what do these compression numbers mean?

[Rusty Hartman]

97 mercruiser 4.3l. this motor hydro locked. i found a bad flapper valve on the starboard side. no indication of water in the oil as far as i can tell, but i haven't pulled a valve cover. no creamy foam under the breather cap and the oil on the dipstick looks good.


starboard port readings front to back
160 195
180 190
180 195

starboard side had a few teaspoons of water. i've had the motor back together and running but pulled it apart to do a manifold conversion so i took these reads.

this motor has had a dieseling problem. are the port numbers indicative of carbon deposits? i've noticed the port side tends to run considerably cooler...leading me to think there's a carbon build up. or....did i slightly bend the rods on the starboard side? the starter stripped out preventing the motor from starting after it ingested water. water ingestion also didn't kill the motor...it just wouldn't start after idling up to the boat slip.

btw, these numbers are cold readings. i couldn't run the motor because it has no manifolds. it was suggested that i take the readings with the motor warm which i obviously couldn't do....thanks in advance.

 

[chimmike]

Re: what do these compression numbers mean?

The 160 is a little teeny bit low, but honestly those numbers are pretty solid. Carbon deposits occur on all pistons, some worse than others, but carbon should not affect compression. Not sure that anything is wrong, honestly, but that could be minor valve leak, could be headgasket was a little unhappy?

 

[Rusty Hartman]

Re: what do these compression numbers mean?

The 160 is a little teeny bit low, but honestly those numbers are pretty solid. Carbon deposits occur on all pistons, some worse than others, but carbon should not affect compression. Not sure that anything is wrong, honestly, but that could be minor valve leak, could be headgasket was a little unhappy?

ok thanks. i bring up the idea of carbon deposits affecting compression because i read it here, honestly. just trying to cover all the bases before i put it all back together.

 

[79Glastron]

Re: what do these compression numbers mean?

I can see why it had a dieseling problem, those 190,195 numbers are high. I would imagine that motor is probably carboned up and needs to be cleaned out. I'd say that 160 is probably the "normal" number.

 

[thumpar]

Re: what do these compression numbers mean?

Was this on a warm engine? Some of those are kind of high. Any other history on the motor?

 

[Rusty Hartman]

Re: what do these compression numbers mean?

Was this on a warm engine? Some of those are kind of high. Any other history on the motor?

motor was cold. the boat has always been exceptionally well maintained. never been in saltwater. i've only owned it since september