Tangential to this topic, a friend had a few "experiments" he suggested might be interesting, since I have an injured #6, and a bunch of blow-by with more of it, one would assume, from #6 than from others.
It is possible to measure the volume of blow-by at the crankcase vent tube, for science.
His hypothesis 1: Unplug spark plug #6. With less/no half-assed combustion going on in 6, perhaps less blow-by would be measured at the vent?
His hypothesis 2: Remove rockers and rods from #6. Valves remain closed so there's nothing to blow by. This idea comes from the fact that GM holds valves closed for their cylinder deactivation / AFM scheme on their V8's.
Thoughts/discussion on these HYPOTHESES?
Note: This is not seeking a solution to anything. This will solve nothing. It is just in the interest of answering "what would this do?". Retired engineers just wondering about things....
Very low compression on #6
Re: Very low compression on #6
67 Coupe Powerglide, Lynchburg, VA
Re: Very low compression on #6
Hard to say about pulling the plug wire on #6 and blowby measurement. Never tried it, if a cylinder was that low on compression (vs others), I went after the problem. On removing rockers / pushrods: DON'T. This would leave no resistance on the lifters and running like this could cause serious damage.
Reality is it appears you have a damaged piston. The cure would be replacing that piston, which would require head removal, blower shroud removal, and top cover removal. Again, how you want to operate the car should determine how far you want to go with the repairs (ONE piston, re-use cylinder if not scored, vs ALL pistons and full shebang).
While your friend had interesting "scientific" suggestions, stick with KNOWN mechanical advice.
Reality is it appears you have a damaged piston. The cure would be replacing that piston, which would require head removal, blower shroud removal, and top cover removal. Again, how you want to operate the car should determine how far you want to go with the repairs (ONE piston, re-use cylinder if not scored, vs ALL pistons and full shebang).
While your friend had interesting "scientific" suggestions, stick with KNOWN mechanical advice.
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Wagon Master
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Re: Very low compression on #6
On removing rockers / pushrods: DON'T. This would leave no resistance on the lifters and running like this could cause serious damage.
On a roadside repair I've loosened rockers and removed pushrods before in a cylinder that has dropped a valve seat. Just to get home and prevent further damage and eliminate the chances of the seat exploding and moving to other cylnders. I believe the pushrod tubes prevent the lifter from coming too far out of its bore and causing zero oil pressure, as can happen on a V-8..
On a roadside repair I've loosened rockers and removed pushrods before in a cylinder that has dropped a valve seat. Just to get home and prevent further damage and eliminate the chances of the seat exploding and moving to other cylnders. I believe the pushrod tubes prevent the lifter from coming too far out of its bore and causing zero oil pressure, as can happen on a V-8..
Re: Very low compression on #6
Are you positive both #6 valves are opening fully ,a flat cam lobe will affect breathing , a broken valve spring will also affect compression pressure reading. A cam lobe going flat takes time to show up , a broken spring would be felt in one trip,
Watch closely at rocker movement while rotating the engine,do the #6 rockers move equally compared to #4 cylinder rockers? A flat cam lobe would barely move a valve open.
A broken spring would fail to hold the valve closed tight enough to hold compression pressure. A broken spring is not always apparent without removing the spring for inspection.
I don't see any thing suspicious in the #6 cylinder picture.
Paul
Watch closely at rocker movement while rotating the engine,do the #6 rockers move equally compared to #4 cylinder rockers? A flat cam lobe would barely move a valve open.
A broken spring would fail to hold the valve closed tight enough to hold compression pressure. A broken spring is not always apparent without removing the spring for inspection.
I don't see any thing suspicious in the #6 cylinder picture.
Paul
Re: Very low compression on #6
Lot of suggestions, BUT 67Monza said he has lots of blow by and low compression on # 6. SURE sounds like a piston, or piston ring problem at #6. Sad to say this is a case of removing the drivetrain and then the head and probably #6 piston and cylinder. This is a situation were "magical thinking" happens to try and avoid what is an time consuming and possibly expensive situation.
As Dennis said --- either just repair the issue, or if the engine is tired and worn out, then do a full engine rebuild!!
If the inside of the engine is clean, and it ran fine, then just doing the one cylinder (if that is the problem) may do the job if you don't drive the car that much.
I have seen folks do "in chassis" engine work, but I'm too old for that. Yes pulling a the drivetrain is a lot of work, but so is pulling engine tin, head, cylinder and piston with the engine IN the chassis!!! Just my
As Dennis said --- either just repair the issue, or if the engine is tired and worn out, then do a full engine rebuild!!
If the inside of the engine is clean, and it ran fine, then just doing the one cylinder (if that is the problem) may do the job if you don't drive the car that much. I have seen folks do "in chassis" engine work, but I'm too old for that. Yes pulling a the drivetrain is a lot of work, but so is pulling engine tin, head, cylinder and piston with the engine IN the chassis!!! Just my
Re: Very low compression on #6
I hoped that my "note" implied that I know what needs to be done.
For clarification: I intend to repair things correctly. However I have numerous projects and can not begin repairs right now.
In the interim, I thought some banter and thoughts on these "hypotheses" might be interesting, and I do not mind experimentation provided it is non-destructive. The tangent isn't about whether to fix or not to fix, black-or-white, but rather, discussion of the odd ideas. I like hearing each thought on it. chief's comment above made me think a little bit more of my previous assumptions. I enjoy pondering this stuff beyond just solutions to a problem.
Perhaps I see a problem as an opportunity to go on a journey of sorts, more than just fixing it.
For clarification: I intend to repair things correctly. However I have numerous projects and can not begin repairs right now.
In the interim, I thought some banter and thoughts on these "hypotheses" might be interesting, and I do not mind experimentation provided it is non-destructive. The tangent isn't about whether to fix or not to fix, black-or-white, but rather, discussion of the odd ideas. I like hearing each thought on it. chief's comment above made me think a little bit more of my previous assumptions. I enjoy pondering this stuff beyond just solutions to a problem.
Perhaps I see a problem as an opportunity to go on a journey of sorts, more than just fixing it.
67 Coupe Powerglide, Lynchburg, VA
Re: Very low compression on #6
Perhaps I see a problem as an opportunity to go on a journey of sorts, more than just fixing it.
YES!
YES!
Re: Very low compression on #6
No problem with looking at, and trying to eliminate, different possibilities, but you did the leak down test and that typically indicates a piston/ring, or valve leak, or head gasket. If you have time - check the following:
If the intake or exhaust valves are leaking, then normally you hear hissing at the intake or exhaust during a leak down test.
I do like your idea of seeing if air comes out of the PCV vent while doing a leak down as that would confirm leakage past the piston/rings.
If it is a leaking head gasket, usually with the engine running you will either hear hissing at idle (really bad leak) or more commonly a "chirping" sound as you open the throttle under load.
Anyway ----- Since it will be awhile before you can take the engine apart, go ahead and check the crankcase or other areas for leaks, but ultimately you will have to pull the head to confirm gasket or valve leakage (if a valve is leaking, then usually it or the seat is damaged beyond fixing with a valve adjustment), if they look good, then it's time to pull the piston and cylinder for inspection.
Just my free advice -- and others can correct me. As a buddy says "Advice is only worth what you pay for it - SO beware of advice you pay nothing for."
Good luck.
If the intake or exhaust valves are leaking, then normally you hear hissing at the intake or exhaust during a leak down test.
I do like your idea of seeing if air comes out of the PCV vent while doing a leak down as that would confirm leakage past the piston/rings.
If it is a leaking head gasket, usually with the engine running you will either hear hissing at idle (really bad leak) or more commonly a "chirping" sound as you open the throttle under load.
Anyway ----- Since it will be awhile before you can take the engine apart, go ahead and check the crankcase or other areas for leaks, but ultimately you will have to pull the head to confirm gasket or valve leakage (if a valve is leaking, then usually it or the seat is damaged beyond fixing with a valve adjustment), if they look good, then it's time to pull the piston and cylinder for inspection.
Just my free advice -- and others can correct me. As a buddy says "Advice is only worth what you pay for it - SO beware of advice you pay nothing for."
Good luck.
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