00 LS V8 issues

[2000mnLSV8]

Recently i took my 2000 ls v8 with 158,000 miles for a short drive (during the drive there did not notice any issues) and parked it for about two hours outside and then started it up again. i keep it in a warm garage and that day i parked it outside and it was about 10 degrees out. Anyways i started my car and at first i noticed a sound that almost sounded like a light knocking in the engine and i proceeded to drive it the short distance home and the engine began to misfire quite bad, enough to make the check engine light flash. I got it home and noticed it once again idled rough and misfired from idle to around 1500 rpms and also misfires a little the whole rpm range above 2500 rpms. So the next day i proceeded to take it to my mechanic because i knew it wasnt the coils or the spark plugs because i just replaced them about 2000 miles ago, and the valve covers are not leaking oil to foul the coils. The next day the mechanic told me that the #4 cylinder is the one that is misfiring and he replaced the plug, coil, and injector for that cylinder and it did not stop the misfiring. He also did a compression test and that cylinder failed. He thinks that i have a bad compression ring on that cylinder and that it is what is causing it to misfire, and he quoted me a $6000 engine replacement. What i find odd is the car is not burning oil and also the car has had a vacuum leak for some time now that got a little worse before the issues started. i have also noticed an odd smell from the exhaust from the moment it started misfiring, it does not smell like burning oil though. I have smelled it before on a taurus with a vacuum leak. I was recommended to run an additive to my oil to help clean out the engine incase there is a carbon build up or something around the ring but it has made no difference. The car still drives fine, just runs rough and idles horrible and also lost a lot of power which is a given since its only running on 7 cylinders i also have the engine code P0304 cylinder #4 misfire on. Im not ready to give up on the car yet, it just doesnt seem like a compression ring problem, anyways any ideas about what is going on are greatly appreciated! Also recently did a muffler delete and put some magnaflow tips on and also put a kurts kustom air intake on it, dont think any of that would cause an issue though

 

[joegr]

If it really failed a compression test, then it has to be the rings, the head gasket, the valves, or the timing. It is possible that the compression ring has failed but the oil control ring is still good.

All in all, with yours being a gen I, I would suspect that a timing chain tensioner has failed and your bank 1 exhaust cam has jumped time. Fortunately, this is really cheap and easy to check. You just need to pull the bank 1 valve cover off and have a look. Since your "mechanic" didn't do this before making a $6000 diagnosis, I suggest that you find a better mechanic first.

If you do find that it is a tensioner, you should go ahead and replace the others too.