4.6L Mercury

I don't know the number. I do know that all cylinders should be within 10%, or so of each other.

Google, "compression check".

Thanks for that. I did google. compression ratios come up all the time. I guess I'll go find a rebuild manual.

If the engine is healthy it should have a minimum of 125 psi in each cylinder when conducting a dry, cold engine compression test with the throttle plate wide open. Then the highest and lowest pressures should be no more than 10% or 12 psi difference between them. Then if you have any doubt run a wet test and see how much the pressures change. If the change is massive I would certainly pass on that engine as it needs work big time!

Thanks for that detailed information.

After this test, If positive, I'm going to run her till she finally croaks. Hopefully another 250K miles. Love this car.

If this cylinder is really bad, you might get a good feel without any equipment. Do as bwrench suggests, and disconnect the fuel pump relay. Then crank the engine with all plugs in place. You will be getting a , " rr-rr-rr-rr-rr" sound. If you have a truly bad cylinder, one or those, "rr s will be quicker beat with an increased pitch, as the started get a little less load on that bad cylinder.

Have used if over the years with some success as an early screening device. Was really obvious on air cooled VW engines. (Just passed 200,000 miles on my Ford Explorer)

steve (Not too smart)

Ballpark number is compression ratio times atmospheric pressure at your location (or just use 14.7). A better check on cylinder health is a blow down cylinder test. You can get a tester from Harbor Freight for about $40 US. You will also need a source of compressed air at 100 psi. Bring each cylinder to TDC and rig a way to stop it from rotating. Hook up the tester and put pressurized air into the cylinder being tested. If the gauge shows a significant drop in cylinder pressure listen for air escaping through the carbuerator (leaking intake valve), the oil filler port (leaking rings), or the exhaust pipe (leaking exhaust valve).

Thank you.