Engine Dead Cylinder, Broken Valvespring, Diagnostic Weirdness

[MFE92]

Sorry for the cross-forum post, but anyway...

Engine
is a 306 on a stock block, forged pistons, Cobra intake, GT40 irons, Comp 270HR cam (.533 lift), Twisted Wedge springs, (.542 max lift), stock stamped rockers, and 48,000 miles since a professional rebuild. 4000 miles of those miles are on a road course.

I was running an errand on the way home from work, did a quick 2nd gear WOT up to the point where I needed to turn off the road, and the engine suddenly developed a dead miss. It didn't make any noise when it did it, so I figured no biggie, I burned another plug wire on a header.

I swapped all the plug wires, still had a miss. So I ran a Cylinder Balance Test through 3 levels. Results were:

Cylinder Balance Test
First pass: Cylinder 7
Second pass: Cylinders 6, 7, 8
Third pass: Cylinders 6, 7, 8.

That was a bit of a head scratcher, but I figured OK, cylinder 7 is dead. I replaced its spark plug, no change.

Vacuum test
I hooked a vacuum gauge up to it and cranked it over. It pulled about 15 inches except as the starter sped up past the weak cylinder, where the needle dropped.

Compression test
I ran a compression test with these results:

1 = 145 (but it went to 155 when I added oil through the plug hole)
2 = 150
3 = 155
4 = 165
5 = 0 (!...WTF? 7 was the dead cylinder on the CBT)
6 = 165
7 = 160
8 = 160

I squirted some oil in #5 and did it again, it pumped up to 30 momentarily and then free-rotated, absolutely no compression at all

valvetrain
I pulled the DS valve cover and observed the intake spring on #5 is broken, but the valve still appears to open and close normally.


So...
What does the crowd think I'll find as I tear deeper into it? And any guesses on why the CBT showed a dead 7 when its compression is fine, and didn't flag 5 even though it's not contributing at all?

 

[Chuckman]

just throwing this out there as a theory to see if im thinking right (and i may be totally wrong), but maybe its not seeing 5 in the cbt because there's no change when it cuts 5 out since its not doing anything in the first place? iirc, the cbt only checks for a weak cylinder and not a completely dead one. you may actually have more than one problem, and its all on the same side of the motor, so maybe a chain effect somewhere?

 

[Noobz347]

I think either the valve on #5 is not closing or that the piston ring is totally smoked.

I tend to lean more toward the ring since you were able eek a whole 7 psi out of oiling it.

 

[MFE92]

just throwing this out there as a theory to see if im thinking right (and i may be totally wrong), but maybe its not seeing 5 in the cbt because there's no change when it cuts 5 out since its not doing anything in the first place? iirc, the cbt only checks for a weak cylinder and not a completely dead one. you may actually have more than one problem, and its all on the same side of the motor, so maybe a chain effect somewhere?

I think you're right

 

[MFE92]

I think either the valve on #5 is not closing or that the piston ring is totally smoked.

I tend to lean more toward the ring since you were able eek a whole 7 psi out of oiling it.

And I hope you're wrong!

 

[Noobz347]

And I hope you're wrong!

Wouldn't be terribly bad so long as the piston wall isn't scored. Hell, you could swap it from the bottom by removing the oil pan.

 

[MFE92]

Relief! I changed the spring out for one of the other cylinders' springs and compression-tested it again, #5 came right up to 160. Looks like I'm good for now if I can find a proper set of springs for it that'll work with the stock rockers and a .533 lift cam. Any ideas?