How to Perform Compression Test???

[peterb]

I have a 1997 175 Johnson Ocean Runner. I bought a compression gauge and would like to check the compression on each of the six cylinders. I understand that I want the results to be within 15% of one another.<br /><br />I have read that I should do the following:<br />1) Warm up the engine. <br />2) Turn it off.<br />3) Remove either the clip from the safety switch<br /> or the wires from the spark plug heads (so<br /> that the engine does not actually start)<br />4) Perform the compression test, one cylinder at<br /> a time. The reading should be taken after 3<br /> or 4 healthy cranks.<br /><br />Here are my questions:<br />A. Are any of the above noted instructions wrong?<br />B. Should I remove all six spark plugs at the<br /> same time or only one at a time (from the<br /> 1 cylinder that I want to test)?<br />C. I have read that I should have my throttle at<br /> WOT while eprfroming the test. Does this<br /> mean that I leave the trollte in netutal, but<br /> then pull it out and put it in the WOT<br /> postion (so that it is in both the WOT and<br /> neutral positions?)<br />D. Am i forgetting anything else important?<br /><br />Thanks

 

[chicken louie]

Re: How to Perform Compression Test???

i think you should remove all plug wires so it will not start

 

[JB]

Re: How to Perform Compression Test???

Remove all spark plugs, as well as the safety kill switch. Better yet, leave the ignition switch off and turn the engine by jumping the battery directly to the starter hot lug.<br /><br />Turn her over at least 5 times, and the same number for each cylinder.<br /><br />Opening the throttle plates may make a small difference, but not important. Just do it the same for all cylinders.<br /><br />Good luck.

 

[BoatBuoy]

Re: How to Perform Compression Test???

Make sure your battery is well charged. Weak battery will result in overall lower compression readings.

 

[peterb]

Re: How to Perform Compression Test???

I do not know how to start the engine by jumping the battery directly to the starter hotlug.<br /><br />Also, my kill switch does not seem to be working (I remove it while the engine is running and it still goes).<br /><br />Therefore, unless I could cause some trouble, I would prefer to turn her over the conventional way. My one concern with this is that I have heard that pulling all of the plugs and disconnecting the spark plug wires from the plugs may not be enough & that I should also somehow prevent the engine from starting by unplugging the gang plug that goes into the ignition module.<br /><br />Does anyone have any idea what this means?<br /><br />Please advise<br /><br />Thanks

 

[rodbolt]

Re: How to Perform Compression Test???

hello<br /> the ign problem I worry about is as the engine is cranking over you will be blowing a chemically correct air/fuel mixture out of the plug holes. therefore you must ground the ign or otherwies disable it. leaving the plug wires dangling with a non disabled ign is asking for a problem. if you unplug the stator charge coil wiresbrwon and brown yellow it effectively disables the ign.<br /> I use an s48 spark tester so I can control and observe the spark at the same time as I test compression.<br /> good luck and keep posting

 

[clough]

Re: How to Perform Compression Test???

Just a thought, but you also could unplug the gas line and run the gas out of your engine (if its an outboard) while you warm it up. That way, it will spin all day without starting - Hey, kind of like my boat! I am getting ready to post some questions on it (my motor) pretty soon.

 

[peterb]

Re: How to Perform Compression Test???

If I go the tenjed4 route, do I still have to ground the ignition?<br /><br />Rodbolt mentions unplugging the stator coil wires? Does anyone know where these wires are located? Are they under the flywheel?

 

[peterb]

Re: How to Perform Compression Test???

Any/all additional input would be greatly appreciated

 

[MarkHPCA]

Re: How to Perform Compression Test???

Disonnecting the input plug to the ignition would be OK. I would also worry about why the kill switch isn't working, maybe just a bad contact?

 

[marinemech]

Re: How to Perform Compression Test???

rempve cover on top of moter at the back above cover which says OIS ignition there is a plug that comes out from flywheel area (large round cover) disconnect that plug from power pack ignition is now dead or remove all plugs and hook up a spark tester to all plug wires to ground high voltage side of pack<br />that moter has what the call staggard compression all clyinders will probably not be equal<br />why are you checking comp??? is moter down in power

 

[Chesapeake Explorer]

Re: How to Perform Compression Test???

You have got it right.although the 15% I think should be 15 pounds of each other on the compression. I run my engine get it warm pull all plugs and insert the compression gauge and run the motor about 5 cranks on each cyl. 15 % of 120 would be 18 lbs or 102 ..thats a wee bit low 15 lbs would be 105 of 120 for example but thats also pulling hairs Under 15 lbs betweeen cyls keep a eye on things thats all.Try usining some engine tuner to decarbonize.Now if you way off between cyls that can put excess pesssure ob the crank say off by 25 lbs on a cyl. Also a engine thats too far off will not run smooth to idel.

 

[peterb]

Re: How to Perform Compression Test???

Thanks for all of the great feedback.<br /><br />I performed the compression test. I warmed up the engine in my drive way until it idled at about 600 RPM's.<br /><br />I then removed all of the spark plugs. I performed the test with the engine in neutral (and did not have it at WOT).<br /><br />Here are my readings<br />Left side - 115, 110, 100<br />Right side 115, 115, 100<br /><br />All of the plugs were wet except for the top right plug. I will perform a spark test on all six cylinders tomorrow.<br /><br />Someone told me that my engine has staggered compression.<br /><br />I did the compression test because it will very soon be coming off of warrantee (it is a rebuild, and I can only get it up to about 4800 RPM's (about 43 MPH). between the boat (a 20 foot center console), engine and gas, it is pushing about 3,000 pounds.<br /><br />I am happy with the engine in that it always starts for me. It has been idleing a little rough and that is why I will be checking the spark.<br /><br />Any feedback would be appreciated.

 

[ledgefinder]

Re: How to Perform Compression Test???

I absolutely would worry about that large a difference in compression (115 versus 100). 10 pounds difference is plenty to allow. <br /><br />You might have someone more experienced do the check as well. If the 15 psi difference hold sup, I'd take it back to the rebuilder. If they won't take care of it, I would have someone pull the heads & see what the cylinder walls look like. If the walls & rings are OK, then run it.<br /><br />I haven't seen any that much different that didn't have scored walls, or broken rings, etc.. <br /><br />Your engine should not have 'staggered compression'.