350 vortec problems

Ok Guys, This is the first time doing something such as this. I need a little help. I have a 1999 Chevy tahoe w/the 350 vortec motor. Last Friday it started missing really bad, so I started with the obviousthings first. I replaced all plugs and wires correctly. This did not cure the problem. My mechanic that usually does the work I cant, said that the spider had gone bad. He replaced it along with intake manifold gaskets, and informed me that I still had little or no compression in the number eight cylinder. I took it to another friend of mine to see what else we could find out. This is a list of thimgs we have tried as I am trying not to have to put a new motor in it yet. We did a leak down test on the cylinder and found that there was no leakage. <10%.  Compression at start up runs around 120 until it warms up and drops off to about 75%. Took off valve cover gaskets and inspected springs clamps etc, and everything visually checked out. Number 8 spark plug shows carbon residue due to I guess incomplete ignition. motor has 165000 miles on it. no apparent water in the motor, valve cover gaskets are new, no lifters ticking or sticking. we even tried swapping plugs to see if the miss moved which it didnt. wires were checked again in the dark to see if any there was any visible sparking. I am looking for anything that might cause this missing on number 8 before I replace the motor. Thanks Greg  

you might have rigs that went south, thats what it ended up being when my truck did the same crap.  

Greg, start the truck and warm it up as usual to get #8 warmed up to where it is low on compression again.  Then, yank the spark plug and use an oil can to squirt some oil into the combustion chamber, doesn't take much.  If you have bad rings, the oil will act as a temporary gasket and seal around the rings.  If that is the case, you will notice a huge jump in your compression numbers.  This is a quick and easy way to check for bad rings.  Good luck  

I agree with the  suggestion mentioned above. I also had similar problem and and it helped to determine the missing compression numbers. You also mentioned that you did leak down testing. Can you share the leakdown tester that you used?  

95-99 00-tahoe limited have a spider injection system. but they have 8 individual injectors. gm extended the warranty on them to 10/100k.  the first thing that will occur is a blinking check engine light. any time the light blinks this means to much fuel is getting past the engine and if left untreated will cause permenent damage to the cat.  
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Check compression on all 8 cylinders and see how much difference there is. If one cylinder is fairly lower then the others you probably have a bad valve. The bottom line is rings or valves you unfortunitly are doing an engine. 165000 isn't bad mileage out of a motor if your not the original owner.Good luck. Hope I'm wrong!!