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 Compression on a Mercury
#10892321
06/03/15 03:46 PM
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Joined: Jul 2010
Posts: 1,153
Fisher77
OP
Extreme Angler
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OP
Extreme Angler
Joined: Jul 2010
Posts: 1,153 |
I am looking at a boat with a 1986 Mercury 175 Black Max on it. Before I buy, I want to make sure the engine internals are sound. Can anyone tell me what the compression should be on this engine? What psi is ideal, and what psi would indicate needing to rebuild it. Any help is appreciated.
Thanks
“You may all go to hell and I will go to Texas.”....Davy Crockett "Victory or Death"....William Barrett Travis 
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Re: Compression on a Mercury
[Re: Fisher77]
#10892545
06/03/15 05:29 PM
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Joined: May 2003
Posts: 11,586
gary purdy
TFF Guru
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TFF Guru
Joined: May 2003
Posts: 11,586 |
A motor 30 years old is a roll of the dice but all 6 at 100lbs compression is good, under 90 I would be concerned. +or- 10% is the general rule
The world will not be destroyed by those who do evil, but by those who watch them without doing anything. Remember what He has done and remember that He is not done yet!-- GOD IS GOOD
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Re: Compression on a Mercury
[Re: Fisher77]
#10892547
06/03/15 05:30 PM
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Joined: Apr 2004
Posts: 14,069
Stump jumper
TFF Guru
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TFF Guru
Joined: Apr 2004
Posts: 14,069 |
I have the same motor in a 1985. I do not remember where mine was before I had the heads milled. I am thinking 120s. Right now it is 140. Make sure all cylinder are with a few lbs of each other.
2200 Bay Champ/200 Mercury Optimax
2017 Tundra TSS 4x4 Crewmax 5.7L
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Re: Compression on a Mercury
[Re: Fisher77]
#10892786
06/03/15 07:12 PM
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Joined: Nov 2006
Posts: 2,848
TLW
Extreme Angler
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Extreme Angler
Joined: Nov 2006
Posts: 2,848 |
if ya can get hold of another gauge it would help... need something ta compare it to unless that one is a KNOWN good gauge.....100 lbs is a little weak at any age for 'em.. need 115 minimum.. has tha motor been open at anytime.. in other words does it still have all 6 "chrome" cylinders?...
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Re: Compression on a Mercury
[Re: Fisher77]
#10893248
06/03/15 10:10 PM
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Joined: Jul 2010
Posts: 1,153
Fisher77
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Extreme Angler
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OP
Extreme Angler
Joined: Jul 2010
Posts: 1,153 |
Thanks for the replies.
Do you test dead cold, or at operating temp?
“You may all go to hell and I will go to Texas.”....Davy Crockett "Victory or Death"....William Barrett Travis
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Re: Compression on a Mercury
[Re: Fisher77]
#10893341
06/03/15 10:47 PM
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Joined: Jan 2005
Posts: 16,934
Pat Goff
TFF Guru
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TFF Guru
Joined: Jan 2005
Posts: 16,934 |
Warmed up is better, but dead cold will show a cancer case.
Pat Goff Seadrift TX ![[Linked Image]](https://i.ibb.co/X48Kbbq/scooter-shallow.jpg)
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Re: Compression on a Mercury
[Re: Fisher77]
#10894328
06/04/15 12:18 PM
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Joined: May 2003
Posts: 11,586
gary purdy
TFF Guru
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TFF Guru
Joined: May 2003
Posts: 11,586 |
Reason for my compression #`s. I found most will not warm up the motor or check at full throttle. I personally do not do a cranking compression test any longer. Leak down and running compression tells me so much more.
The world will not be destroyed by those who do evil, but by those who watch them without doing anything. Remember what He has done and remember that He is not done yet!-- GOD IS GOOD
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Re: Compression on a Mercury
[Re: Fisher77]
#10899157
06/06/15 02:01 PM
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Joined: Jul 2010
Posts: 1,153
Fisher77
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Extreme Angler
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OP
Extreme Angler
Joined: Jul 2010
Posts: 1,153 |
Thanks for the help everyone.
“You may all go to hell and I will go to Texas.”....Davy Crockett "Victory or Death"....William Barrett Travis
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Re: Compression on a Mercury
[Re: Fisher77]
#10905675
06/09/15 05:30 PM
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Joined: Dec 2014
Posts: 1,665
Muzzlebrake
Extreme Angler
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Extreme Angler
Joined: Dec 2014
Posts: 1,665 |
When I was shopping for a used boat I took my compression gauge with me to test the boat's motor. Anybody objected to testing the motor the conversation ended right there and on to the next one. I would be looking for 115-120 each hole and not over 5 psi difference between any hole. 100-110 is iffy and below 100 is a no go for me. Also a motor can have good compression and still have poor leak down. Leak down is a bit harder to check and needs a special gauge and you have to know how to put each piston at TDC
From Genesis: "And God promised men that good and obedient wives would be found in all corners of the earth."
Then He made the earth round...and He laughed and laughed and laughed!
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Re: Compression on a Mercury
[Re: Fisher77]
#10906769
06/10/15 12:56 AM
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Joined: Aug 2009
Posts: 7,009
leanin post
TFF Celebrity
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TFF Celebrity
Joined: Aug 2009
Posts: 7,009 |
sometimes carbon build up can affect one or nmore cylinders I had a motor go from 85 to 110 after a decarb.. one cylinder was low. If them rings aint spinning they aint sealing.
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Re: Compression on a Mercury
[Re: Fisher77]
#10922792
06/17/15 04:06 PM
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Joined: Feb 2004
Posts: 2,998
DAN-O
Extreme Angler
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Extreme Angler
Joined: Feb 2004
Posts: 2,998 |
Mine is a 1997 Mercury 150 EFI. It has compression of 125-130 in each cylinder.
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