Incorrect valve lash can create misfires

[DallasTahoe]

I've been searching this subject and I've found several different answers, anywhere from 1/8 of a turn to a full turn after reaching zero lash. So what is the CORRECT amount of preload on the lifters? Also if the valve lash and preload are not set correctly could it cause misfires and/or low performance? Specifically losing speed but not rpms when going up a hill??@SunlitComet what say you sir??


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[floater]

I just got my motor rebuilt. The engine builder told me to tighten the rocker nut till the push rods just stop spinning between your fingers then one full turn.

 

[DallasTahoe]

I just got my motor rebuilt. The engine builder told me to tighten the rocker nut till the push rods just stop spinning between your fingers then one full turn.

Readjusted valve lash on all cylinders on the passenger side and my misfires disappeared!!!

 

[DallasTahoe]

Sooooooo I believe I fixed it!!!
I pulled the passenger side valve cover and reset the valve lash on all 4 cylinders, put it back together and put my obd2 adapter on, started it and it started right up and I could feel a noticeable difference. Checked the scanner after I let it get up to temp and sure enough no more misfires on cyl 6, now I just need to readjust bank 1 and I should be good!!


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[1998Suburban]

Glad you found a solution! Had you done any engine work that required pulling heads, valve train, etc? I know the engine would run like crap if the lash was way off. I had a '93 4.3L that needed new valve stem seals. Fel-Pro instructions said 1/2 turn from snug, FSM said 1 full turn. I did 1 full rotation and she barely ran, I split the difference and set them to 3/4 turn and she ran well.

 

[DallasTahoe]

Glad you found a solution! Had you done any engine work that required pulling heads, valve train, etc? I know the engine would run like crap if the lash was way off. I had a '93 4.3L that needed new valve stem seals. Fel-Pro instructions said 1/2 turn from snug, FSM said 1 full turn. I did 1 full rotation and she barely ran, I split the difference and set them to 3/4 turn and she ran well.

Yea I replaced the heads last month and I was chasing this misfire demon ever since, I read a post on one of the other forums similar to this one and they said something about it being a factor, then after looking at a post where @SunlitComet posted some material everything just kind of clicked. Just finished bank 1 valve lash and in a bit ill have her back together and scan her... She's been a mean *****, but now she's my *****... Lol

 

[SUBURBIAN]

Generally speaking, stock engines do well with 1/2 to 3/4 turn past zero lash. Zero lash is the point in tightening down the rocker nut where the cam lobe, lifter, pushrod, rocker arm and valve are all just barely in contact with each other. If the nut is tightened to the point where the pushrod will not spin between your fingers any more, it has been tightened past zero lash. Then when another full turn or half turn or whatever is applied, it's even farther past zero lash. This is when you'd see runability issues, because cylinfer (s) can't fire correctly when a valve is hanging open. Compression can't be built as it should.

 

[willxfs]

Good information. Thanks for keeping up and posting with the resolution. I'm trying to find a subtle misfire (no check engine light) on a newly built 383. Scanner shows a few misfires on passenger side bank-2,4,6,8. This might help.

 

[DallasTahoe]

Good information. Thanks for keeping up and posting with the resolution. I'm trying to find a subtle misfire (no check engine light) on a newly built 383. Scanner shows a few misfires on passenger side bank-2,4,6,8. This might help.

Yea try it it couldn't hurt especially if they are all on the same bank...