EXHAUST VALVE LEAK?

[nama7]

My friend was removing the valve cover to see if everything looked good. He found that the valve guide seals were old and coming apart. He changed the seals while the head was still installed on the engine. He got it installed and running, but not like it should. The 318LA sounded and acted like it had a burnt exhaust valve on #6. Pulled the intake, head, and all things associated with tearing it down. Cleaned the combustion chamber and it looked good. There was no evidence of a burnt valve. Watched a few videos of a "leak test" using water, mineral oil, and or gasoline. There was no leaking noticed. Also blew air into the exhaust and intake ports with fluid still in the combustion chamber to see if any bubbles were seen. There were no leaks found and no visible indication of a leak. Is this a reliable test? The only thing found that may cause the problem were two bolts holding the valve rockers in place on #6 cylinder were very loose. Just barely needed a wrench to take them off. #6 was the problem cylinder. Could those loose bolts have caused the problem? I sure hope so. In hindsight I guess that should have been the first thing to check. But it wasn't. Your thoughts on the problem and the test used please.

 

[CBODY67]

As the engine has hydraulic valve lifters, how was the rocker shaft re-installed? The issue with the loose shaft bolts near the allegedly misfiring cylinder are interesting, but probably NOT the reason for the misfire on that cylinder as I suspect the stiffness of the shaft would keep things as they need to be.

Replacing the valve guide seals, on the engine, if done properly, should not have caused any issues either. I've heard of no issues resulting from such a labor operation if done correctly. Main thing is to ensure the valve stem keepers are all put back so that they are fully seated on the valve stem and the spring keeper is engaged with the locks correctly, to me.

Putting the rocker shaft back onto the cyl head has some issues, especially with hydrualic lifters with oil in them. When the engine stops, not all of the valves are in the same place. Some open, some closed, or anywhere in-between. When the rocker shaft is removed, the plungers inside of the lifters will extend, even if they were compressed with the shaft installed. When the shaft is re-installed, those plungers have to be re-compressed to their prior position. We know that fluid will not compress, so the shaft re-torques have to be done in incremental steps, to gently re-compress the plungers.

On an engine with adjustable hydraulic lifters, as a small block Chevy or others, when adjusting the valves with the engine running, the procedure is to back-off the adjustment nut until a "tick" is heard (space between the end of the pushrod and the rocker arm), then slowly tighten things back down until the click is gone (zero pre-load) and then tighten the adjustment in no more than 1/4 turn preload segments. Done slowly, the particular cyl can "miss" as the lifter plungers are moved against oil pressure. After a few strokes, they compress and the engine runs smoothly again. If NOT done in small doses, PUSHRODS can be bent.

Additionally, if the specified torque sequences are not followed, with the shaft being the main intermediary item between valve spring pressure and the hydraulics of the valve lifters, the shaft might become a bit "tweaked", too, in the re-torque process. Yet, to me, the pushrods are designed to be "a weak/sacrificial link" in the valve train. Strong enough to do the job they need to, but also the first to "give" to prevent other damage, if needed. Easy to replace, too. With no other issues which might be at play, that's what I would check.

Just some thoughts,
CBODY67

 

[65Port]

Adding to what CBODY67 mentioned, did you check the push rods on that side to make sure they are true? Was the engine huffing or was there another noise leading you to believe it had a burnt valve?
Is the exhaust heat riser stuck closed (if still installed)?

 

[nama7]

Thanks for both replies. I believe the seals to be installed correctly. There was no smoke being burned from leaking. The push rods are all straight. Any thoughts on the leak test?

 

[rags]

Any thoughts on the leak test?

decent but if i had i torn the head off for a possible valve leak i would have just pulled the valve out of the head and inspected the seats and stem directly. before taking the head off i would have done a compression test on at least that cylinder bank. if the cylinder showed low compression, i would have removed the rocker shaft and put air down the cylinder to find the leak. inspect the valve springs? why think it has a bad exhaust valve? more likely a stuck lifter after what all @CBODY67 describes above.

 

[nama7]

In reply to @CBODY67, the engine had not been run in years. The seals were found to be brittle and so new ones were installed. They were replaced while the heads were installed on the engine. For whatever reason the bolts must not have been torqued at that time. Once everything was back together and engine installed in the car the engine came to life again. The noise of the engine was loud, of course, so I trailered it and got all new exhaust system installed. After I got it back and ready for a good tune-up is when the miss was heard. I guess the best thing to do would be to remove the exhaust valve to check for indications of a leak. I think I might just as well replace the lifter too.

 

[66 Monaco 500 365]

Another thought, is it possible that the exhaust valve spring was broken while it was compressed to change the valve seal?

 

[nama7]

Another thought, is it possible that the exhaust valve spring was broken while it was compressed to change the valve seal?

I took both heads to the machine shop to have a valve job done. I hope to get them back this week. I read somewhere where another make engine rocker arm bolts were getting loose. Supposedly it was because of a bad lifter. I plan to replace at least the lifters on #6 cylinder.

 

[CBODY67]

Not sure how it might be possible for a "bad lifter" to influence the hold-down bolts on a rocker arm shaft. Period. Let your machine shop operative inspect the bottoms of the valve lifters and possibly "bench check" them for proper operation. Then, as the heads are already removed, look at the cam lobes for "difference" and "sameness" between the wear patterns on the individual lobes. Then compare the #6 lobes to the others.

 

[nama7]

I took both heads to the machine shop a week or so ago and he found 3 bad springs, the valves needed to be ground, the freeze plugs needed to be replaced, and I am pretty sure he also replaced seals. He put all new springs in. I should get them back tomorrow all painted and ready to go. It will cost $385, but they should be like new.

 

[rags]

It will cost $385, but they should be like new.

that's a good deal. $385 well spent.