89 yj dead miss cyl #5

[mcgrnr0013]

i really need some help, i got a 89 yj with the 258 inline 6, with weber 32/36 carb, today i was tracking down some vacuum leaks and after i got those fixed i started to adjust my carb now that the engine wasnt gettin all that extra air. right after i revved it up and it backfired through the intake and it started misfiring i tweaked the carb a little more and smoothed it out but it still has a dead miss. first i checked my spark, all the cylinders are getting spark, then i checked my compression and it was cyl1-80 2-100 3-100 4-115 5-90 6-120 so then i started pulling plug wires while it was running and figured out that it was my number 5 cylinder that is misfiring. if anyone may have any idea what it may be please help

 

[JPNinPA]

It could be as easy as a stuck valve or sever carbon buildup. Those things just need cleaning. But I thought all should be around 120, or at least within 5 of one anothe. So cylinder #1looks problematic as well.
Other possibilities are leaking rings. Failing or broken valve springs.

 

[mcgrnr0013]

well tomorrow i am going too pull the valve cover off and turn the engine over manually and see if my valve and everything is operating properly. what all would be required to replace the piston rings? would i have to pull the head off to remove them? ill update tomorrow after i check out the valvetrain

 

[PoliceMonkey161]

Before you take the valve cover off, I would recommend getting a oil squirt can with some heavy weight oil in it. Squirt just a little of oil in your 2 cylinders that are lower than the others and retake your compression test. If your compression raises, you have worn rings (the oil temporarily fills the gap between wall and rings). If no changes occur, you either have a completely broken ring or will need valve work done. Next, I'd remove the cover and check the position of the valves. Eventually, you'll likely be taking the head off regardless, this only gives you a starting point.

Replacing the rings (if in fact that's the problem) is not difficult but is seldom just a matter of replacing the rings and oil skirts. At a minim, you would have to remove the oil pan to remove the pistons, but most folks remove the whole block. Normally, if the pistons are the problem, you would remove the block and have the bores checked for size, re-decking, bore alignment, crank shaft ware etc..

 

[mcgrnr0013]

thanks for all the help guys, i really appreciate it. and today i think i solved my problems. i ended up taking the head off due too a hissing sound when i turned the engine over manually so i assumed it was possibly a leaky head gasket or my valve stem seals were shot, and actually the valve stem seals were in pretty awful condition. sooo anyways i went ahead and removed all the valves while i had the head off and was going to clean and polish them up and all slid out nice and smooth until i got to the cyl #5 intake valve. i had to take a hammer and a punch and gently tap it out. my assumption is that the valve had bent slightly just enough to keep the valve from closing all the way, so i ordered a new gasket set and new valve stem seals and a new intake valve and im hoping that this was my issue because everything else checked out okay, here are some pics of it after i had it tore down

 

[carnuck]

The '79-81 head (with tin cover) had an issue of cracking valve guides that suddenly bound valves and caused them to smack the piston as it came up.