Main Menu
Advertisement
Affiliates
Advertisement
Newest Members
D DAVID G, Mach22, tomsmith, Jamie Dubroc, TX1984
106784 Registered Users
Top Posters
TexDawg 87554
hopalong 78853
Pilothawk 75988
JDavis7873® 67386
John175 � 66189
FattyMcButterpants 60834
Derek 🐝 59302
Tritonman 57847
LoneStarSon® 54012
SkeeterRonnie 53181
facebook
Forum Stats
106784 Members
61 Forums
817445 Topics
10326705 Posts

Max Online: 36273 @ 01/23/13 02:34 PM
Topic Options
#10892321 - 06/03/15 10:46 AM Compression on a Mercury
Fisher77 Offline
Extreme Angler

Registered: 07/11/10
Posts: 1150
Loc: Lubbock, TX
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



Top
#10892545 - 06/03/15 12:29 PM Re: Compression on a Mercury [Re: Fisher77]
gary purdy Offline
TFF Guru

Registered: 05/24/03
Posts: 11326
Loc: Littlefield Tx & Lake A. H. US...
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

Top
#10892547 - 06/03/15 12:30 PM Re: Compression on a Mercury [Re: Fisher77]
Stump jumper Offline
TFF Celebrity

Registered: 04/19/04
Posts: 7703
Loc: Rockwall
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

Top
#10892786 - 06/03/15 02:12 PM Re: Compression on a Mercury [Re: Fisher77]
TLW Online   content
Extreme Angler

Registered: 11/16/06
Posts: 2258
Loc: ft worth
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?...

Top
#10893248 - 06/03/15 05:10 PM Re: Compression on a Mercury [Re: Fisher77]
Fisher77 Offline
Extreme Angler

Registered: 07/11/10
Posts: 1150
Loc: Lubbock, TX
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



Top
#10893341 - 06/03/15 05:47 PM Re: Compression on a Mercury [Re: Fisher77]
Pat Goff Online   content
TFF Celebrity

Registered: 01/29/05
Posts: 5388
Loc: Marble Falls/Seadrift
Warmed up is better, but dead cold will show a cancer case.
_________________________
Pat Goff
Marble Falls/Seadrift TX



Top
#10894328 - 06/04/15 07:18 AM Re: Compression on a Mercury [Re: Fisher77]
gary purdy Offline
TFF Guru

Registered: 05/24/03
Posts: 11326
Loc: Littlefield Tx & Lake A. H. US...
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

Top
#10899157 - 06/06/15 09:01 AM Re: Compression on a Mercury [Re: Fisher77]
Fisher77 Offline
Extreme Angler

Registered: 07/11/10
Posts: 1150
Loc: Lubbock, TX
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



Top
#10905675 - 06/09/15 12:30 PM Re: Compression on a Mercury [Re: Fisher77]
Muzzlebrake Offline
Extreme Angler

Registered: 12/26/14
Posts: 1652
Loc: Euless
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!


Top
#10906769 - 06/09/15 07:56 PM Re: Compression on a Mercury [Re: Fisher77]
leanin post Online   content
TFF Celebrity

Registered: 08/18/09
Posts: 5717
Loc: TEXAS
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.
_________________________
Waco Kid’s guide service. Crappie fishing on lake Waco, Tradinghouse, and Aquilla
Crappieman1969@yahoo.com 254-315-3999

Top
#10922792 - 06/17/15 11:06 AM Re: Compression on a Mercury [Re: Fisher77]
DAN-O Offline
Extreme Angler

Registered: 02/21/04
Posts: 2643
Loc: MANSFIELD, TEXAS, USA
Mine is a 1997 Mercury 150 EFI. It has compression of 125-130 in each cylinder.

Top



© 1998-2017 OUTDOOR SITES NETWORK all rights reserved USA and Worldwide