[MarkS57]

Originally Posted by dataway

I'm curious what you guys think ... can a very late opening intake valve increase vacuum enough on the intake stroke to pull additional oil past rings and valve seals? I mean like 50-60 degrees late.

Yup, in this thread, the right side exhaust was putting out smoke, that's what led me to the right side cylinder bank; in my case, the intake wasn't opening at all.

https://forums.maxperformanceinc.com...=795907&page=2

[geeteeohguy]

Originally Posted by supersport#69

Mechanic said when he blew air in some air came out the exhaust so he thought exhaust valve might be burned or some carbon blocking.

Was the cylinder up on top dead center when he 'blew air in '? It needs to be to be an accurate test. In my past experience with these engines over 40+ years, a burnt exhaust valve is the most common cause of low compression. You may be in for a simple valve job. Not a big deal. An actual leak down tester with the gauge would be the right way to verify engine condition here.

[supersport#69]

I’m not sure if it was top dead center or not. He’s a pretty old school mechanic locally. I assume he knows.
Believe me, I’m open to somebody testing it too as knowing it’s just a valve job might ease some stress.
Out of curiosity if it’s just a valve job and I get the heads rebuilt, wouldn’t that put pressure on the bottom end too or it should be ok?

[geeteeohguy]

Originally Posted by supersport#69

I’m not sure if it was top dead center or not. He’s a pretty old school mechanic locally. I assume he knows.
Believe me, I’m open to somebody testing it too as knowing it’s just a valve job might ease some stress.
Out of curiosity if it’s just a valve job and I get the heads rebuilt, wouldn’t that put pressure on the bottom end too or it should be ok?

IIRC, your engine has less than 65,000 miles on it. If all it needs is a valve job, that and a timing set (which you said had been done) would be good enough for me. I pulled my '67 GTO's engine down at 173,000 miles because it had two burned exhaust valves. Did a complete rebuild because of mileage. In retrospect, the bearings and cylinders were actually still in spec. I could of cheated and just done the valve job. Have driven it another 80,000+ miles since then. You could borrow a leak tester from an auto parts store possibly and do the test yourself. You need an air compressor and to get the cylinder at TDC....that's it. The fact that adding oil didn't significantly increase the PSI on the compression test is indicative of a burnt valve. And as I've stated, have seen it many times on these engines in their original survivor state.

[Schurkey]

Originally Posted by mgarblik

a wet compression test is easy to do and would give you an idea of piston or valve issue. 2-3 squirts of engine oil in the suspect cylinder and then repeat compression test. If compression comes up 25 psi or more on the retest than dry, likely a piston ring/broken land issue. If it stay's the same or comes up 5-10 PSI, most likely a valve seating issue. It's an OK test and requires no special tools. Good luck with it.

I quit doing "wet" compression tests decades ago. No one could explain to me how the oil was supposed to go uphill to seal the rings on the intake manifold side of the cylinder.

Originally Posted by Formulas

My compression gauge hose plugs into my shop air hose I use it LIKE a leak down tester to tell if rings / piston or intake / exhaust valves and also to rat out bad head gaskets because bubbles will show up at the radiator with the cap off,
In fact you should have the radiator cap off and fluid topped off just to see if there's nothing to see then great how much did that cost?

I used to do that, prior to about 1986--'87 when I bought an actual leakdown tester.

Noise out the intake or exhaust, or bubbles in the cooling system are a dead giveaway of engine problems. Noise out the oil fill cap is not, without a gauge you have no idea how good/bad the rings are. I've tested engines that sounded horrible for noise at the oil fill cap, but the gauge said they were OK--and others that sounded OK but the gauge and engine performance verified that the rings were bad.

"Most" compression tester hoses will need to have the valve core removed before using them with a leakdown tester, or plugging them in to "shop air".

Originally Posted by getmygoat

Note that many mechanics don't understand how Pontiac rockers are adjusted. That mechanic may have overtightened the loose rocker . I would suggest getting a torque wrench and retighten that rocker to 20 lbs as called for in the pontiac spec. If the rocker is too tight it could be hanging open. Worse, a too tight rocker will flatten your cam.

The Pontiac engines that require 20 ft/lbs on the rocker arm "adjusting nut" have studs with a positive stop. The nut goes down until it seats on the taper of the stud, tightening it more doesn't change the "valve adjustment" and overtightening it cannot make the valve "hang open". It's hard on the stud and nut, though.

Originally Posted by supersport#69

Another note to add the mechanic blew air in the cylinder and he said a little air came out the exhaust.
He think maybe a burned exhaust valve but not certain.

Any air leaking out the valves is bad news. The engine may still run--the valve may seal under compression pressure--but it's an indication of a true problem.

Pop the valve stems directly in-line with the valve guides with a soft hammer--or a hammer and brass punch--so the valve snaps open and closed. Doesn't take much. The valve doesn't need to open more than an eighth of an inch, if that. If there's carbon interfering with the valve seal, that'll break it up.

More likely, it's a defective valve/seat/guide.

Originally Posted by PontiacJim1959

Depends on how extensive the rebuild. You might get away with just grinding the seats/valves and be OK. But if they are off, why not do it right and go through them?

The iron heads I had rebuilt, cleaned, magnafluxed, surface heads, Ferrea stainless valves, bronze guides, Viton valve seals, 3-angle valve job, new springs, retainers, locks, 7/16" Big Block rocker arm studs, freeze plugs & water nipple ran me $1,100.

I made that mistake, too.
For $1100, he might as well double-up and invest in Edelbrock heads as flush money down the toilet on ancient iron castings that weren't all that great 40 years ago.

Originally Posted by supersport#69

That’s not too bad I guess. Is it worth doing hardened seats?

When it was me, I had hardened inserts installed. I should have just bought the Edelbrock heads.

Originally Posted by supersport#69

if it’s just a valve job and I get the heads rebuilt, wouldn’t that put pressure on the bottom end too or it should be ok?

Depends on the rest of the "bottom end". If it's good...it's good.

I've seen a couple marginal engines get a valve job; all eight cylinders run for the first time in a long time. I found out that those pistons/rods/bearings were only quiet and smoke-free before because there was no cylinder pressure on 'em. Others overheated because with the engine making full power, the corroded/clogged radiator could no longer keep up with the heat load. "It never overheated before you worked on it!" Yeah, but it only made half or 2/3 the power it was supposed to, and you were driving it like my mother. Now that it runs right, you're burning rubber and smiling about it every day.



You've got one cylinder (at least) that's had the rings and cylinder walls gas-washed for awhile. You'll know more when you get the heads off.

["QUICK-SILVER"]

Originally Posted by Schurkey

You've got one cylinder (at least) that's had the rings and cylinder walls gas-washed for awhile. You'll know more when you get the heads off.

Shurkey,

You mind explaining how the cylinder got "gas-washed" when the intake valve wasn't opening any at all?

Clay

[Schurkey]

Originally Posted by "QUICK-SILVER"

Shurkey,

You mind explaining how the cylinder got "gas-washed" when the intake valve wasn't opening any at all?

Clay

I'm going off what was described in the first post.

Originally Posted by supersport#69

The spark plug on the 35 lb low cylinder had raw gas on it and no signs of anything burning at all in that cylinder.

If the intake valve-to-valve seat seal isn't PERFECT, cylinder vacuum will pull raw gas past the intake valve. Raw, liquid fuel is rolling around the floor of the intake manifold especially when the the choke is on and the manifold is too cold to properly vaporize the fuel. That's why almost all intake manifolds have some sort of heating--generally via exhaust gas in the exhaust gas crossover; but sometimes heating with engine coolant, or even an electric grid under the carb.

There's some evidence of poor valve-to-valve seat sealing.

Originally Posted by supersport#69

Here’s the weird thing. When turning it over I could hear air coming out somewhere (maybe carb) from low cylinder. When checking the 150 lb next to it I couldn’t hear anything.

[geeteeohguy]

Until a bonafide leakdown test is performed, everything is speculation. Perform a leakdown test and go from there.