Still bad compression on my motor

Posted by SeymourBoobeez@reddit | projectcar | View on Reddit | 12 comments

Welp, I had my head rebuilt and was checking compression, and I’m still at around 60 on cyl 1. I didn’t hook up my intake manifold, so it was shooting coolant and gas at me and I didn’t bother checking the other cylinders. Really got me down, but I guess it is what it is. Gonna pull the motor next and just send it for a whole rebuild I guess!

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12 Comments

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rudbri93@reddit

fill the cylinder with compressed air and see where you hear it leaking out.

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SeymourBoobeez@reddit (OP)

I don’t really understand how I’d check that? Wouldn’t I just be blowing air at the piston? And I did not, but I had the entire head rebuilt at a machine shop.

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rudbri93@reddit

If the valves are closed and it was sealing properly with good compression, the air would go into the cylinder and not leak out. This will show yiu if air is leaking out an intake or exhaust port (valve sealing issue) or out the crankcase (ring sealing issue).

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SeymourBoobeez@reddit (OP)

So in theory, wouldn’t I need the oil pan off to feel if the air is blowing past the rings? I understand I could take the valve cover off to feel for the valves

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rudbri93@reddit

No because the air comes up through the heads through drain back holes and such. Thats why they pull pcv from the valve covers. If its coming through the valves itll be in either the intake or exhaust port or both.

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SeymourBoobeez@reddit (OP)

So what I think im understanding from that is to blow air with the head on, and air should be coming out from somewhere else not the valves? I dont have any PCV, but I do have a breather cap on my valve cover. This is a big block 439

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jeremymightbe@reddit

Sometimes things need to move around a little to seal. Before yanking the engine, I’d do a cylinder leakage test to see what the actual issue is. You may save yourself a whole lot of work and money.

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SeymourBoobeez@reddit (OP)

Well having the head rebuilt didn’t change the compression at all on Cyl 1… what exactly will a leak down test tell me? I honestly don’t know

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jeremymightbe@reddit

A leak down test requires a “leak down tester”, which is a cool little tool that allows you to pressurize the cylinder (you also need an air compressor). It will then tell you what percentage of the pressure is lost. The important part though, is that you can adjust things during this and try to get things to seal, because the lost air pressure will either be leaving past the piston rings, the valves, or the head gasket. You can pop open the oil-fill cap and listen for the air blowing past the rings. You can adjust the valves (if possible) and listen to whether it’s leaking out the intake or exhaust. And of course spray some soapy water around the head gasket to see if something went way wrong when installing that (cracked gasket? Missing head bolt, etc). It’s just a great diagnostic tool before you condemn the piston / piston-rings as bad. It seems you’ve already spent money rebuilding the head(s) without any change, so this tool will hopefully allow you to zero in on the true problem. Good luck!

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SeymourBoobeez@reddit (OP)

Ahh I gotcha, idk why I was thinking the one that goes on the radiator cap haha. For my motor, I do NOT have adjustable rockers, it’s just one long bar that gets bolted to the head. I torqued everything to what my manual says too. The HG was just changed, and honestly that previous one wasn’t even that bad :( it came off as one whole piece, real clean. I’m gonna have to watch a video how to do a leak down then, just to make sure I’m doing it right.

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BarnBuiltBeaters@reddit

You can also put oil into the cylinder, if your pressure rises, you need new rings.

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SeymourBoobeez@reddit (OP)

I tried doing the marvel mystery in through the spark plug hole, did not raise compression at all.