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Leak Down Numbers = Engine Refresh?

- - - - - Leak Down engine rebuilt

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#1
CARLOS TEIXEIRA

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So I did a Leak Down test on my 2000 and got the following numbers:
1. 5%
2. 18% (leaking air from throttle body and maybe a little from oil cap and exhaust/bad valve seals and rings?)
3. 20% (leaking air from oil cap/bad valve seals?)
4. 6%

I am considering refreshing/rebuilding engine. Just starting to research and would love any advice. At a bare minimum I am planning on replacing all gaskets, seals, valve seals, and rings using Mazda OEM parts. Planning on replacing oil and water pump, and timing belt too. And probably bearings. Anything else I should definitely check/replace while in there? I have never messed with an engine before but want to learn. Also trying to keep this budget friendly and as simple as possible. Goal is to get 2 and 3 with a leak down of 8% or less.

Thanks for any feedback and Happy Holidays!

Carlos Teixeira


#2
Steve Scheifler

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The numbers from 2 & 3 are more likely rings than valve seals, but they may just be a bit sticky rather than worn. Are you testing at 100psi? With full (or even a little extra) pressure on it try turning the crank back & forth just about 8 degrees each direction. Not enough for the air pressure to spin the engine, just enough to rock the rod slightly over center so it puts a little thrust on the rings against the cylinder, watching your leak side number. Having plugs in the other holes can help hold it a little (and helps when identifying where the leak is going anyway).

If you tear it down still replace the valve seals if they are old/original. If this head is a previously rebuilt one with only race miles on it then there shouldn’t be a need. When you buy seals don’t feel you need to get them from Mazda, others are allowed, are perfectly good and cost a lot less.
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#3
Dave D.

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Did you test cold or hot?? Are you sure you got to TDC on each cyl ? Valve stem seals have nothing to do with leak down numbers, valve SEATS do.


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#4
CARLOS TEIXEIRA

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The numbers from 2 & 3 are more likely rings than valve seals, but they may just be a bit sticky rather than worn. Are you testing at 100psi? With full (or even a little extra) pressure on it try turning the crank back & forth just about 8 degrees each direction. Not enough for the air pressure to spin the engine, just enough to rock the rod slightly over center so it puts a little thrust on the rings against the cylinder, watching your leak side number. Having plugs in the other holes can help hold it a little (and helps when identifying where the leak is going anyway).

If you tear it down still replace the valve seals if they are old/original. If this head is a previously rebuilt one with only race miles on it then there shouldn’t be a need. When you buy seals don’t feel you need to get them from Mazda, others are allowed, are perfectly good and cost a lot less.

Yes, I tested at 100 psi.  This is original engine with 90,000 on it.  Never been rebuilt.  I’ll mess around with 2 and 3 again and see what I get.  Thanks for input.  


Carlos Teixeira


#5
CARLOS TEIXEIRA

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Did you test cold or hot?? Are you sure you got to TDC on each cyl ? Valve stem seals have nothing to do with leak down numbers, valve SEATS do.

Dave I tested hot at TDC for each cylinder.   I don’t know enough about seals or seats just that if numbers are right I wanna fix it. Thanks.  


Carlos Teixeira


#6
Jim Drago

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So I did a Leak Down test on my 2000 and got the following numbers:
1. 5%
2. 18% (leaking air from throttle body and maybe a little from oil cap and exhaust/bad valve seals and rings?)
3. 20% (leaking air from oil cap/bad valve seals?)
4. 6%

I am considering refreshing/rebuilding engine. Just starting to research and would love any advice. At a bare minimum I am planning on replacing all gaskets, seals, valve seals, and rings using Mazda OEM parts. Planning on replacing oil and water pump, and timing belt too. And probably bearings. Anything else I should definitely check/replace while in there? I have never messed with an engine before but want to learn. Also trying to keep this budget friendly and as simple as possible. Goal is to get 2 and 3 with a leak down of 8% or less.

Thanks for any feedback and Happy Holidays!

 

I would test those two cylinders again.. most of the time i find stock engines with mostly comparable leakdown on cyls.. Also try each 1-2 times. If still high, drop a few teaspoons of oil in the plug hole, if it is rings it will seal up.  Then you will know

 

Jim


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#7
CARLOS TEIXEIRA

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I would test those two cylinders again.. most of the time i find stock engines with mostly comparable leakdown on cyls.. Also try each 1-2 times. If still high, drop a few teaspoons of oil in the plug hole, if it is rings it will seal up.  Then you will know

 

Jim

I recently retested and got similar numbers.  Ive been following the planet miata procedure/video for leak down and double checking for TDC. I’ll try the oil trick!  Thanks Jim


Carlos Teixeira


#8
CARLOS TEIXEIRA

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So i retested and got similar numbers.  Put a little oil in 2 and 3, and both leak numbers improved a little.  So im guessing worn rings are at least part of the problem.  With a little oil, 2 improved to 10% leak down and 3 improved to 15%.  


Carlos Teixeira


#9
Steve Scheifler

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Rings always leak a little, the other two would probably be near zero with the same oil test. It’s a little unusual for only some cylinders to have badly worn rings unless due to something like a leaky injector washing them down, or are cracked/broken from something like detonation. Even overheating tends to do all not just a couple.

By pulling the manifolds it is vey simple to determine exactly which if any valves are also contributing. Listen at TB and tailpipe first and start with the loudest.
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