned, measured, lubed, etc. Did they do this?
OK lets hear it when did #2 come along? I can't click on the link.
I agree and can speculate what was or wasn't done. Things don't look correct for either bank and I suspect they all look the same. I just had my exhaust valves, guides done everything else was well within spec But replaced the valve springs and retainers. My son revved my engine to around 8k+ rpms (dropped it into 2nd instead of 4th going full speed down the front straight at Tbolt NJMP). Although watching the video it was only monetarily you figure for a half second over rev it had at least 60-70 ignitions at that speed. I had less damage to the piston and a few slightly bent valves.
I always replace my valve springs with uprated versions and Ti retainers. The cost is nominal for added security and will eliminate valve float. But I don't think that is what happened here.
Understandably this is a higher compression 3.6L engine with different shaped pistons but this is what most money shifted engines that survived I have seen look like. Never has it been just one valve. Others were non recognizable after the sintered spring retainer let go. That makes an unbelievable mess. I nearly lost a VS retainer which would have been catastrophic but thankfully the stock retainers held up despite one being only half cracked. Not sure actual mileage on this engine but I suspect well over 15k
! & 3 valve spring hit on piston top
Thanks for all the inputs and advice - very much appreciated. I have asked the workshop to relook at the top of the valve, springs, retainer, rocker, etc. and see if they can spot anything. Let's see what they come back with. In terms of the exhaust valve guides, they did change all 6 pieces during the rebuild, so maybe the one for this particular cylinder wasn't properly installed. I've also told them to change all the valve springs, as well as the head studs, and to check why the car was running rich (although I suspect this may have been caused by a faulty fuel distributor which was changed not long after the rebuild).
In terms of the previous pistons and rods, the rods were Carillo, but I don't know what pistons were used.
The tensioners were not replaced. The rattling sound before the rebuild were coming from the improperly fitted pistons to the rod small ends, and it ran smoothly after the rebuild (at least it did, until the valve hit the piston).
I've seen a cracked rocker arm that had similar results for the piston/valve, but that cracked during downshifting (the guy said he did not miss the gear nor overrevved the engine). I don't understand (as stated by guys before) how a workshop could fit back heads without cleaning them throughly.
One additional point just dawned on me. If they repalced the pistons with new that would or should mean new rings as well. This would require the removal of the cylinders and honing of the cylinders after inspecting them for roundness. It doesn't appear they honed the adjacent cylinder. Was this done? do you have receipts for new rings? What brand were used? Were they sent out or did they do it in house? Although using new rings on cylinders that weren't honed would result in oil seepage and burning. I have taken apart engines that were built like this after a short time and they looked bad but the oil was easily removed with a rag and didn't look like yours.
Yes, the piston rings were replaced with new ones (Mahle). It doesn't look like the shop honed the cylinders though. Here's a pic from the rebuild. Is honing absolutely necessary?
for me this looks like a top end - not a full rebuild. so your shop did some easy work that failed. may be they messed up the heads / valves / guides / valve spring clearance only.
03-19-2024, 05:56 AM
Exhaust valve hit piston
