Low Compression Top Cylinder NS25C3

[NissanOutboard]

Hi Gents,

I tested the compression on my motor today trying to troubleshoot some issues.

The top cylinder is reading extremely low around 15 PSI
The bottom cylinder is reading healthy at 115 PSI

Obviously, this is a massive issue -- any ideas on how to diagnose the problem on the top cylinder..?

Thanks.

 

[racerone]

Remove cylinder head.----Lucky if it is a blown head gasket.-----Do you install a new water pump impeller about every 5 years ??

 

[NissanOutboard]

I got the engine second hand. I changed the impeller as soon as I bought it. I am going to remove the cylinder head. I noticed this shitty weld job on the block…any thoughts on to why this is here? Blown the block before? Is it even worth trying to fix this thing @racerone

 

[NissanOutboard]

Took the head off and here is what it looks like - I’m not expert here so let me know what you see..seems like a blown head gasket? What else should I do?

 

[MattFL]

It's hard to tell from the pictures, but it looks like on the top/left of the block there is a carbon stain where perhaps the head gasket was leaking.

What's odd is how clean the lower head and piston are compared to the top. That gives the impression there may have been water getting into the lower cylinder.

Edit --> Looking closer at the head picture, there is definitely a big leak. Look at the top-right of the head picture, about 2:00.

 

[racerone]

The head gasket was blown as suspected.----Get this motor running ASAP as water inside a motor is bad.----The head is warped.----They all are.----So get it flat before installing a new gasket.

 

[NissanOutboard]

Thanks for the replies guys... so a new gasket is on the way. When you say leak do you mean a compression leak from the blown gasket or do you mean there is a water leak?

How do I find where the water is getting in the motor? Should I clean the top of the pistons? How do I flatten out the head?

Thanks for your patience.

 

[racerone]

??--Leaks both ways.-----The water jacket is full of water when engine is running.----Air leaks out.-----When engine is stopped water will leak into the cylinder for a few seconds.-----Do not delay , get this engine running ASAP !

 

[NissanOutboard]

Thanks - head gasket is on the way from china should be here in a week

 

[racerone]

Put lots of oil through this motor now.----If water got on the bearings there will be damage done.

 

[NissanOutboard]

How should I put the oil through it now? Or do you mean once I put the head back on run a heavier oil ratio?

 

[racerone]

Position motor with carburetor facing up.-----Pour some oil into the carburetor throat.-----Pull motor over slowly.----Repeat.-----Now lay motor on one side then the other to distribute the oil.--Pull recoil a few times.---Motor may be harder to start once repaired and smoke a lot.-----But it will prevent internal rust damage.

 

[NissanOutboard]

Sounds good. I guess I couldn’t just spray some WD-40 in there?

 

[MattFL]

You need a lot of oil to displace any water. But before you put that back to together, you must make sure the head is flat and the top of the block is flat (and clean). If they're not flat, it's just going to blow the gasket again right away. Also I would use an OEM head gasket if it's available, not aftermarket from China.

 

[NissanOutboard]

That makes sense -- how do I go about making them flat?

 

[pvanv]

Gasket from China? I wouldn't...

 

[NissanOutboard]

I hear ya guys. I couldn’t find an OEM gasket they were all shipped from overseas…

 

[MattFL]

Measure it with a strait edge. You can DIY the head if you're careful using sand paper and something very flat, like a piece of glass or granite table, etc.. with very even pressure you can drag the head around in a figure 8 until it's flat. The block is going to be more tricky. It can be done the same way, but you need to dismantle the block to do it. The best and proper way is let a machine shop surface them, but you have to decide if the motor is worth the expense.

 

[TOHATSU GURU]

The head gasket is available from any Tohatsu or Mercury dealer anywhere in the world.