Really need help - cylinder won't build compression
Thread Starter
Staging Lane
iTrader: (4)
Joined: Aug 2004
Posts: 59
Likes: 0
From: Kitchener, Ontario
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
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
Thread Starter
Staging Lane
iTrader: (4)
Joined: Aug 2004
Posts: 59
Likes: 0
From: Kitchener, Ontario
Originally Posted by mmmchickenboy
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.
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
Thread Starter
Staging Lane
iTrader: (4)
Joined: Aug 2004
Posts: 59
Likes: 0
From: Kitchener, Ontario
Originally Posted by REDWS6ROCKET
sounds like you bent a valve to me. take off the rockers and look at the valves is one setting lower than the other.
Mark
Thread Starter
Staging Lane
iTrader: (4)
Joined: Aug 2004
Posts: 59
Likes: 0
From: Kitchener, Ontario
Originally Posted by mmmchickenboy
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.
Just trying to understand what I'm up against before I decide if I want to pull the head.
Thanks,
Mark
Trending Topics
Launching!
Joined: Apr 2003
Posts: 272
Likes: 0
From: Cincinnati, Ohio
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.
Pull the head, cochise.
LS1 Tech Stories
The Best V8 Stories One Small Block at Time
6 Common C5 Corvette Failures and What's Involved In Repairing Them
Pouria Savadkouei
Retro Modern Bandit Pontiac Trans AM Comes With Burt Reynolds' Autograph
Verdad Gallardo
Top 10 Greatest Cadillac V Series Performance Models Ever, Ranked
Pouria Savadkouei
Top 10 Most Powerful Chevy Trucks Ever Made!
Hennessey's New Supercharged Silverado ZR2 Has 700 HP
Verdad Gallardo
Coachbuilt N2A Anteros Is an LS2-Powered C6 Corvette In Italian Clothes
Verdad Gallardo
Awesome K5 Blazer Restomod Comes With C7 Corvette Power
Verdad Gallardo
10 Camaros You Should Never Buy
10 LS Engine Myths That Refuse to Die
Verdad Gallardo
Originally Posted by 1999_SS_M6
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.
Pull the head, cochise.
Thread Starter
Staging Lane
iTrader: (4)
Joined: Aug 2004
Posts: 59
Likes: 0
From: Kitchener, Ontario
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
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
. (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...
perhaps instead of fuggin up the pushrod, you "modded" the rocker?
swap the whole thing, from the "holder strip" on up...
Thread Starter
Staging Lane
iTrader: (4)
Joined: Aug 2004
Posts: 59
Likes: 0
From: Kitchener, Ontario
Originally Posted by MrDude_1
try swapping the rocker shafts from one side to the other.
perhaps instead of fuggin up the pushrod, you "modded" the rocker?
perhaps instead of fuggin up the pushrod, you "modded" the rocker?
Mark
Thread Starter
Staging Lane
iTrader: (4)
Joined: Aug 2004
Posts: 59
Likes: 0
From: Kitchener, Ontario
Originally Posted by MrDude_1
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.
Mark
Originally Posted by Mark99Hawk
That what it seems...next question: can I get the lifters out without pulling the heads?
Mark
Mark
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.
Originally Posted by Mark99Hawk
That what it seems...next question: can I get the lifters out without pulling the heads?
Mark
Mark
Launching!
Joined: Apr 2003
Posts: 272
Likes: 0
From: Cincinnati, Ohio
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.
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.