Compression Test Technique

[tpondal]

I'm going to take a shot at dialing in my 89 Sportfire w/ 89 135 hp Merc OB for spring. I've got at least 6 months 'till it's time to launch. Seems like that should do it. First up, I need to check the compression on this motor. Do I simply disconnect the plug wires, remove one plug, attach the guage, and turn the motor over a few times? Then repeat for each cylinder. I saw in some earlier posts that 125 psi is a standard reading. Should I expect this reading on an 89 V6? Is the difference between cylinders important? Thanks in advance for your help

 

[tpondal]

I'm going to take a shot at dialing in my 89 Sportfire w/ 89 135 hp Merc OB for spring. I've got at least 6 months 'till it's time to launch. Seems like that should do it. First up, I need to check the compression on this motor. Do I simply disconnect the plug wires, remove one plug, attach the guage, and turn the motor over a few times? Then repeat for each cylinder. I saw in some earlier posts that 125 psi is a standard reading. Should I expect this reading on an 89 V6? Is the difference between cylinders important? Thanks in advance for your help

 

[175checkmate]

Also to go along with the compression test, a better way to get great information as to the health of the engine is a leakage tester. There is a gu on scream and fly that makes a great one. Also come's with instructions. Here is the link
http://forums.screamandfly.com/forums/showthread.php?s=&threadid=44030&highlight=leak+down+tester

1975 Checkmate Tri-mate 2, 2.4 200+ / Sky jacker 6" plate / 25p Laser 2 prop

 

[nw2chkmtob]

just did mine take all the plugs out and make sure the throttle is wide open. i also ran mine for a while cause some problems dont seem to appear until its hot. i`m working on a leakdown tester myself. just missed one on ebay that went for 20.00. oh well.....bill

 

[JW]

Be careful! You can harm the electronics of many motors if you crank the engine with the spark plug wires open. On mine, I had an ignition cut-out switch attached to a lanyard for safety. When I tested my compression, I would pull the lanyard, which shut off the switch so the ignition wouldn't fire the plugs. For those that do not have an ignition cut-off switch and want to take extra care, leave all of your spark plugs in each cylinder not under test screwed in a few turns but still very loose. For the cylinder under test, put the plug into the wire and ground the base of the plug against the block so that the plug can fire normally, in a safe area away from gas fumes! A third option is to buy a spark plug wire testing rig. You can hook your wires to this rig, and it will spark across this rig out of the way so that you can have all of your plugs out of the motor. Once again, with this rig, make sure there are no gas fumes.
Sure, you can just pull the plugs out and leave the wires dangling and you MAY never have a problem testing that way. I didn't dare though.......I've had to replace electronics before and it was EXPENSIVE

Checkmates owned: 1995 Pulsare 2100BR/1995 Mercury Promax 225 2.5 EFI. 1995 Persuader 183/4.3 LX V6. 1992 Pulse 171/1992 Mercury XR6 150 hp 2.5 liter.

 

[tpondal]

Thanks for the feedback! I'll take this advice and perform the test over the weekend. I'll post the results next week for some help with interpretation.