'91 Merc 115 Compression Results

I read a ton of postings on this subject but didn't find a decent answer. I looked at a very nice 17' Larson with a 1991 Merc 115 OB. Ran strong on the water. Checked compression: 120, 95, 120, 120. All plugs were dry and greyish/brown. I hat to pass on the boat, but I need to know if the motor is a deal breaker.<br />Thanks.

Re: '91 Merc 115 Compression Results

Howdy, sawbones.<br /><br />Welcome to iboats. <br /><br />Bad engine. #2 is 25psi (greater than 15%) out from the others.<br /><br />Your search should have turned up about a hundred or so posts saying that all cylinders should compress within 10-15% of the others.

Re: '91 Merc 115 Compression Results

Hi JB,<br />Thanks for the response. I did find lots of responses on what the values should be but not what they meant in terms of repair or trash if one was low. I did squirt some oil in the low hole and it came up to 110. I talked to two local mechanics and they both said it sounds like rings/or scored piston/cylinder. Between the two estimates the average was $1500 to repair. One said worst case he could rebuild the entire power head for $4000. Based on that info I negotiated the price down to $3700 and made the deal. The boat is in awesome shape aside from the low compression so I won't feel too bad if I have to put some money into the engine.<br /><br />Next question - since it runs strong, can I run it the rest of the summer and then have it repaired in September without doing further damage? I plan to replace the impeller and install a temp gauge so I know what is going on from a cooling situation.<br /><br />Thanks for the input.

Re: '91 Merc 115 Compression Results

Sawbones.<br /><br />I had a 150 Evinrude that started off at 75PSI and worked itself over 90PSI after running it for 2-3 good tours. If you are planning to keep the engine, check it after a couple runs to see if the rings have loosened up a bit. If not, try decarbonizing as per instructions.<br /><br />I had a good experience with decarbonating another engine to get the compression numbers near even.<br /><br />Good Luck ... CF

Re: '91 Merc 115 Compression Results

Do a search on decarb. You may try spraying Deep Creep in the cylinders and let them soak overnight, then run the engine for a while on muffs. (The engine will smoke like you would not believe). Then run it out on the water wide open for a while. After you bring it home, check the compression again with the motor warmed up. Sometimes a decarb will unstick a ring and return compression to an acceptable level. Good luck and keep us posted.