No compression...Valve Seal or Piston ring

[My Black Beast]

Hey guys, my number 4 cyl has decided it no longer wants to work. I did a compression test and got between 188 and 200 (btw...isn't that a bit high?)for all of my cylinders except number 4. It had a big fat 0. I have been having a problem lately with the intake rocker on that cylinder in which the pedestal that the Crane rocker sits on has been cracking. It's happened three times now. I guess that would suggest somethings going on with the valve.

If it's the valve, what should I do about it? Do I need a complete valve job or do you think I need just the seals? I have very little money and can't afford new head since I am buying a new house. I would rather not mess with these my self right now so how much would it cost to have those done?

Thanks guys, Mike

 

[rd]

Pull the valve cover and see if something broke, like a pushrod, or the rocker arm.

If it is just the valves, pull one head and fix it.

Been there, done it. Got the valve and pushrod on my desk.

 

[My Black Beast]

Well I pulled the valve cover the other day after the problem occured and the pedestal was cracked. I was hoping that was the only problem and replaced it and the pushrod, just in case, and put everything back together. Everything else looked fine.

Do you think that I should go ahead and have both heads done or just the one?

 

[TheUser]

I'm no expert with that issue, but I'd suggest the valve is not opening as opposed to just a poor seal. I would think that if it were opening at all, you'd get at least some compression.

I have been having a problem lately with the intake rocker on that cylinder in which the pedestal that the Crane rocker sits on has been cracking. It's happened three times now. I guess that would suggest somethings going on with the valve.

I'm not clear exactly what is cracking from reading that quote. As I understand it, it's called a pedestal rocker simply because you bolt it down rather than using a head w/ a "stud" in which you'd place the rocker over a stud and use a nut to hold it down.

What I'm asking is: Is it the rocker breaking or the head?

Sounds like maybe the valve is stuck for some reason

 

[My Black Beast]

The thing that is cracked is the pedestal the the rocker sits on and the bolt goes through the rocker and the pedestal into the head. How do you fix a stuck valve?

 

[TheUser]

The thing that is cracked is the pedestal the the rocker sits on and the bolt goes through the rocker and the pedestal into the head. How do you fix a stuck valve?

hmm...I've never seen one of those before. I've only pulled the heads on one car and that's my 91 GT w/ the GT40P's and I guess it had pedestals now that I think about it, but I can't recall excatly what they looked like and can't find any pictures (grr!!), but they didn't look like that.

Either way, maybe you're torquing them too much or incorrectly?

I'm not sure what to do about a stuck valve, but searching may help you:SNSign:

 

[Dbeck002]

It doesnt sound like a piston ring, even with very poor rings you will see compression build and then drop slowly as air escapes through the worn rings. Reading zero means there is a large gap somewhere coming from one of the valves.

Is there a lot of carbon buildup on your valves?

 

[90mustangGT]

Hook air pressure to the cylinder such as though you were going to change valve springs. Better yet, use a leakdown tester if you have one. Pull the surrounding plugs. Be sure that the cylinder being tested is in the compression stroke. When air is applied, check where it is going.

If it's comming out of the crankcase (listening through oil fill hole) then it is most likley a piston ring.
If it's comming out of a surrounding piston, it is most likley a head gasket.
If it's coming out of the intake or exaust, then you have a valve issue.
If it's comming out of the radiator, it's either a head gasket or craked head/block.

 

[Rick 91GT]

Hook air pressure to the cylinder such as though you were going to change valve springs. Better yet, use a leakdown tester if you have one. Pull the surrounding plugs. Be sure that the cylinder being tested is in the compression stroke. When air is applied, check where it is going.

If it's comming out of the crankcase (listening through oil fill hole) then it is most likley a piston ring.
If it's comming out of a surrounding piston, it is most likley a head gasket.
If it's coming out of the intake or exaust, then you have a valve issue.
If it's comming out of the radiator, it's either a head gasket or craked head/block.

bingo

 

[My Black Beast]

Update!!!

Ok, thanks for the responses. I did a wet compression test and it only went up to about 10 lbs. I then decided to pull the valve cover off again, and to my surprise, I had a broken valve spring on the exhaust on #4. I am not sure if that was there a few days ago when I replaced that pedestal or not. Also the pedestal was cracked again on the intake valve.

Now my question is: What could have caused that to happen? I would hate to replace that and it break again.

What is the best way to replace the valve spring? Do you think something is wrong with the valve itself?

BTW, I think the pedestals are only on Crane Cam rockers. The only place that carried them were a Crane Cam dealer and they only had 1. I then ordered a package of them from Crane.

 

[dastang2]

i am thinking you have a bent valve or maybe a wrong pushrod if you replaced one or 2 recently. if it is stuck, it's putting a lot of stress on the springs,rockers pushrods and everything else valve related. if it is stuck, it is probably bent and you may also have broken valve guides now as well. i am just summarizing what i think about this and what could be happening here

 

[My Black Beast]

Anyone else?... bump