Correct procedure to check compression

[mphsmaestro]

Am familiar of how to check auto engine compression but don't know the steps for outboards. Have '87 Mariner 150 with 2002 powerhead and want to check compression. Seller showed receipt where he had mechanic pull head to unstick rings on #2. Thought that rather strange for a 2 yr. block.<br />Any help would be appreciated.

 

[JB]

Re: Correct procedure to check compression

Ahoy maestro.<br /><br />Welcome to iboats. <br /><br />Different wrenches will offer variations on the following, but the important thing is that all cylinders are tested exactly the same way and under exactly the same conditions.<br /><br />1. Warm the engine up.<br />2. Remove all spark plugs.<br />3. Open throttle completely<br />4. Pull safety lanyard to disable ignition.<br />5. With a fully charged battery, turn the engine over at least 5 times to test compression on each cylinder. Use the same number of strokes on each cylinder.<br /><br />All cylinders should compress 100psi or more, and the spread must be less than 15% of the average.<br /><br />Good luck.

 

[KCLOST]

Re: Correct procedure to check compression

Excellent procedure, But don't be afraid to crank up to 10 revolutions before you are sure you have a good seal with the compression guage you are using...

 

[rodbolt]

Re: Correct procedure to check compression

one of the big things I also look for is that the needle on the gauge swings up about the same amount per stroke as all the rest. if I have 5 cyl that will hit 130 psi on the second stroke and the 6th takes 8 strokes to hit 130 I will leakdown test the 6th. what your looking for is a difference in any cylinder. all should be about the same for any test. cold,hot, throttles open or closed, do all the cyl with the same procedure.

 

[funpilot]

Re: Correct procedure to check compression

OK (embarrassed look down at his shoes...)<br /><br />JB, would this work with a Mercury SportJet 120? I'm aphrensive about frying the electronics cranking the engine with the sparkplugs out and not grounded. I mean, if simply removing the lanyard will disable the ignition properly, I'll certainly keep that in mind the next time I want to check compression!<br /><br />You learn so much here browsing...<br /><br />funpilot

 

[JB]

Re: Correct procedure to check compression

Howdy Funpilot.<br /><br />This really deserves its own topic.<br /><br />If the safety "kill" switch is correctly wired the ignition is disabled and there is no risk in turning the engine over with the plugs out.

 

[dajohnson53]

Re: Correct procedure to check compression

Originally posted by JB:<br /> <br />3. Open throttle completely<br />

Really stupid question I know, but here goes:<br /><br />What exactly does "completely open throttle" mean? My outboard, like every one, cannot start or turn over unless throttle lever is in neutral/idle position.<br /><br />Therefore, when I do the compression test, I leave the trottle lever in neutral, but advance the "cold start" lever as far as possible. <br /><br />Does this completely open the throttle, in other words, is is proper technique for testing compression?<br /><br />Thanks.

 

[Docdoc]

Re: Correct procedure to check compression

D.<br />The procedure described as WOT (wide open throttle) allows the engine to pull in the maximum amount of fuel/air mixture on each stroke. Warm up position usually opens up the throttle about 1/3 to 1/2. If your engine has a neutral start switch (and almost all do). Rig up a jumper wire on it to allow starting in gear. That will give you much more accurate compression reading. Also, either take the prop off, or stand clear of it when doing the compression test. You can get a nasty souvenir if you don't.

 

[JB]

Re: Correct procedure to check compression

Leave it in Neutral and use a jumper to the starter solenoid to operate the starter.

 

[bigbrownbuku]

Re: Correct procedure to check compression

usually i will manually open the throttle plates, prevents nasty propellor souvenirs.