How do you do a leak-down test?

pastorbud

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I’m looking for a link or a sticky on how to do a leak-down test on a late 1980s Force 125, and how to use that info to diagnose a compression problem .
 

tpenfield

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You can google that. not sure if there any stickies.

You will need:

Leak Test gauge
Compressor good to 80-100 psi.

2 cycles are easy, since every revolution is TDC on compression stroke.

The noise of the air leak will tell you if it is rings/cylinder or head gasket.
 

Zink357

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There is a sticky at the top of this forum. It is the FAQ one. Not sure what leakdown you're talking about though for compression test or if it is even in that sticky. Good luck.
 

flyingscott

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What is your compression? A leak down test is only done after every other test has been exhausted.
 

Jiggz

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What exactly are you looking and what is the problem? A leak down test of what? Compression problem on what?
 

pastorbud

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Late 1980s Force 125. It has been a while since I did the compression test, so I don’t remember the numbers, but it had low compression on one cylinder. I was hoping to find out if the problem is with the head gasket or the rings. Head gasket replacement I could take a stab at. Rings or another matter!
 

flyingscott

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If you have low compression save yourself the hassle and pull the head. No need to do a leak down test because if it is the gasket, you will need to replace it. And if it is scored well you can decide from there.
 

Jiggz

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There you go. As Flyingscott said open and inspect and you just have to take it from there. Take pics of the cylinder and piston on the low compression cylinder on your next post.
 

jerryjerry05

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When I was a newbie, my boss who knew EVERYTHING about outboards said
"don't waste time doing a leakdown test ,a compression test tells you all you need to know.
If the comp is low?? remove the head.
That ok? remove the port cover and inspect the rings.
 

pastorbud

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I got all 18 bolts out of the head. Only broke two! The head still won’t come off. I’ve smacked it a little with a dead blow hammer and a piece of wood. What’s the safe way to break it loose? Can I pry anywhere?

BTW, before I took out the bolts, I warmed it up and ran compression test. Cylinders 1-3 are just over 120. #4 is at 30 psi! And I had a little water come out cylinders three and four, And the number four Spark Plug was a little wet with some rust, so… Yeah there’s some gasket problems.
 

pastorbud

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Here’s a hillbilly question. Suppose I put the bolts back in finger tight, and the spark plugs back in, and try turning the engine over to let the pressure pop the head off. Too risky?
 

Bluestream

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I got all 18 bolts out of the head. Only broke two! The head still won’t come off. I’ve smacked it a little with a dead blow hammer and a piece of wood. What’s the safe way to break it loose? Can I pry anywhere?

BTW, before I took out the bolts, I warmed it up and ran compression test. Cylinders 1-3 are just over 120. #4 is at 30 psi! And I had a little water come out cylinders three and four, And the number four Spark Plug was a little wet with some rust, so… Yeah there’s some gasket problems.

Too bad you took the head off; should have left it alone. A leak down test is easy to do, and offers more info that a simple compression test
 

pastorbud

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Well, I’m getting mixed signals on that. But if the compression is low, it’s either the head gasket, the rings, or the reed valves, I should think. And in any of those three cases the head still has to come off to do the repair.

Anybody got any ideas on how to break it loose without warping or cracking something?
 

pastorbud

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Okay, two things, rings or head gasket, but either way the head has to come off to make the repair, right?
 

flyingscott

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Too bad you took the head off; should have left it alone. A leak down test is easy to do, and offers more info that a simple compression test

When you have a cylinder at 30 psi. What will a leak down test tell you that the compression test didn't. And compression tests are FAR EASIER than a leakdown test.
 
Last edited:

jerryjerry05

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The head needs to come off for a good visual.

Bluestream. Maybe elaborate on your statement and tell the "benefits" of the leakdown.
If the motor had valves? I'd agree with you.

Did the broken bolts come out or they still in?
Removing the bolts.
PB Blaster, Krol, or any penetrating liquid a torch and a couple pry bars.
Spray and heat the studs a bunch
Then try to insert the prybars and work the head off the studs. Try to do as little damage as possible to the
mating surfaces. Damage to the block can be hard fixing. The head can be welded fairly easy.

The reason they broke is the corrosion from dissimilar metals close or touching each other.
Every time the motor gets .hot the corrosion gets a tiny bit worse.
When you reinstall, marine grease.

The trying to blow the head off probably won't work.
 

pastorbud

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Got two broken bolts still in head. They broke off level with the top of the head. After I get the head off, there should be a short stub sticking up. I’ve never tried a welding a nut on there, because I don’t know how to weld. But I have a friend who does. I may call him. I’m not looking forward to trying to drill those grade 8 bolts. Maybe the heat from the welding will make the stub more likely to come out, and I won’t have to drill and a tap.
 
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