Inaccurate Compression readings due to low battery?

[Dave The Plumber 2]

Merc 5.7
I got a 125 readind on one of my cylinders when I turned it over with a low battery, but it went up to 175 with a fully charged battery. Is this common?

 

[alldodge]

I moved your thread from FAQ section, don't post anything there

Welcome
What motor are you working on?

 

[Dave The Plumber 2]

I moved your thread from FAQ section, don't post anything there

Welcome
What motor are you working on?

It's a mercruiser 5.7. After I let the battery charge up so that I was getting a healthy turnover, I'm getting #1-170,#2-170,#3-165,#4-175,#5-160,#6-135,#7-165,#8-170 and # 6 is 160 with oil. Should I be concerned with 6?

 

[alldodge]

Moved thread to Merc
Always start a thread with what your working on, there are a lot of different ones.

It's best to have the motor warmed up, throttle held full open then crank to take readings. With a cold motor I wouldn't need to look further into it. Put a couple squirts of oil in the cylinder to see if that changes it.

If the oil fixes the number, then there is a ring issue. Could do a leak down test to verify

 

[Scott Danforth]

It's a mercruiser 5.7. After I let the battery charge up so that I was getting a healthy turnover, I'm getting #1-170,#2-170,#3-165,#4-175,#5-160,#6-135,#7-165,#8-170 and # 6 is 160 with oil. Should I be concerned with 6?

Verify your gauge

Run the motor for about 5 minutes and retest.

 

[Dave The Plumber 2]

Moved thread to Merc
Always start a thread with what your working on, there are a lot of different ones.

It's best to have the motor warmed up, throttle held full open then crank to take readings. With a cold motor I wouldn't need to look further into it. Put a couple squirts of oil in the cylinder to see if that changes it.

If the oil fixes the number, then there is a ring issue. Could do a leak down test to verify

Thanks

 

[Dave The Plumber 2]

Verify your gauge

Run the motor for about 5 minutes and retest.

 

[Dave The Plumber 2]

What is running the motor and retesting going to show me?

 

[alldodge]

The short answers don't help but....
Same as I said, motor needs to be warmed up before running a compression test

That said I'm not so worried about the high numbers (normal is max 150). So your seeing more then 150 which is not so much, it's probably just a gauge accuracy issue. IMO it doesn't really matter from my thoughts if your above 150, it's more of an issue that the difference between 175 and 135

 

[Scott06]

What is running the motor and retesting going to show me?

When the engine is warm and up to operating temp the pistons rings valves expand with temp. normally a compression test is done after an engine is up to temp. it is more representative of the engine condition

 

[QBhoy]

What is running the motor and retesting going to show me?

The engine and its internal components will expand and seal when at operating temperature. The oil pressure or even presence also make the compression too.

 

[tank1949]

It's a mercruiser 5.7. After I let the battery charge up so that I was getting a healthy turnover, I'm getting #1-170,#2-170,#3-165,#4-175,#5-160,#6-135,#7-165,#8-170 and # 6 is 160 with oil. Should I be concerned with 6?

I had a compression tester's Shrader valve loosen, which caused inconstant readings. I luckily discovered loose valve before pulling heads.