running compression test on an OB

[JerseyJim]

I bought a compression tester this weekend, and it did not come with any instructions on how to use it, and as stupid as it may seem, I've never used on before.<br /><br />I screwed it into the spark plug hole and had someone crank the starter. What is the correct reading, the first compression, or one after it has been cranked a few times? The tester is one that the guage will stay at the highest reading until you press the pressure relief valve and it zeros out again.<br /><br />Basically the first compression of the piston read 75, but i had her crank it for a second or two, and on the third or so time around it went up to 110. I did this on all the pistons, and got similar readings. Is this the correct method? Should I crank it until i get the highest reading? Or is the first pop on the piston the correct reading?<br /><br />thanks in advance...

 

[NathanY]

Re: running compression test on an OB

You need to pull it over until it peaks out. Make sure that the throttle is wide open, and both plugs are out. If possible, do it with the motor warm. That will allow everything to expand, and give you a more accurate reading, like what the compression would be if the motor was running.

 

[JerseyJim]

Re: running compression test on an OB

this is on my v6 200hp - it has 6 plugs.... should i remove all 6 plugs? i just pulled the spark plug wires on all 6 so it wouldnt run....will having the throttle wide open affect compression? it is just the carb openning at WOT?

 

[dinsdale]

Re: running compression test on an OB

Not sure about details but everything I have read says that the actual reading means less than having all cylinders having close readings.

 

[clanton]

Re: running compression test on an OB

If you have 110 on all cylinders, after the piston comes to top 3/4 times, all you need now is spark, gas, and oil. Specs for 92-99 is 110 to 135 warm block, throttle wide open, hot battery. No more then 15 lbs between cylinders.