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Yes, my truck is down, & it has been the most confusing week I've had in a long time.
I know its a long story, but it took me a whole week to realize I was not crazy.
I was cruising down the hwy about 60 mpg when it suddenly started backfiring from the carb. I pulled off & parked... engine sounds OK but has a dead cylinder. Just off idle it rattles like a rocker fell off. So I pull the valve covers.. everything OK. All valves pumping normal, but has no compression in one cylinder during cranking. Had the truck towed home.
That night, I pulled all the plugs & ran a compression test. All cylinders at 115 to 135, all plugs look OK, looking into the holes all pistons look OK. WTH?
Reassemble, start, bad misfire. WTH??
Pull plugs, recheck, comp test, all OK! Pull valve covers, check everything, all OK!! WTH???
Reinstalled the plugs, but this time I crank without starting. no compression in one cylinder again! WTH????????
I finally notice that #5 intake rocker seems to have an extra bounce when cranking!
Never seen this in 20 years working as a mechanic. Re-run compression test on #5 & no compression for about 5 turns, then it bumps to 125 & the rocker is only bouncing 1x. Then it looses compression again.
My first thought was a sticky valve. Pull plugs, pull rockers, hook air compressor to #5 cylinder to see where air come out. Cylinder now holds compression.
Getting ready to pull the head, I pull intake, & start to pull lifters. Can't pull out the #5 intake lifter. The bottom has been destroyed, but somehow, the cam lobe still looks good!
I finally get the lifter out of the bore, & I am unable to push down on the plunger. I disassemble it & find the spring inside was offset, & had jammed the plunger all the way up. I am guessing that this also blocked the oil port so there was no lube to the cam.
So it turns out that the lifter froze, & wore out the bottom at such an angle that as it rotated, it held the valve open for a few rotations, then let it close for a few, causing the intermittent compression.
Engine has 4900 miles on it, new comp cam, lifters, & springs, from a kit.
So I posted a video of the double-rocker action. If you listen close, I get compression on the first turn, then none after that, as it starts the double bump. I then stop, release the pressure, try again, & nothing.
