Texasmark
Supreme Mariner
- Joined
- Dec 20, 2005
- Messages
- 14,558
We were on the subject the other day and I quoted from a 1996 (as I recall) published Merc. Service Manual for 60 Jet through 125 HP 3 and 4 cylinder Mercs of that and later era. I stated that the manual said that one COULD (nice rubber word) expect to have problems if below 120 PSI or over 15% variance between cylinders. One guy who is in the know and posts regularly here said he never saw that. I try to annotate my sources/references (including...opinion, best guess, I think) when that's what they are. I gave away the manual with my last boat but I now have 2 newer manuals, one covering carbureted engines of the (2002 era my interest) and the other covering direct injection engines. So from the two current Service Manuals:
First is the carbureted manual, Document number...lower left corner of page, dated Nov. 2015, covering 75 Marathon and 75 Sea Pro plus the same engines listed above, from 1994 up :90-8MO110566. Page 1C-19, titled Compression Check:
Warm up engine
Hold throttle plate at WOT....(Gotta get some air in the cylinders if you expect to compress it. My addition)
All plugs out
Crank engine till peak reading obtained...(didn't mention that this info used to contain: At 200 rpm or better aka fresh battery)
Record data:
"All within 15 psi of each other" or "A reading below 120 PSI might indicate a total engine wear problem." (Notice the rubber word MIGHT...my addition again.)
The Injected V6 Serv. Man. #90-859494 dated May 1999 for 115, 135 and 150 Optimax , Page 1C-5 states: Same basic setup for test as above but did add using a fully charged battery notation.
"Normal compression for all cylinders should be 110-130 PSI. Cylinders should not vary more than 15 PSI between one another. A variance of more than 15 PSI would indicate the need for a power head inspection/disassembly."
Still no indication anywhere of what a new engine coming off the assy line and in initial test/setup measures or one just broken in.
HTH
PS: While we are on the subject of manual verbiage, they have added paragraphs about where to run your RPMs, effect of trim position when checking, effect of altitude, load, dirty bottom, and temperature on WOT performance and corrective action (prop pitch changes) you are to take if below the min RPMs. Wink! Must have been reading boating forums!
First is the carbureted manual, Document number...lower left corner of page, dated Nov. 2015, covering 75 Marathon and 75 Sea Pro plus the same engines listed above, from 1994 up :90-8MO110566. Page 1C-19, titled Compression Check:
Warm up engine
Hold throttle plate at WOT....(Gotta get some air in the cylinders if you expect to compress it. My addition)
All plugs out
Crank engine till peak reading obtained...(didn't mention that this info used to contain: At 200 rpm or better aka fresh battery)
Record data:
"All within 15 psi of each other" or "A reading below 120 PSI might indicate a total engine wear problem." (Notice the rubber word MIGHT...my addition again.)
The Injected V6 Serv. Man. #90-859494 dated May 1999 for 115, 135 and 150 Optimax , Page 1C-5 states: Same basic setup for test as above but did add using a fully charged battery notation.
"Normal compression for all cylinders should be 110-130 PSI. Cylinders should not vary more than 15 PSI between one another. A variance of more than 15 PSI would indicate the need for a power head inspection/disassembly."
Still no indication anywhere of what a new engine coming off the assy line and in initial test/setup measures or one just broken in.
HTH
PS: While we are on the subject of manual verbiage, they have added paragraphs about where to run your RPMs, effect of trim position when checking, effect of altitude, load, dirty bottom, and temperature on WOT performance and corrective action (prop pitch changes) you are to take if below the min RPMs. Wink! Must have been reading boating forums!
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