Really need help - cylinder won't build compression
 

History: I did a misshift (3-2) and had 3 bent pushrods.

So...I replaced the 3 bent pushrods and did a compression test. 7 cylinders are fine but number 7 is not (won't build any compression at all - reads 0 on the guage). Number 7 did NOT have a bent pushrod...

So...here are the tests we did:

- swapped rockers and pushrods to another cylinder. No change.
- took the rockers off - cylinder definitely holds air with the rockers off so I don't think it is a bent valve.
- as soon as I tightened the intake valve rocker the cylinder would no longer hold air or build compression.
- if I loosed the intake rocker a few turns the cylinder builds compression.

So I'm very confused. A lifter can't magically get longer...neither can a cam lobe or a pushrod.

Tried firing it up - misfires galore on that cylinder especially at idle. Seems ok over 2500rpm for some reason.

I really need some help here guys...I don't want to put the car away for the year if this is something I can fix...

Thanks,

Mark

sounds like you bent a valve to me. take off the rockers and look at the valves is one setting lower than the other.

even though the pushrod i'snt bent, i would have to say bent valve. remove the rockers from #7 and do the compression check.. if you still don't get anything then it has to be a valve stuck open for some reason.

I don't think it is a bent valve.

Here is why:
- when I back the rocker off about 1 full turn the cylinder builds compression (not a lot but it is better than 0)
- when I put compressed air in the cylinder with the rockers off it does not come out the intake or exhaust (which would indicate a bent valve to me...)

Thoughts?

Mark

They all look to be the same height - as far as I can tell anyway...spring looks fine too.

Mark

by just looking at them you wont be able to notice a different unless it was severe. you should use a caliper or some other precise measuring tool. even if its bent just a little bit it wont seal right.

But why would it hold air and act exactly the same as the other cylinders when I put compressed air into the cylinder? That's the part I don't understand...if it is a bent valve it shouldn't seal at all I would think?

Just trying to understand what I'm up against before I decide if I want to pull the head.

Thanks,

Mark

sometimes we spend too much time trying to convince ourselves that the situation is not what we're afraid it is... Be objective, and forget that you don't want it to be a bent valve... what else do you think it would be when you've disassembled the upper valvetrain?

Pull the head, cochise.

I agree, if its not building compression with just the intake valve on (but sloppy) then your going to have a bent valve and you need to fix it and any other ones that are like that.

I don't have a problem with it being a bent valve but I want to know why the cylinder holds air with the rockers off...

I took it for a spin around the block today - runs fine at anything higher than idle. Once it idles for a bit it misses more and more...also has another clacking noise now that it didn't have before.

Lifter?

Mark

Regardless, you'll have to pull the heads to check the nice kissing marks on you pistons :kiss:. (or at least if you have them)

try swapping the rocker shafts from one side to the other.


perhaps instead of fuggin up the pushrod, you "modded" the rocker?


swap the whole thing, from the "holder strip" on up...

Tried swapping the pushrods and rockers...no change.

Mark

in that case, the only thing left is the valve..... or the lifter is screwed in such a way that it doesnt compress... making it so the lifter preload pushes on the valve.

That what it seems...next question: can I get the lifters out without pulling the heads?

Mark

nope.

i love the LS1s design. very easy to work on, but the one thing i dont like is that.
you have to pull the heads to remove/replace the lifters. and if you're doing that, you might as well replace the valves.... you'll get better flow out of good aftermarket valves anyway.

yes if you pull the ca out with out the cam tool and they drop to the oil pan. but that isnt a very good idea. LOL. but in all seriousness pull the heads and put new lifters in it. if you see marks on the valves i would replace the valves that hit the pistons even if they are not bent. just my opinion though.

Yeah if the heads come off anything that needs replaced will get replaced...new springs will go on too.

Just trying to avoid pulling the head off...

I don't find it that hard to imagine how a valve could seal with the rocker off vs. with it on...

Picture 2 valves... one of them with a perfect 90 degree orientation between the valve face and the valve stem, one of them with a slight bend in the uppermost part of the stem.. When no component is installed (the rockers you keep mentioning) to push the valve off the seat by putting pressure against the misaligned section of stem, it seals.. The upper valvetrain components can be putting just enough force against the bent stem to cause it to lift off the seat.

I know you don't wanna pull that head, but in time you've wasted trying to ignore the obvious, you could've already had the repair done, and been burning tires.

Ok, yet another question: do I absolutely need to pull the manifolds to get the head off or can I just pry them back a bit?