Correct procedure for compression test

[ali'i]

I want to do a compression test on a 1981 Johnson 70 hp, (never done a compression test on anything) The Clymer manual I have says, do it with the engine warm, well that's the first problem, the engine won't start, so does it really make a difference whether its warm or cold? It also says disconnect the "power pack-to armature plate connector to disable the ignition system" Is this also necessary considering the motor doesn't start anyway? Thanks

 

[kfa4303]

Re: Correct procedure for compression test

Hi ali'i. To do a compression test, you'll of course need to thread the tester into the stock spark plug socket. You'll then need to turn the motor over some how. Ideally, the motor would still be running and could start with ease. However, if the motor won't start, you can use the electric starter to simply rotate the flywheel for a few seconds to take a reading. If your starter is also dead, then you'll have to get that fixed first, or you'll never get a decent/accurate reading on a motor that large, by turning the motor over manually. On smaller motors (<40hp) you can use the pull start mechanism the turn the motor over 5-6 times to get a reading. Here are some handy links too.

1995 Mercury 225 HP offshore outboard running and compression test - YouTube

Compression Test Mercury 15 hp Outboard - YouTube

 

[Bosunsmate]

Re: Correct procedure for compression test

Warm means you get a more accurate reading but cold will tell you if you have a significant problem.
You can just ground the sparkplugs or sparkplug leadsto the motor instead of disconnecting the powerpack.
Or if you have got a lanyard safety line just pull that.
You just dont want to have the motor turning over and no where for the spark to go back to engine ground as it can load up the ignition system and blow parts

 

[jbjennings]

Re: Correct procedure for compression test

dooesn't matter if it's warm or cold..... take all the plugs out and ground the wires on the block. Take the highest reading the gauge gets to.

 

[ali'i]

Re: Correct procedure for compression test

Forgot to mention in my original post that the motor turns over fine, (electric start), it ran fine a few weeks ago, then just sat unused, and now won't start, (but that's a whole different issue). if the plug is removed, do I need to still have it connected to the wire and ground it (them), I only have two hands, and can't hold plug grounded, while at the key turning motor over, didn't know I could cause damage by turning over the engine with plug removed. And will a cold engine check have lower or higher compression than on a warm engine check?

 

[Bosunsmate]

Re: Correct procedure for compression test

I just keep the plugs in and use a block of wood or something to keep the sparkplug up against a clean bit of metal on the block like a bolt.
Warmer will mean higher reading.
Sounds more like you have a spark issue rather than a compression one, so when you have the spark plug grounded if you can get someone to look for spark in both plugs.
You can do it at the engine if you run a little jumper wire at the solenoid, saves having two people, its what i do as i live in the country on a farm

 

[ali'i]

Re: Correct procedure for compression test

re: You just dont want to have the motor turning over and no where for the spark to go back to engine ground as it can load up the ignition system and blow parts"" that's got me worried!.. so when turning engine over, do I have to try keep all 3 plugs grounded at the same time?, If I just remove them, there will be no spark, right? or is spark not the issue?, sign me confused.

 

[Bosunsmate]

Re: Correct procedure for compression test

Yes try and keep all three grounded somewhere on the engine. If you have a safety lanyard line, man overboard line just pull that as that will disable spark.
It wont necessarily blow things if you dont do it but there is a risk of it. What happens is that the charging system keeps building up charge but if the sparks arent grounded the spark hasn o where to go so that can lead to parts getting overcharged and blowing. So you want to keep the sparkplus/or boots grounded so that the spark can get to ground.
Its not hard, just use sticky tape or piece of wood to hold the threads on the sparkplugs to a bare bit of metal on the engine somewhere

 

[ali'i]

Re: Correct procedure for compression test

Yes try and keep all three grounded somewhere on the engine. If you have a safety lanyard line, man overboard line just pull that as that will disable spark.
It wont necessarily blow things if you dont do it but there is a risk of it. What happens is that the charging system keeps building up charge but if the sparks arent grounded the spark hasn o where to go so that can lead to parts getting overcharged and blowing. So you want to keep the sparkplus/or boots grounded so that the spark can get to ground.
Its not hard, just use sticky tape or piece of wood to hold the threads on the sparkplugs to a bare bit of metal on the engine somewhere

Its funny, not one of the videos I have been viewing online has shown anything about grounding spark plugs, maybe I just viewed the wrong ones. Sounds easier to just disconnect the power pack to armature wire as manual says, that would suffice, right? or not? Now to just figure out which wire that is.

 

[Bosunsmate]

Re: Correct procedure for compression test

Yes you can do that, i just have pieces on my shelf ready to use to ground them so thats why i ground them instead.
Yes those videos need improvement. They may of pulled their safety line and not said that.
You mostly get away with it.
However i went to look at a mates mates motor the other day. A V4, it had all new electronics on it but it also had an as yet undiscovered exhaust gasket leak. Water was getting into the bottom cylinder emulisifing with fuel and thus the sparkplug couldnt fire as the charge couldnt get to ground.
It had blown the brand new coil within forty seconds, they were lucky it didnt do the powerpack too.
This build up on charge may of being what lead them to replace all parts previously

 

[kfa4303]

Re: Correct procedure for compression test

Hi ali'i. To test the compression, you don't need to have spark one way or the other. The compression is created in the cylinders by the pistons rings and head gasket. If any/all of them are leaking, you'll get low compression. In fact, it's best to remove the plugs from the leads all together so they can't accidentally spark and ignite any fuel that may be present. As far as checking the spark itself, grounding them to the block is NOT a very good method. Yes, it will work in a pinch, but it's not very accurate and often a pain in the butt to get to work. I would suggest you save yourself the trouble and get a proper spark plug tester from the auto store for about $5. There are a couple different styles. One style plugs inline to the existing leads and lights up to show you have spark, while other styles use a small thumb screw to create an adjust able gap that you can use to determine the strength of the spark, which should be able to jump a 1/4" gap with ease.

 

[ali'i]

Re: Correct procedure for compression test

Warm means you get a more accurate reading but cold will tell you if you have a significant problem.
You can just ground the sparkplugs or sparkplug leadsto the motor instead of disconnecting the powerpack.
Or if you have got a lanyard safety line just pull that.
You just dont want to have the motor turning over and no where for the spark to go back to engine ground as it can load up the ignition system and blow parts

So if I just leave the key turned off, is that good enough to avoid maybe doing damage to the electrical, and with key off, will jumping the solenoid still till turn engine over to check compression? Don't even want to look at the motor until I'm sure what to do or not to do.

 

[Bosunsmate]

Re: Correct procedure for compression test

Yep thatl be fine, good idea

 

[ali'i]

Re: Correct procedure for compression test

Yep thatl be fine, good idea

looking at the solenoid, it has 2 big bolts on opposite sides, and 2 small bolts on top beside each other, is it the 2 big ones that I connect to turn the motor over?

 

[Bosunsmate]

Re: Correct procedure for compression test

No, connect from the big positive to the little positive. Put a little switch in there if you have one, i use push button switch.
Make sure you connect to the little positive, not the little negative ( the little negative will have a wire going straight to a ground on the engine.

 

[JB]

Re: Correct procedure for compression test

http://forums.iboats.com/engine-frequently-asked-questions-faq/compression-test-jb-391323.html

 

[ali'i]

Re: Correct procedure for compression test

Bought a tester and going to do the test today, but just to update my first post, I got the motor running, turns out, if you kink the fuel line by pushing the battery up against it, the motor won't run, who'd have thought? But still worried about doing electrical damage,, so rather not try jumping solenoid, and no kill switch, and trying to keep 3 plugs grounded on a shaking motor while 8ft away turning the key doesn't sound do-able, so which is the easiest wire to unhook, read somewhere to just un-hook the coil, would this do the trick? if not, which wire, in layman terms.? Thanks

 

[Bosunsmate]

Re: Correct procedure for compression test

i find the plugs normally slip onto a headbolt pretty well.
i wouldnt unplug a coil

 

[ali'i]

Re: Correct procedure for compression test

My concern, as I am doing this by myself, is making sure all 3 plugs stay grounded while cranking the motor from 8ft away, what if 1,2 or all 3 become ungrounded as I am turning the key.? Most of the engine is painted, and don't I also have to keep them separated to avoid spark jump, (as I read somewhere)..

 

[Bosunsmate]

Re: Correct procedure for compression test

yep separate them.
Dont stress too much if they come off, You a only doing a compression test so you a only turning the engine over a few times.
Its when a sparkplug goes dead and the engines running for a while that it is the riskiest