[TestDrive]

Welcome to ecomodder, Mogal.

With so many questions about anything but Catalytic converters and/or 02 sensors - you should have started a new thread. Maybe some kind moderator will do it for us?

Closed loop refers to times when the ECU readings from the 02 sensor(s) into account in order to maintain a stoichiometric air/fuel ratio. Open loop refers to times when the ECU ignores O2 sensor readings and provides a richer fuel mixture eg. cold engine, wide open throttle.

As to how to drive by vacuum gage - if you're trying to maintain a rate constant rate of cruise, you want the highest vacuum reading you can obtain and still maintain the cruise rate. Arguments begin when you start talking about best reading while accelerating. If you've got a scan gage or other gage that will show when the ECU goes into open loop, the lowest vacuum reading where the ECU doesn't go into open loop is probably close. If you don't have that instrumentation - 5-7hg is probably a reasonable guess.

With the engine warm, you want the highest possible hg at the lowest, smooth idle.

I doubt changing the O2 sensor will make a difference as there was no code. Keep the old one as a spare.

120-125 psi is low. Not sure of the specifications for a metro of unspecified engine size, but probably ~180-200 psi. Was the correct test procedure used to get that reading? Was the engine fully warmed up? Did you do both a wet and a dry tests. Were all plugs removed? Battery fully charged? Starter in good condition (not cranking slow)? Throttle wide open?

The water trick won't cure the low compression (assuming you've got low compression). I'm not a fan of the water trick in any case.

As a general rule, you should shift into neutral. If you just push the clutch in you risk premature failure of the clutch throw-out bearing (may have a different name on metro).