My ranger 2.3 l has about 240000 miles on it so I decided to to a compression test. I got what i think are pretty good numbers: 170, 175, 170. Number one cylinder reading was a little low 155. So I went ahead and pour 2/3 spoons of oil on in it When I started cranking the truck the needle on the compression gauge will jump up and right away will go to zero. Has anybody experienced this before? The compression gauge is brand new.

 

check valve in compression gauge is stuck open.

Compression gauge has a check valve that allows pressure in but not out.

Because engine compression is temporary, you would need to watch the gauge while cranking and couldn't get an accurate reading, so these gauges have a check valve, well should have and need to have a check valve.
Usually there is a button you press to release pressure before testing the next cylinder, thats the check valve.

Yes, 170's would be expected from 2.3l

 

Thanks again Ron. It was a defective gauge. I ended taking back to the store and exchanging it for a new one.