[OT] Non-Jag Engine Mystery

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I have one of my BMWs in a million pieces, due to a mysterious failure about 10 days ago. The victim is a 2001 325i with 225K miles on it. It was running fine, then all of a sudden it wasn’t. Computer complained about misfire on cylinder 2. I’ll spare you the details of the diagnosis, but bottom line is: A piece of one of the cylinder 2 exhaust valves went missing:

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I am completely baffled about what happened here. There is not so much as a tiny scratch anywhere on the piston, chamber, the remaining part of the valve, or the valve seat. The valve is NOT bent at all.

The piece, as you can see, is not small, and the metal is quite thick where it broke. And it DID break - it is NOT burned. As I said, the failure was instantaneous, and occurred under conditions of low speed and very light load. The only explanation I can come up with is a latent defect in the metal itself, that simply waited 225K miles to fail! Any other ideas?

Other than that small problem, the engine appears incredibly healthy. At 225K miles, the compressions, other than that one cylinder, all fell between 175-178 PSI, which is, to me, remarkably high, and almost unbelievably consistent. There is some VERY minor scoring near the very top-most travel of a few cylinders, and at some point in its life cylinder 1 obviously swallowed something that left two very small divots in the piston and chamber, but otherwise the pistons, bores, valves and seats all look great, and the bores still show clear cross-hatching! The head itself is incredibly clean - ZERO sludge found anywhere, and ZERO corrosion in the water jacket. It all came apart very easily (if you ignore the ~6 hours it took to remove the 16 nuts holding the exhaust manifolds and primary cats in place).

Once all the parts and special tools arrive, it will get put back together with 24 brand new valves, and the tappets will be rebuilt, as will the VANOS. The cams, cam bearings and tappets all already look like new, but I figure there MUST be at least some crud in a few of the tappets, and they are very easy to disassemble and clean. When it’s done, it should be ready for another 225K miles!

But I’d really like to know WHY this happened!

Regards,
Ray L.

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Backs up my machinist’s comments that modern engine design and materials essentially means that they’ll run forever. I’m sure you’re right; pre existing material failure. The bit will be in your exhaust system somewhere.

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With millions of exhaust valves, all opening and closing trillions of times, Id say the law of (metal fatigue) averages caught up with ya.

Though different metal, old Bugs would spit out, then eat, their accelerator pump jets, and never once did I ever see any internal damage from it.

I did, however, have to remove the muffler on one, to get rid of the rattle, caused by the munched-up remains of an accelerator pump jet!

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Im betting it’ll be wedged in the front of the cat/cats.

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Any sign of foreign object damage in the valve seat? Sometimes things just break.

My first Fiat 124 Sport Spider had a seeping oil pan leak. When I pulled the pan I discovered a fair sized chunk of piston skirt laying there. The car seemed to run OK other than burning a quart about every 300 miles. I pulled the engine and replaced it with a known good one from a salvage yard.

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John,

As I said, NO damage to the bore, chamber, or valve seat. I figure the piece broke off, and got immediately blown out the exhaust port, and into the cat.

Regards,
Ray L.

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Interesting abstract in this book about fatigue behavior in exhaust valves. I’m not about to purchase the book or deep dive into the subject, but your teardown and report sorta got me looking around as to the why. Maybe this explains it?
Chris

https://www.jstor.org/stable/44633761?seq=1#page_scan_tab_contents

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Yeah, I suspect that paper would answer the question.

Regards,
Ray L.

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Yes, I agree. I vote for a little piece of metal in the catalytic converter…

LLoyd

A step backward, after making a wrong turn, is a step in the right direction.

Kurt Vonnegut

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Being that newer engines run at much higher temperatures it is heat metal failure as the engine is hottest right there.Jaguar had this problem also on newer xj’s i have a couple of those valves.Yes that piece if not in the engine lodged in a port will be in the cat or burned through in the exhaust baffles.I found a piece wedged in an intake manifold small port wedged once.

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Failure analysis was a big part of my interest as I trained to be a Metallurgist - way way long ago. The short answer has been identified, metal fatigue. You can get an exact answer by looking at the fracture surface with an electron microscope to see if there is an inclusion or some other origination cause, but unless you have equipment access, the root cause will remain hidden. Glad the piece was blown out of harms way.

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SequelXKE is on the right track. With a good magnifying glass have a look at the fractured surface. Can you see any larger structure than the crystal structure? That would be your stress concentrator…

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Well, it’s almost back together! I’ve got it together enough to do the first cold compression test - all cylinders are now 205-215 PSI, up from 175-180 before the head rebuild! Not bad for an engine with 225K miles!

Regards,
Ray L.

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Just shows to go ya, modern engines are orders of magnitude better built and engineered than ever before.

It’ll prolly go at least another 100K!

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Got it up and running today. It started quickly, but at first it was unhappy because I had forgotten to re-connect the idle control valve. Once that was fixed, it ran fine, with no OBD faults. Tomorrow I’ll take it for a drive, and see how it runs on the road. I’m hoping for noticeably more power than before…

Despite the complexity, I find these cars reasonably easy to work on. The dozens of harnesses, hoses and pipes can be daunting at first, but once you understand what everything is, it all makes sense. When I put it back together this time, I was able to get the “explosion in a spaghetti factory” of wiring and hoses under the intake manifold much better organized than it was from the factory, which made putting it back together a lot easier.

Regards,
Ray L.

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