| Author |
Message |
    
neil jones
Member
Username: jansteam40
Post Number: 45
Registered: 02-2009
| | Posted on Sunday, January 31, 2010 - 10:35 am: | 
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does anyone know what the pressure should be when doing a compression test on a 1999 merc 305 engine |
Bondo
Senior Member
Username: bondo
Post Number: 2176
Registered: 12-2006
| | Posted on Sunday, January 31, 2010 - 10:46 am: |
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Ayuh,... Over 100psi,+ under 200psi...
You're not looking for a particular #,...
It's much more important that the spread between the Highest,+ Lowest is within 10% of each other... |
neil jones
Member
Username: jansteam40
Post Number: 46
Registered: 02-2009
| | Posted on Sunday, January 31, 2010 - 10:57 am: |
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ok thanks bondo |
Ricardo E.
Senior Member
Username: ricardomarine_vp
Post Number: 1063
Registered: 03-2009
| | Posted on Sunday, January 31, 2010 - 11:03 am: |
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Ditto Bondo!
Perform both dry "cold" and "warm" tests for more info.
You can also perform an "oil wet" test to eliminate or define piston ring issues and/or valve related issues.
Start with cold dry test!
Record the readings. |
Kghost
Senior Member
Username: kghost
Post Number: 1235
Registered: 07-2008
| | Posted on Sunday, January 31, 2010 - 11:40 am: |
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I would guess aprox 140 - 150 psi |
Bt Doctur
Senior Member
Username: bt_doctur
Post Number: 1284
Registered: 06-2006
| | Posted on Sunday, January 31, 2010 - 02:50 pm: |
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Make sure the battery is fully charged, the throttle plates wide open, and all the plugs out. |
PugetSounder
Advanced Member
Username: pugetsounder
Post Number: 580
Registered: 04-2007
| | Posted on Monday, February 01, 2010 - 02:35 pm: |
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It helps to know what the compression was at new just to get an idea how worn the engine is. If somebody is telling you an engine has 100 hours but the compression is ~125 lbs across all cylinders and it was 160lbs new, they are blowing smoke up yer you-know-what. That engine could have well over 1000 hours on it. Eventhough all cyls might be within 10 lbs and could be a good engine, you don't know what else they are lying about. I don't know what a SBC 305 is new but I'm sure yu can dig that up somewhere. |
makomark
Senior Member
Username: makomark
Post Number: 2120
Registered: 09-2008
| | Posted on Tuesday, February 02, 2010 - 03:41 pm: |
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Initial compression numbers will vary due to two drivers: 1) the manufacturing process and 2) the break-in process. these variables contributed to GM revising their recommendations for compression checks over a decade ago. |
neil jones
Member
Username: jansteam40
Post Number: 47
Registered: 02-2009
| | Posted on Wednesday, February 03, 2010 - 02:20 pm: |
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thanks for your input guys keep em coming |
JUST-IN-TIME
Senior Member
Username: justintime
Post Number: 7487
Registered: 09-2006
| | Posted on Tuesday, February 16, 2010 - 08:53 pm: |
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do a leak down
compression test are for 2 strokes |
Ricardo E.
Senior Member
Username: ricardomarine_vp
Post Number: 1144
Registered: 03-2009
| | Posted on Tuesday, February 16, 2010 - 10:34 pm: |
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I once over-heard a young mechanic (he was probably 34/35 years old) try to explain this to a small group of seasoned mechanics... I was one of them!
When several logical explanations were offered to him......, he sort of changed his philosophy on that idea.
I am not sure what Neil is trying to accomplish here, and/or if he has the skills to perform the tests and to interpret the readings (dry/wet/cold/warm/etc) ........, but he will be able to gain some basic knowledge by performing a few correct Comp Tests w/ a correct L/D test. |
JUST-IN-TIME
Senior Member
Username: justintime
Post Number: 7490
Registered: 09-2006
| | Posted on Tuesday, February 16, 2010 - 10:39 pm: |
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yes, but if you have a slightly leaky valve seat, your compression will still be good!! |
Ricardo E.
Senior Member
Username: ricardomarine_vp
Post Number: 1145
Registered: 03-2009
| | Posted on Tuesday, February 16, 2010 - 11:08 pm: |
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"yes, but if you have a slightly leaky valve seat, your compression will still be good!"
I must disagree.... the compression (pressure) will not be good!
The bad valve(s) is allowing the pressure to escape! Cylinder pressure will be low!
The piston's sealing rings may very well be capable of rendering good cylinder pressure....., but the reading will be low due to the failing valve/seat......... hence, an inadequate cylinder pressure reading!
With the correct procedure, an experienced mechanic will have determined that at least one valve(s) is part of, or all of, the culprit!
From there, and with possible further testing, he will determine his next step! |
JUST-IN-TIME
Senior Member
Username: justintime
Post Number: 7493
Registered: 09-2006
| | Posted on Tuesday, February 16, 2010 - 11:22 pm: |
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ok
well i have seen this first hand
the valve has a very very slight bend to it
the compression was only 5 psi different!
until i did the leak down test!! |
chiefalen
Advanced Member
Username: chiefalen
Post Number: 639
Registered: 03-2008
| | Posted on Wednesday, February 17, 2010 - 06:38 am: |
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Then the vacuum test will indicate that. Some people hook a vacuum gauge up to the dash. |
JUST-IN-TIME
Senior Member
Username: justintime
Post Number: 7500
Registered: 09-2006
| | Posted on Wednesday, February 17, 2010 - 02:46 pm: |
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yep, the vacuum gauge works like a charm also
most tech's never use them!!! |
