Checking compression??

[Vinnie]

What is the "proper" way to check compression?

I have always done it on a cold engine, plugs out and throttle open.. I have seen motors where the starter wont say engaged with the plugs out..

How much difference is there between a cold and a hot engine?

Just curious what the consensus is...

Thanks in advance..

:cheers:

[YELLOWSS]

i have done it warm and cold. just a few pounds different on a healthy motor. if isn't WOT it just a few pounds too...just make sure you do it the same so you results dont vary. the biggest thing that will alter the test is a low battery!!!!!!!! a compression test is a good way to tell if it is shot or not.. a leak down or blow down test is a much better test to see how much ware it has. JMO

[flabum1017]

i have done it warm and cold. just a few pounds different on a healthy motor. if isn't WOT it just a few pounds too...just make sure you do it the same so you results dont vary. the biggest thing that will alter the test is a low battery!!!!!!!! a compression test is a good way to tell if it is shot or not.. a leak down or blow down test is a much better test to see how much ware it has. JMO


:iagree:

Also, the Lemmonrude loopers tend to spit the starter as it builds compression; the trick to stop that is to leave all the other plugs in except the one your'e testing. And of course, plug wires and kill switch disconnected..............

[BagMan]

i have done it warm and cold. just a few pounds different on a healthy motor. if isn't WOT it just a few pounds too...just make sure you do it the same so you results dont vary. the biggest thing that will alter the test is a low battery!!!!!!!! a compression test is a good way to tell if it is shot or not.. a leak down or blow down test is a much better test to see how much ware it has. JMO

How much would low battery alter test?? Would it be possible for an engine that would otherwise read 150 actually read around 125 with low juice, or would it not effect to this degree?? I would assume all six cylinders would still read equivalently low however.

[sschefer]

How much would low battery alter test?? Would it be possible for an engine that would otherwise read 150 actually read around 125 with low juice, or would it not effect to this degree?? I would assume all six cylinders would still read equivalently low however.
If your battery is low and won't turn 300 RPM with all the plugs out you risk damaging the starter and other components. And yes the compression will be lower if the engine is turning less because the exhaust timing will be delayed allowing compression to bleed off for a longer period of time than normal, (layman's anaology).

[[email protected]]

Could someone please post the actual steps in doing a compression test on a V-6 Merc. Does someone maybe have a pdf file or something on the actual step by step procedure? Thanks for listening to a newbie. Craig :o :D

[CALVIN]

After a fresh rebuild when is a good time to take a leak down and or a compression test..what is the usual difference..before and after a fresh rebuild and after the rings have seated?

[BUZZIN' DOZEN]

Another common mistake, if your striving for accuracy, is to use a different length adapter than the spark plug, I have seen plenty people use a short one which gives up a few pounds.

[Jay Smith]

I agree with Roy, do it the same way every time.

TIP : Also if you testing compression make sure the ignition is disabled ( Kill switch pulled ) It can damage switch boxes turning a motor over and over not allowing the coils to dispel their energy or attach to a multi spark plug tester , as long as the coils can "spark" the switch boxes will remain safe or totally disabled ...

IMO Best test for ring/bore health is leak down test..

Jay

[[email protected]]

After a fresh rebuild when is a good time to take a leak down and or a compression test..what is the usual difference..before and after a fresh rebuild and after the rings have seated?


Good question!

[Jay Smith]

I find that completed break-in is according to the ring/bore and hone combo's. A steel bore with chrome rings can break in pretty quickly, A Nicom/nick bore with TiN rings can take longer.. Also it has allot to do with the surface prep on the bores ( the hone finish )

Good luck,
Jay

[tlwjkw]

One thing thats well worth tha 10 bucks when doing this a "Remote Starter Button". Puts ya right there at tha motor and no 'lectrics uh oh's. Plus it comes in real handy stabin' gearcases........

[flabum1017]

Could someone please post the actual steps in doing a compression test on a V-6 Merc. Does someone maybe have a pdf file or something on the actual step by step procedure? Thanks for listening to a newbie. Craig :o :D

Really a no-brainer...... Here's how I've been doing them for the past 25 years:

1) Warm up the motor and remove the plugs. Make sure the battery is fully charged.

If it's a omc looper, loosen all the plugs and remove the first plug from the cylinder you are going to test. After that cylinder has been tested, put the plug back in and pull the plug for the next cylinder...etc.... These engines will spit the starter bendix after 2 revolutions if you leave all the plugs out and try to check the compression on one cylinder.

2) Insert compression gauge into cylinder #1.

3) open throttle all the way

4) key off and the kill switch lanyard removed. If the motor is not hooked up to a boat, ground the plug wires either directly or using a spark checker. (If using a spark checker, make sure the spark checker is not behind the motor where the fuel/air mixture can be ignited as it leaves the spark plug hole :eek: )

5) attach a starter remote button tool to the starter solenoid between the positive battery cable and the yellow wire.

6) Using the remote button, crank the engine over 6 revolutions (or pumps). I have found if you don't build up full compression after 6 pumps, there is something wrong with that cylinder. Also, using the same number of revolutions per cylinder assures each cylinder is read the same way.

7) write down the reading on the compression gauge and repeat the process for each cylinder.

Compare the results. If you have a difference of more than 10% between the highes and lowest reading, you got a problem.

Compression gauge selection is important here too. Don't get those cheap Discount Auto or Harbor Freight gauges..... get a good one from Snap-On, Mac, Matco, Sears or Napa. A crappy tool will just have you running around in circles. My Snap-On gauge has lasted me over 20 years now.

Leak down tests are more accurate, but a compression test is a fast way to check the general health of the cylinders. If you get a low reading on any cylinder, a leakdown test is not necessary..........just pull the head and see what's wrong. If the numbers are good and you want a better idea of the condition of the cylinders then do the leakdown test.

[flabum1017]

Another common mistake, if your striving for accuracy, is to use a different length adapter than the spark plug, I have seen plenty people use a short one which gives up a few pounds.

Be careful of using the longer ones as it may be hit by the piston if it extends too deep through the plug hole. Make sure the adapter threads are not longer than the plug threads :thumbsup:

[[email protected]]

Really a no-brainer...... Here's how I've been doing them for the past 25 years:

1) Warm up the motor and remove the plugs. Make sure the battery is fully charged.

If it's a omc looper, loosen all the plugs and remove the first plug from the cylinder you are going to test. After that cylinder has been tested, put the plug back in and pull the plug for the next cylinder...etc.... These engines will spit the starter bendix after 2 revolutions if you leave all the plugs out and try to check the compression on one cylinder.

2) Insert compression gauge into cylinder #1.

3) open throttle all the way

4) key off and the kill switch lanyard removed. If the motor is not hooked up to a boat, ground the plug wires either directly or using a spark checker. (If using a spark checker, make sure the spark checker is not behind the motor where the fuel/air mixture can be ignited as it leaves the spark plug hole :eek: )

5) attach a starter remote button tool to the starter solenoid between the positive battery cable and the yellow wire.

6) Using the remote button, crank the engine over 6 revolutions (or pumps). I have found if you don't build up full compression after 6 pumps, there is something wrong with that cylinder. Also, using the same number of revolutions per cylinder assures each cylinder is read the same way.

7) write down the reading on the compression gauge and repeat the process for each cylinder.

Compare the results. If you have a difference of more than 10% between the highes and lowest reading, you got a problem.

Compression gauge selection is important here too. Don't get those cheap Discount Auto or Harbor Freight gauges..... get a good one from Snap-On, Mac, Matco, Sears or Napa. A crappy tool will just have you running around in circles. My Snap-On gauge has lasted me over 20 years now.

Leak down tests are more accurate, but a compression test is a fast way to check the general health of the cylinders. If you get a low reading on any cylinder, a leakdown test is not necessary..........just pull the head and see what's wrong. If the numbers are good and you want a better idea of the condition of the cylinders then do the leakdown test.


Thank you kindly Sir. Just what I was looking for. And thank you to all replies.
:thumbsup: Craig

[Vinnie]

Really a no-brainer...... Here's how I've been doing them for the past 25 years:

1) Warm up the motor and remove the plugs. Make sure the battery is fully charged.

If it's a omc looper, loosen all the plugs and remove the first plug from the cylinder you are going to test. After that cylinder has been tested, put the plug back in and pull the plug for the next cylinder...etc.... These engines will spit the starter bendix after 2 revolutions if you leave all the plugs out and try to check the compression on one cylinder.

2) Insert compression gauge into cylinder #1.

3) open throttle all the way

4) key off and the kill switch lanyard removed. If the motor is not hooked up to a boat, ground the plug wires either directly or using a spark checker. (If using a spark checker, make sure the spark checker is not behind the motor where the fuel/air mixture can be ignited as it leaves the spark plug hole :eek: )

5) attach a starter remote button tool to the starter solenoid between the positive battery cable and the yellow wire.

6) Using the remote button, crank the engine over 6 revolutions (or pumps). I have found if you don't build up full compression after 6 pumps, there is something wrong with that cylinder. Also, using the same number of revolutions per cylinder assures each cylinder is read the same way.

7) write down the reading on the compression gauge and repeat the process for each cylinder.

Compare the results. If you have a difference of more than 10% between the highes and lowest reading, you got a problem.

Compression gauge selection is important here too. Don't get those cheap Discount Auto or Harbor Freight gauges..... get a good one from Snap-On, Mac, Matco, Sears or Napa. A crappy tool will just have you running around in circles. My Snap-On gauge has lasted me over 20 years now.

Leak down tests are more accurate, but a compression test is a fast way to check the general health of the cylinders. If you get a low reading on any cylinder, a leakdown test is not necessary..........just pull the head and see what's wrong. If the numbers are good and you want a better idea of the condition of the cylinders then do the leakdown test.


I see you mention to leave the plugs in an OMC,,, What if a MERC starter disengages after about one rotation? I had to leaver 5 plugs in to get a reading!!

Thanks

[flabum1017]

I see you mention to leave the plugs in an OMC,,, What if a MERC starter disengages after about one rotation? I had to leaver 5 plugs in to get a reading!!

Thanks

I've never had a merc do it, but if it does, then leave the other 5 in..........

[[email protected]]

What difference does it make if the engine is cold or warm? Thanks, Craig :thumbsup:

[Jay Smith]

Warm with bores freshly "oiled" up will yield MORE compression..

Jay

[[email protected]]

OK...thanks Jay!! I am selling my 200EFI and I know everyone wanted compression numbers on it but I could not run it first to warm it up. Cold numbers ranged from 135 to 140. I did a leakdown test as well and they were all around 3 to 7. Are those numbers OK? 90 hours on a fresh rebuild. Thanks, Craig :thumbsup:

[flabum1017]

Those are good numbers............. assuming it's mostly stock with slightly cut heads

[racerx]

I had a newer 260,put out 145 lbs compression all stock cold,warmed it on the hose,put out 138lbs compression after..

[[email protected]]

Those are good numbers............. assuming it's mostly stock with slightly cut heads


It was rebuilt all stock with the exception of the oiler removed and metal cage bearings. I think I do remember being told that the heads were cut slightly but not anything significant that I needed to run special octane. I do run 93 anyway to be safe. Thanks for the replies, Craig :cheers:

[Jay Smith]

Craig sounds like ya got a motor that is at its peak for compression and seal...

Jay

[[email protected]]

Thanks! Great to know. Craig :thumbsup: