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How to compression check with engine out

 
Old 12-18-2018, 07:31 PM
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backitoff
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Default How to compression check with engine out

Just purchased an 60,000mile S4 motor that has been stored for 4 years. I have been assured it’s in good health but want to check it out. Didn’t really want to take heads off, but do intend to remove sump to check bottom end. Would first like to check compression or leak down but have only ever done this on other cars with engine in situ using starter motor. Is this possible with engine out? Sorry in advance if it is a dumb question.
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Old 12-18-2018, 07:33 PM
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Originally Posted by backitoff View Post
Just purchased an 60,000mile S4 motor that has been stored for 4 years. I have been assured itĺs in good health but want to check it out. Didnĺt really want to take heads off, but do intend to remove sump to check bottom end. Would first like to check compression or leak down but have only ever done this on other cars with engine in situ using starter motor. Is this possible with engine out?
We do it all the time. The S4 has the starter bracket attached to the block which makes it easy. Don;t expect great numbers seeing its been a long time since it has run but at least you can verify it has compression in all cylinders
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Old 12-18-2018, 10:51 PM
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smart thing to do first is,
remove the plugs and put a bore scope into each cylinder,
verify no debris are in the cylinder, and the cylinder walls are not scored.
add half an oz of ATF to each cylinder and turn it over slowly this will oil the rings.
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Old 12-19-2018, 01:43 AM
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Leakdown test is what you can do here.


xx/100 is the end value, plus you can hear where the leaking is from..and decide what you wanna do about it.

(There will always be leaking, 65/100 and up is healthyish. High 70s is pretty good.)
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Old 12-19-2018, 05:18 AM
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When in college bought a Audi motor from a junk yard to put in a car. They had it laying on the ground with the water hoses run to a bucket and the fuel line going to a jug of gas to show me ir ran before I bought it.
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Old 12-19-2018, 12:05 PM
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Originally Posted by Speedtoys View Post
Leakdown test is what you can do here.

xx/100 is the end value, plus you can hear where the leaking is from..and decide what you wanna do about it.

(There will always be leaking, 65/100 and up is healthyish. High 70s is pretty good.)
A healthy, cold engine would record in the neighborhood of 98/100.

The numbers you quote indicates the engine would need a rebuild.
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Old 12-19-2018, 01:19 PM
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Huh, not in the airplane world. I dont recall what values I had, but my compression values were 210-215.

I dont think even a new engine will give you a 98.

8-15% is a fantastic engine value, down to 80 is where the nice examples of our 30yr old engines are, even as lot as 75/100 most of us likely lean towards.

And..a 65/100 may only be 5-8Hp short of "fresh" and seem just fine...but Id wonder if a LOT of the 928 population is closer to here, than 75.

Last edited by Speedtoys; 12-19-2018 at 07:51 PM.
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Old 12-19-2018, 10:09 PM
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If all you have is a compression gauge, I would try pulling the plugs and only put the compression gauge in. You should be able to spin it by hand well enough to get some kind of measure. Not totally sure what the numbers mean with the motor cold and spun slowly, but still useful and a simple solution. If you don't even have a compression gauge you can still get some kind of feel for it just by putting one plug in at a time and spinning by hand.
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Old 12-19-2018, 10:14 PM
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Too slow by hand, you'll leak down too much before you get to TDC.

A ring gap is pretty large at 30 RPM.
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Old 12-19-2018, 10:47 PM
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I agree the numbers will be off, because of leak and temp. But if your cyl is no compression or even substantially low you will feel it even without the gauge. Recently needed a truck and picked up one with a bent valves for a song. Spun the motor by hand and could easily tell what was going on just by feel.
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Old 12-19-2018, 10:54 PM
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Ya...kinda wandered off the "is it broke" part.

Could do that.
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Old 12-20-2018, 08:59 AM
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Originally Posted by Speedtoys View Post
Huh, not in the airplane world. I dont recall what values I had, but my compression values were 210-215.
We are talking leakdown numbers, not compression results.

I dont think even a new engine will give you a 98.
I've seen plenty of older engines record 100/100.

​​​​​​
8-15% is a fantastic engine value, down to 80 is where the nice examples of our 30yr old engines are, even as lot as 75/100 most of us likely lean towards.

And..a 65/100 may only be 5-8Hp short of "fresh" and seem just fine...but Id wonder if a LOT of the 928 population is closer to here, than 75.
You really have no idea of what you're talking about, and yet, you keep posting like you do.

Why don't you do some research on automotive leakdown numbers and only then come back and report your findings.
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Old 12-20-2018, 09:20 AM
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Great advice, thanks. Would this still be possible with the cam belt removed?
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Old 12-20-2018, 10:21 AM
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Originally Posted by backitoff View Post
Great advice, thanks. Would this still be possible with the cam belt removed?
Simple answer- No!
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Old 12-20-2018, 10:48 AM
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Originally Posted by backitoff View Post
Great advice, thanks. Would this still be possible with the cam belt removed?
Slightly longer answer:

No. Hell no. Abso-freaking-lutely no.

Do NOT, under any circumstances, turn the crank with the belt off.

Period.

Unless, that is, you want to replace some valves.

There is no position that the cams can be in where there are no valves open.
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