Hey guys I have a '95 Z28 LT1 M6 w/ 125K, recently my father was bouncing off the rev limiter and since then theres been a mis-fire. It has a new opti, plugs, plug wires. There are no trouble codes. I checked the vacuum at idle it was at 14 hg steady, when I rev it, it dropped to 10 for a sec then jumped up and held steady at 15. I also did a dry compression test, all the cylinders are 150 psi. I'm guessing maybe he jumped the timing chain, is there anyway to test for this? Thanks guys for any help.

 

not really that i am aware of. not other than taking off the timing cover and seeing if the timing chain is loose. common cause of backfiring is loose timing chain

 

If he jumped the timming chain even one tooth it wouldn't run at all. I've got a 94 with 115k and I'm good friends with my rev limmiter and have never had any problems after hitting the rev limmiter.

 

Any other explanations then for the low vacuum/compression?

 

It's still possible for the engine to run with it off a tooth on the cam gear. Numerous people have mistakenly installed a cam this way and it still ran (just poorly).

Jumping a tooth is not real common, but possible. It would be troublesome to try to prove this without teardown. You would have to know specific information for valve lift/position in relation to piston position and have the tools to measure/track them.
Low vacuum with no other signs of head gasket problems might make the jumping a tooth scenario more plausible, but make sure you have ruled out any other possible sources of vacuum leaks.

 

ok Well I was starting to tear apart the front when I right after I got the crank pulley off I noticed one of the plug wires had a chunk missing out of it. I'm guessing when I installed the opti a few months back I pulled one out of the wires out of the groove and it made contact with a pulley. So I replaced that wire and voila the miss is gone, I still have a problem of oil in my coolant and coolant in my oil. Anyone think it could be the oil cooler? Im getting ready to search so the answer is probably already out there. Thanks guys for all the replies.