Oxygen sensor broken?

Okay, that is good.

Remove the oil fill cap while the engine is running and see if you feel vacuum over the hole. You should feel positive pressure coming out, not a vacuum.

Roy

If you feel suction, you have a leaking intake manifold gasket. There should be no vacuum at the fill for the oil.

https://www.2carpros.com/articles/replace-intake-manifold-gasket

Roy

NOTE: On models with coded audio anti-theft systems, refer to Technician Safety Information for code procedures.

1. On models with air bag system, disarm airbag system as outlined under Technician Safety Information.
2. Disconnect battery ground cable.
3. Remove air cleaner assembly.
4. Disconnect all electrical connectors, hoses, cables, fuel lines and electrical equipment that will interfere with removal of and intake manifold.
5. Ensure following are removed:
a. Ground strap.
b. ISC valve connector.
c. Cold start injector connector.
d. Throttle position sensor connector.
e. If equipped, EGR VSV and gas temperature sensor connectors, Vacuum sensor connector.
f. Disconnect wire clamp from vacuum pipe and remove engine wire from manifold.
g. Vacuum sensor hose from gas filter.
h. Fuel return hose from air pipe.
6. Remove manifold stay, then water bypass hose from air pipe.
7. Remove intake retaining bolts, ground strap, intake manifold and gasket.
8. Reverse procedure to install. Tighten to specifications. Reset audio anti-theft system, if equipped, as outlined under Technician Safety Information. On models with air bag system, rearm airbag system as outlined under Technician Safety Information.

Not for an internal vacuum leak. If it were external, yes.

If you smoke it and you see some smoke come out of the oil fill, then yes, it is bad.

Roy

The fact that you have vacuum says that the intake manifold is leaking into the crankcase.

Roy

Okay, then you are good. Back to the catalytic converter testing.

Roy

You need a back pressure gauge to install where the O2 sensor goes before the catalytic converter.

You run the engine at 1,200 RPM's and see what the back pressure measures. Anything over 1.5 pounds is no good.

You could also disconnect the exhaust from the manifold and drive the car as well.

Roy

The next thing I would check is the timing. Check the valve timing on the belt and make sure the marks are correct.

I attached a diagram of the marks for you to check.

Roy

The reason I asked is the compression was very high. That tells me that maybe the chain is advanced from the normal setting.

Roy

If you are talking about carbon, yes it could.

There are top engine cleaners for that to remove the carbon around the intake valves.

I would try that because carbon will absorb the fuel and starve the cylinder of needed fuel.

Roy