problem with compression but now fixed; have questions now

[jruppert]

Car has been running like crap since engine was put together this spring. Today I bought a compression gauge. I was getting 150 across all cylinders but one. #8 cylinder had 0. Removed valve cover and hooked up an air hose and could hear air hissing out of lower intake. I go to loosen both intake and exhaust rockers. I go back to run my compression check. Bam! I have 150 psi. Go back to tighten each rocker and end up loosing compression. I removed my pushrods and compared with others. They are exactly the same length. So what gives? I ended up installing a washer under both intake and exhaust rockers. Rechecked compression and had 150 psi. I am stumped on this one.

I figured maybe the pushrods were different seeings that I purchased these hardened ones in the fall and had just installed them this spring. Could the lifter be holding them open? I really could not see the valves opening while tightening these down.

 

[89CopCoupe]

.........

Well it's apparent something on the valvetrain for the #8 hole is keeping a valve open (assuming you are smart enough to test with both valves closed )

All any of us could do is give you an educated guess, which it seems you are capable of making on your own.

Ya just gotta tear into it and mic stuff to find the reason.

 

[jruppert]

Yeah I loosened both valves and had my 150 psi. Its gotta be either the lifter or the valves being a hair longer. I have no clue. All I know is that if I shim the rocker up just a hair then it runs with 150 psi. Maybe I need to just invest in adjustable rockers.

 

[89CopCoupe]

Yeah I loosened both valves and had my 150 psi. Its gotta be either the lifter or the valves being a hair longer. I have no clue. All I know is that if I shim the rocker up just a hair then it runs with 150 psi. Maybe I need to just invest in adjustable rockers.

Based on your sig, AFR185's, I don't think they would have installed a valve that was off, but stranger things happen

By shimming the rocker I would believe you have found the culprit since you already pulled the push rods and they check out .... plus the lifter would make a fair amount of racket if it was at fault.

 

[burycm]

Hmm... You are running a roller cam application arent you? and If so, even though it is rare, have you elimiated the idea of a bad lifter? i was having a similar problem on two cylinders on my car, and fixed the problem by adjusting the lifters. Not sure what your deal is, just offering up a few ideas

 

[jruppert]

Yes it is a roller setup. I am beginning to think it is a lifter. I dont know what else it could be.

 

[vristang]

Here's my thoughts.

For it to be a lifter it would have to be stuck in the high position and seized so that it is unable to compress. I don't think lifters usually fail in this manner. If you remove the lifter you should be able to compress it to check that it is not sticking?

If the push rods are all the same length, then thats not it.

If the valve length was different than the other valves, it should be visible, or at least measurable with the valve cover off. A straight edge similar to those used for checking flatness of a head would work well for comparing cyl 8 with the others.

That leaves the cam lobes and the lifters.
Is it possible that the lifters for this cyl are not the same ratio? I think this could be verified visually with a side by side comparison.

The cam lobes could have been machined on the wrong base circle. Who made the cam? One cam grinder started the job, then the second shift cam grinder came in and finsihed up with the wrong base circle for cyl 8. To check this you will have to pull the cam and get some calipers on the base circles.
I am interested in how you find the cuprit.

Hope some of this helps
good luck

 

[coming4you]

hmmm

This isnt an abnormal factor... youve already made it clear that a valve is being held open... And its not abnormal to have to shim the rocker pedestal... this is why they sell .10 and .015 shim kits... All it takes it just the slightest bit of negative torque to the rocker to hold it open... Roller cams (hydraulic) should have zero valve lash... if your tightening down the rockers and the rocker is holding open the valve shim it and be done...

 

[jruppert]

I may pull lower intake and inspect lifter just to make sure. Ultimitely I want to go a custom cam from ed curtis and maybe the spring upgrade. At this point I would either go a 1.6 or 1.7 roller rocker. I would prefer something adjustable also. This is down the road though.

It is amazing how one cylinder can cause the thing to run like crap. I was still running a 14.5 though.

 

[vristang]

I may pull lower intake and inspect lifter just to make sure. Ultimitely I want to go a custom cam from ed curtis and maybe the spring upgrade. At this point I would either go a 1.6 or 1.7 roller rocker. I would prefer something adjustable also. This is down the road though.

It is amazing how one cylinder can cause the thing to run like crap. I was still running a 14.5 though.

If the valves were not closing then every compression stroke and was pushing partially burned exhaust fumes into the intake and every intake stroke was pulling exhaust gas into the cyl for compression and combustion.

Pushing air from the cyl to the intake manifold is bad news for power.

You should be happy with how it runs when its fixed though