Engine compression tests....how easy is it?

[katieclooney]

So I am hunting for a used toon and my mechnaically inclined friends tell me to always do a compression test.
However, i do not know how to do one.
My boyfriend, who is a non boater, knows how to do one on a car engine.

Same process? Do I dare trust him with a boat OB?

[badmoonrising]

Same tool, same process. It's really easy to do.

[curtiscapk]

yep do the same thing, pull plugs,screw in tester, and crank it....

[katieclooney]

Does it make a difference if we test it on land without water running through it?

I am looking at a 70HP evinrude oil injected motor. Owner says its a 90-something so the year is unknown

[badmoonrising]

Remove the coil wire(s), insert tester, crank. If one cylinder is lower than others, squirt in a bit of engine oil. If it comes up = bad rings. If not = rut-roh.

No it does not have to be in the water...but DO NOT start it up not hooked up to a boot or in the water.

Not even briefly.

[pond tuuunes]

Test it wide open throttle, coil wire off. JT

[cleatus]

Test it wide open throttle, coil wire off. JT

+1 on WOT

[Lurch77]

+2 wide open throttle when testing. Not doing this will give a low reading.

[katieclooney]

Ok.......here it comes......your dumb question for the day.....

if the boat is on a trailer and no water, how do you open the throttle?

[MPosey519]

Buy some muffs, hook it up to your hose, crank it up and throw that hammer down!

Won't be under load, but can get to WOT

[cwag911]

Buy some muffs, hook it up to your hose, crank it up and throw that hammer down!

Won't be under load, but can get to WOT

He's doing a compression test, not running the engine. I believe you would have to disconnect the shift cable to be able to crank the engine with the throttle all the way open. It won't crank if it's in gear (neutral safety switch). Someone correct me if I'm wrong.

[curtiscapk]

Buy some muffs, hook it up to your hose, crank it up and throw that hammer down!

Won't be under load, but can get to WOT

He's doing a compression test, not running the engine. I believe you would have to disconnect the shift cable to be able to crank the engine with the throttle all the way open. It won't crank if it's in gear (neutral safety switch). Someone correct me if I'm wrong.

I am glad I am not the only one to think of this. I have always done it in neutral and all plug wires off....

[MPosey519]

NVM, I understand now. Like most of what I say on here, I generally have no idea what I'm talking about

[jafo9]

is WOT necessary? i've done it both ways and it doesn't seem to matter. besides, what you are really looking for is a small delta between cylinders not an exact number. i think using WOT actually got me in trouble on a 4 stroke wave runner. it had a dry sump system (not sure if that made a difference) and it seemed that i forced some gas into the oil. the rings don't leak as i've had the ski for 2 seasons and my oil level doesn't change. this may have been my imagination and the ski may have just been overfilled with oil (the airfilter was clean which makes that less likely). anyway, i think on a 4 stroke i'll just use neutral and crank. if i get a super low number across the board, then i'll consider WOT.

also, BMR, good tip about adding some oil to a low cylinder to test the rings.

another suggestion is to buy a decent gauge. you don't have to get the million dollar snapon tool, but don't buy a piece of junk from harbor freight. if you are into motorized toys, you'll be surprised how often you use it.

[curtiscapk]

When I did mine I took all the plugs out and cranked in neutral... first dry then with oil as bmr said.

viewtopic.php?f=2&t=8709&start=30

scroll to page 3.