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Toyota Autopsy

January 1, 2013

I’m helping replace the engine in a certain red Toyota pick up truck.  This will be the fourth engine in the truck – the first replacement put in several years ago was a basic “reman” engine that just didn’t hold together, the next engine was a high performance custom build, and while the engine builder had a lot of knowledge, it just wound up being plagued with valve train troubles that ultimately led to a catastrophic failure.

As Jim and I swapped out the very broken engine, transferring many of the external parts to the new one, I couldn’t resist tearing into the old engine to see exactly what had failed.   When we pulled the valve cover, careful inspection noted one valve was stuck down – a “dropped” valve that typically results in serious piston damage.   As I began pulling the valve train and cylinder head, Jim remarked that perhaps I should have been a coroner as I was performing an autopsy of sorts.

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Looking at the cylinder head with number 1 cylinder at the top, we can see extensive damage to the number 2 and number 3.

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Seeing a close up, the intake (the larger valve on the left side) was the one which dropped, and the multiple impacts from the piston broke the stem, and much of the seat and pounded the valve back into the seat sideways.  Certainly we can see why the #2 cylinder was toast, but looking at #3 we can see the intake valve is slightly bent and there is a lot of scoring and chipping of the combustion chamber around the valves?

So, what caused all the damage in number 3?

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Taking a look at the block and the pistons.   Number 2 has a hole smashed in the top of the piston – obviously damage caused by the collisions with the valve.   But, look at the damage in number 3, and these odd pieces that were found inside.   These 3 pieces are the valve stem and pieces of the seat from number 2 cylinder.

How did they get into the number 3 piston?  My theory is that after the valve in number 2 dropped and was broken by the piston and driven sideways back into the intake port, the intake opening was now permanently open and the compression stroke pushed the broken pieces back up the intake runner.  This force, coupled with the vacuum of the intake of the other three cylinders, drew the pieces up into the intake plenum and by luck and adjacency, they were then ingested by the number 3 cylinder on a subsequent intake stroke.

I remain amazed that the valve stem could have passed through the open valve.  Perhaps even more amazing was the fact that the engine still ran, limping along on 2 of the 4 cylinders – enough to make it back home.

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