Seems it might be time to get a thread started to discuss why the VM seems to randomly (and most distressingly - more often now as the miles are piling up) drop/break valves off. Many of you know that I experienced a broken #3 fwd intake valve. Sotry and pics here:
viewtopic.php?f=5&t=83692Dieselguy (now Jay) has a broken #4 fwd exh. Both appear to have broken at about the same spot. That is - at the end of the stem near the valve head.
So let’s try to go at this logically. Certainly one would logically conclude something had to impact the valve to cause it. But that could only be caused by the piston, with just a few root causes -
1) possible if the timing belt broke or slipped
2) if a glow plug broke and a piece got stuck behind a valve
In my case (and dieselguy's) we know it wasn't #2. So that would mean it has to be #1.
Or ----- are there some other root causes? I haven't posted any numbers, but I did take some measurements of the valve stems out of my old head. The stem diameter is definitely smaller in the area between the end of the guide and the head. Looking closely at the valve it appears as though that taper is intentional and by design. OK, got it - maybe that's not the problem. But I also noticed that same area of the stem has some pretty heavy corrosion, surface pitting and wear marks (will post pics in the next reply). Not sure if this is leading to any clues or root causes, but it is interesting. What is causing this?
I have done some research on generic causes of broken valves. Certainly you find overheating being a logical root cause. Engine overheats, valve stem gets too hot, expands, gets stuck in the guide and gets impacted by the piston. Is this what we are seeing? I found the analysis/discussion on this site rather good:
http://www.aa1car.com/library/ar1192.htmAnother interesting root cause was leaking valves (at the seat) due to excessive build up of soot. With the valve leaking you can overheat the stem due to combustion gasses slipping by the valve and increasing the temp of the stem to the point of sticking or embrittlement and ultimately failure.
And the last one I found interesting was again related to excessive soot build up. But in this case the near(er) term cause of failure was the valve being held slightly open and ever so slightly contact by the piston upon each crank rotation. In this case the valve gets impacted very slightly, but takes the impact thousands of time and eventually breaks at the weakest point.
Not sure, lots of data. Little to conclude.
Thoughts?