No. Rings get weak due to wear and heat. If the cylinder is out of round enough to kill the compression you'll likely have a knock. Of course if putting rings in it, cylinder health needs to be checked. Did you ever overheat it? That's a ring killer. The post on soaking overnight isn't a bad idea. A stuck ring will cause your problem and being on one cylinder would be high on my list of suspects. The chance of unsticking a ring by soaking is slim, but it beats pulling the piston to find it pops right out with a tap.

When the head was pulled, how were the valve springs? A broken spring will do this, but isn't consistent. shoot a shotglass amount of oil in it and recheck compression. If it comes up a noticeable amount (15-20 psi or more), you're done. The rings are stuck, worn out or broken.

 

I've measured engines with as low as 40 psi. That one is still running with one dead cylinder - it even made a 500 mile drive after we changed out the heater core.

It's definitely not ideal, but the car will be driveable and probably won't do any damage with one cylinder reading 70psi as long as it's not a valve.

 

I drove 500 miles like that without any problems. It didnt really have an overheating problem but it was losing A LOT of water. The temperature never went past 1/2 though only when it was turned off after a few hundred miles it touched 3/4. I believe water problem was due to either a bad head or intake manifold gasket or a faulty radiator cap. I changed all three so hopefully its good now (I posted another thread on this). Anyways, Im bringing the car to the paint shop right now so I will do the oil check when I get it back and go from there.

 

Is it still losing any coolant? If so, it's likely a cracked head. If not, it may have been leaking into the weak cylinder and washed the rings down - meaning, the coolant washed the oil away and allowed the rings to make contact with the cylinder completely wearing the rings and or piston and or cylinder out. If so, it's game over. Time for a rebuild or replacement. Hopefully you or whoever checked the oil for coolant and changed it if needed. This is another case of how a problem never gets cheaper by putting it off. Hopefully I'm wrong, but I think you got bit... The compression test with oil will tell the real story. If that improves compression, I'd skip the overnight soak attempting to loosen the rings as coolant will have loosened any and all carbon already - along with causing other issues.

 

There was no coolant in the oil. Once cylinder (#4) not the one with 70psi was extremely clean. I posted about this awhile ago and heard that could be where the water was getting in and causing the water to constantly be cleaning the cylinder and piston. I changed head gaskets and intake manifold gaskets so Im hoping that solved it. I only fired up the car and took it to the paint shop so I will work on it some more when it gets back. I havent driven the car that much to see if its still losing coolant.

Question about the oil test. Wouldnt putting a shot glass of oil into the cylinder screw up either my spark plug or compression tester?

 

I wouldn't use a shotglass full. Maybe 1/4 shotglass, just enough to get the rings wet.

 

In answer to you and the OP, with 70 PSI, there isn't a lot to hurt the gauge or anything else. Sure, you can start with 1/4, but if the rings are as bad as I think, there won't be much difference, hence, as a last resort, use the shot glass amount as a general guideline. Especially before blaming something else when less oil makes no difference. It's not that much unless compression is 20:1. Normal 8.5 or 9:1 with cooked rings starting at a measly 70 psi will be fine. The middle of last week I had a pickup in my shop that wouldn't even register a change on anything less oilwise. It started at 60, a squirt made no difference, and 2 shot glasses brought it to around 100 as I knew it was cooked going into it as I had a leakdown tester.