Compression Testing

[Mare]

I want to do a compression test on our M7's before the season kicks off. Been told a few different ways of doing this. Would like to know the RIGHT way of doing this test. Step by step. I've been told, to do test on warm engine, been told the opposite and test on cold. Someone said to have both plugs out as well.. so I am wondering as to the correct way. Any help is appreciated. Thanks.

Sleds are 05 and 06 M7's. And what are the numbers I should have if compression is good.

 

[Bnorth]

Both plugs out, kill switch off and hold the throttle wide open. Do it cold and then warm to compare the results. Your warm results should be better as everything has expanded but cold can show if some wear is starting. Important to have the switch off so the fuel pump doesn't run and the injectors don't open. Also run the sled before the cold test as those cylinders will either be dry or covered in oil depending on if you fogged the motor or not, either will skew the results.

 

[Mare]

Thank You. Appreciate it.

 

[marcel.]

Yes for the most accurate psi reading do the test whith a warm engine and kill switch off as I've made this mistake... Oops lol but yeah cat runs a lower compression engine hence why you don't need to burn premium grade fuel. It should test at least 120 psi I believe or roughly around that area.

 

[Bogger]

I'm told anything over 115 is good, more importantly both sides should be within 10%.

When I did mine I removed one plug from each cylinder after it had run but not totally warmed up. I would think if you removed both plugs you would not get a reading as it will never build compression because it escapes from the other hole.... but what do I know....

What impact does holding the throttle open have? I didn't do that, how would that impact my results....better re-check

 

[marcel.]

Yeah I forgot to mention that having both sides with same pressure is very inportant. But yeah I've never heard of holding the throttle down I'm not sure what it would do?

 

[cdnredneck_t3]

I'm told anything over 115 is good, more importantly both sides should be within 10%.

When I did mine I removed one plug from each cylinder after it had run but not totally warmed up. I would think if you removed both plugs you would not get a reading as it will never build compression because it escapes from the other hole.... but what do I know....

What impact does holding the throttle open have? I didn't do that, how would that impact my results....better re-check

It opens up the Throttle Bodies allowing air a less restricted flow path through the engine. And don't forget to give it a few good pulls. The pressure tester increases the cylinder volume so it needs to fill before you get a true reading. A one handed sissy pull will only give you 80 PSI.

 

[Bogger]

I pull till the pressure quits increasing... think it was around 8 pulls and she topped out at 120 dead even both sides...but I didn't open the throttle.

It opens up the Throttle Bodies allowing air a less restricted flow path through the engine. And don't forget to give it a few good pulls. The pressure tester increases the cylinder volume so it needs to fill before you get a true reading. A one handed sissy pull will only give you 80 PSI.

 

[cdnredneck_t3]

Ya. You are fine, just needed to pull a few more times with the throttle bodies restricting flow.