How to compression check with engine out
12-18-2018, 07:31 PM
#1
Instructor
Thread Starter
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.
12-18-2018, 07:33 PM
#2
The Parts Whisperer
Rennlist
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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?
12-18-2018, 10:51 PM
#3
Team Owner
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.
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.
12-19-2018, 01:43 AM
#4
Rennlist Member
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.)
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.)
12-19-2018, 05:18 AM
#5
Rennlist Member
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.
12-19-2018, 12:05 PM
#6
Drifting
The numbers you quote indicates the engine would need a rebuild.
12-19-2018, 01:19 PM
#7
Rennlist Member
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.
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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12-19-2018, 10:09 PM
#8
Rennlist Member
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.
12-19-2018, 10:47 PM
#10
Rennlist Member
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.
12-20-2018, 08:59 AM
#12
Drifting
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.
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.
Why don't you do some research on automotive leakdown numbers and only then come back and report your findings.
12-20-2018, 10:48 AM
#15
Nordschleife Master
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.