Taking A Really CLOSER Look

This time it is a Ford Explorer Sport Trac, vintage 2004, with a 4.0 SOHC and automatic transmission.  It belongs to a fellow employee and it suddenly dropped a cylinder on his commute to work.  The ECM logged a P0305, and Mode $06 didn't point to any other weak cylinders.  There was a mechanical knocking noise at idle, and I didn't let it idle for long.  I already know what's happening here, but it's a good opportunity to play a bit and learn a bit more.  So, out came the scope and the FirstLook sensor and the first capture I wanted was at the exhaust.  Here it is:

In any pressure/vacuum pattern, I look for anomalies, variants that don't fit the pattern.  Obviously, the clipped off peak, fourth to the right of the first injector pulse, is not the same as the others.  This capture shows the pressure pulses exiting the exhaust, and at least one cylinder is not contributing its share. 

My next capture was intake manifold vacuum.

The pattern is uniform between the injector firings, but keep in mind that is the time it takes for ALL cylinders to run through their cycles.  The pattern in between the two injector triggers should also be relatively uniform.  In the old days, I'd have used a mechanical vacuum gauge and this pattern would make that gauge bounce...an indication of valve sealing issues.  OK, if that is the case, I can verify it pretty quickly by checking the pressure pulses in the bottom end...those caused by the pistons moving up and down. 

If the pistons are sealing, the pressure pulses should be uniform, and there should be 6 between the two injector triggers.  This tells me it's not just a valve issue, but a piston sealing issue as well.  In less than 30 minutes, I've collected enough information to justify the teardown needed to see the problem.

With the valve cover off, the broken valve spring on the #5 intake was easy to see.  The question now is, is the valve damaged?  Did it hit the piston?  Or is the piston sealing issue a result of all that unburned gas washing down the cylinder?  The videos below were taken with a borescope, and show the valve topside, then from the combustion chamber.  The last video is the piston top and cylinder wall, and you can see the raw fuel that has collected there.  Cool, huh?