One cylinder with significantly higher compression?

[quagmire]

Okay, here's a weird one.  It's getting closer to storage time, so I decided to check the plugs and compression test the motor before I do a little teardown work this winter.  Plugs all look great, all cylinders were at 150psi except number 2 which was over 180psi.  I pulled off the valve cover for that bank, and both rocker adjusters for #2 are unscrewed significantly more than any of the surrounding cylinders. I rechecked adjustments, and they all seem fine.

A little background too: 
I compression tested it several months back and got about 150psi all around.  I always thought that was a bit low, after calculating what I should have in regards to dynamic compression it is.  I figured out it should be around 172psi cranking.  After that I readjusted my valves because I had a suspicion that they were preloaded too tight when I first built it.   Most were too tight, and I do remember having noise issues after I first did this too.  I remember popping the valve covers off and adjusting them a hair more with the motor running to get it just enough to quiet it down.

I'm not quite sure what would cause this problem, my guess is I have a cam or lifter problem with all of the other cylinders except for #2.  Is it possible the other lobes are all worn, or maybe the other lifters are damage/collapsed?  I have 1100 miles on it now, from what I've heard about cam lobe failure it happens fast and they go flat; not just a little worn.  I am leaning more towards a lifter issue right now.  I bumped the engine over when I had the cover off and all valves seemed to move fine; #2 may move more than the others but it is hard to tell with my eye micrometer.  Oil in the cylinders did not make any difference either.  Also, when I drained the oil last (300mi) it was crystal clean, no metal on any magnets.  Any ideas or insights would be greatly appreciated! - Dan

[Chryco Psycho]

the only though I have is that posiibly the exhaust valve is not opening giving the air nowhere to go

[quagmire]

I was initially thinking that too, which is why I popped the valve cover on that side. Both valves seem to open and close at least as much as the other cylinders if not more.  It's also really weird that the adjusters on both valves for that cylinder are backed out much more than any of the others.

[vinb]

It maybe that the valve stem heights are way high and the machine shop over looked those two valves why the adjusters are up high.
As far as the compression goes it could be that the exhaust is not opening all the way and quickly closing or you have A$$ kicking ring seal in that cylinder..

[quagmire]

The heads are new Edelbrock Performer RPM's and I took them apart and measured everything on both heads.  Didn't find anything out of the ordinary, including the problematic valve guide clearance on these heads.  Both valves are definitely opening, I have a remote starter button under the hood so I can watch them as it cranks.

[gkring]

A trick I use on other motors is to take the rockers off and lay a straight edge on the valve stems. You can spot uneven valve heights pretty quick. Theoretically a loose valve setting on a single cylinder makes that cylinder think it has a smaller cam, both in lift and duration. Smaller cam will create more cranking pressure. have you tried a leakdown test as well to see if it is a mechanical timing thing and not the rings or valve seat issues?

[quagmire]

That's true about loose valve adjustments, but both valves are adjusted a 1/4 turn past zero lash.  Valve heights are all equal.  Why these two are threaded less is beyond me.  Now when you are setting preload on lifters how much resistance to compressing the plunger should there be?  Both times I have checked and adjusted preload there was quite a bit of resistance as I tightened them down, and this was on all of the valves.  I was wondering if the lifters could cause a problem, like they are stuck and not bleeding down at all.  I have not leakdown tested the motor, but I may buy a gauge if it seems like that could help determine the problem.

[71chally416]

Chamber volume might be different in that cylinder, or even the center to center on that rod. That could put that piston higher in the bore.

[Chryco Psycho]

could be different lifters causing the adjuster height difference why both would end up side by side is odd still

[Roppa440]

Just a thought to cover all bases.

How are you setting the lash exactly?

I know many people that have assumed TDC is the position to set lash in on each cylinder. Then they find that when they fire the engine up the lash is too loose.

I only mention this on the small chance you did something like that.

[Moparal]

Are you sure all 8 cylinders was the same the first time you did this?  2 things come to my mind is, 1 the crank is not indexed, and 2 the cc's is off in the head on that cylinder.  Or maybe the gauge went reading bad or hose seating different. Were all the plugs out and the throttle wide open when you did the test?  The engine temp must be the same in the test. if it was warm and you have a hot spot, this can cause a differnt reading on aluminum heads to.

[moper]

What you see in a higher pressure reading is normal for a cylinder where the valves are closiong earlier due to varience in lash/preload. The piston is lower in the bore and movign up when the intake valve closes. The earlier in that stroek the valve seals the cylinder (less lift and druation at the valve) the more pressure get's sealed in and built up. You need to find out why it has to be adjusted that way to yeild the same preload as the others. I would venture to guess it's a lifter issue.

[quagmire]

Yes, I pulled all of the plugs with the engine warm and throttle open.  Last time I checked compression they were all about 150psi, this was before I adjusted preload. That's why I kinda doubt it is a machining issue.  I set preload by tightening a 1/4 turn past zero lash with valve completely closed.  I thought it was weird that #2 required so much less screwing down, but didnt worry much about it at the time.  All of the lifters required quite a bit of force to turn the adjusters down once I got to zero lash.  Is this normal because they have been run and have oil in them? I may buy a second compression gauge just to rule out the possibility that the hose isnt sealing well.

[moper]

Yes, they get stiff with oil in them. Also, if the lifter bores are tighter on that hole, the lifters will be even tougher to bleed down. Basically you are pumping out the oil when pre-loading an installed lifter. Ifyou turn the engine over a turn before you set the preload on a cylinder (by hand) will help squeeze oil out of them before you tighten the adjuster.

[Roppa440]

I set preload by tightening a 1/4 turn past zero lash with valve completely closed.

Sorry but this is still not clear to me. I don't mean to be a pain but how did you determin the valve was completely closed?

For example. When you positioned #1 cylinder at TDC firing did you then only adjust lash on the intake of #2 and the exhaust on #8?

Or did you determin the lifter position on the cam lobe by some other means?

It is just that I have come across all kinds of problems in the past due to people not setting lash by the correct method.