Ok this is my first real post but I've been hanging around the site for the last couple years while I've owned my truck. I've got an 01 with 106k miles on it and blew a head gasket a couple weeks ago. I'll get a sig put together soon but for now all that's on the truck is a 4" exhaust and Bully Dog Power Pup 50HP cal (never gone higher). I bought the truck at 61k. GMVIS shows there's never been any major service work done on it, at the dealer at least.

I did a leakdown to try and find the popped gasket but had some strange results. At the point I did the leakdown, I had everything torn down to the heads and pulled the rocker arms so I knew all the valves were closed. None of the cylinders on the pass side would hold pressure and had significant flow out of the intake manifold. I couldn't do the drivers side because all 4 glow plugs snapped off in the head (life in the rust belt). I pulled both heads and found the blown gasket on cylinder 8 to a coolant port, but everything looked perfect on the passenger side. No obvious cracks, burned valves, or blown gasket. The straight edge test shows that neither head is warped but I'm going to send them out to a shop for inspection anyway.

Now, my question... How could I have major air flow out of the intake tube with all the valves closed and no signs of cracks anywhere?

The truck ran perfect before I pulled it in the garage with the exception of the pressurized coolant system from the head gasket. This one kinda has me stumped.