Compression test

[itstim]

So what is the correct way to do a compression test? Take all the plugs out or just one at a time? And they should all be within what lb/% of each other? What is a max/min lb for a GM 165?

 

[Scaaty]

Re: Compression test

Pull them all, stick a screwdriver in the throttle plate so its open, and look for no more than 10/15% variation...not sure on the min/max, but it doesn't matter if they are all close. One or two off, you will know you have a problem.

 

[Dunaruna]

Re: Compression test

and disconnect the pos wire from the coil so that you don't get 4th of July fireworks.

 

[steve n carol]

Re: Compression test

heres one from the archives;<br />I had one of those engines...135 psi comes to mind. I think it was posted on a red or orange sticker on the rocker cover along with the max RPM. check your book...sl <br /><br /><br />"rickdb1boat <br />Admiral <br />Member # 9947 <br /><br /> posted September 13, 2002 07:15 PM <br />--------------------------------------------------------------------------------<br />ARIZONABASSMAN,<br /><br />Warm up engine,<br />remove all plugs,<br />Ground all plug wires,<br />connect compression Gauge'<br />Hold throttle wide open,<br />Crank at least 4 times for proper reading!<br />Try to crank each Cylinder the same amount of rev's!<br /><br />--------------------<br />Rick"

 

[Dunaruna]

Re: Compression test

oh and start off with a good battery, you don't want it cranking slowly as you get to number six.

 

[Scaaty]

Re: Compression test

My bad on not saying to ground the coil! <br />20 lashes!!!

 

[waterone1@aol.com]

Re: Compression test

All of the above sounds great, I would just add one more piece of advice. Take your time going from cylinder to cylinder, even with all of the plugs removed, the starter will get a little hot if you proceed to quickly.

 

[Don S]

Re: Compression test

Here is a Mercruiser Service bulletin with GM's recommendation on how to do the compression test.<br /><br /> Compression Test