cylinders 5 and 6 low compression

Thumbtack

Petty Officer 2nd Class
Joined
Aug 3, 2007
Messages
118
Merc 7.4L V-8. MPI. Bravo. I forget the engine number but it started as OL3...... i forget the rest. Boat sat for about 1 year, then was used once, then sat for another year. Did a compression test to see how everything was mechanically and here are the numbers 1=145, 3=150, 5=125, 7=150, 2=150, 4=150, 6=125, 8=160. 5 and 6 struggled to reach 125 while the rest were at about 150. 5 and 6 are directly across from eachother so i was wondering if this could have anything to do with a valve being hung due to the boat not being used that much. Could a valve on one side even effect the other side's compression?
 

Don S

Honorary Moderator Emeritus
Joined
Aug 31, 2004
Messages
62,321
Re: cylinders 5 and 6 low compression

Did you run the engine first before check the compression or did you just do the compression test on an engine that has been setting for a year or so?
You really need to start it up and warm it up after setting for any long period of time before doing a compression test.
 

Thumbtack

Petty Officer 2nd Class
Joined
Aug 3, 2007
Messages
118
Re: cylinders 5 and 6 low compression

thanks don, after running (and running fine) the compression was good all around. Thanks again.
 
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