lost power while running - help

[ssbob]

Hello All,

I am new member to this sight, however I have read many helpful threads over the past few years. It has been a huge help! Now I have a specific problem that I encountered on the water. I have a 1991 mec 75HP 3 cycle outboard motor. It has been a strong motor over the years but I think I may have done some damage as I was running it without any 2 cycle oil for 5-10 minutes. While I was running my motor shut down. I thought it was just running out of gas since I was getting close to empty. I switched over to my other tank and it ran well for 30 minutes until I docked the boat for a few hours for lunch and kids activities. When I started to head back home the motor shut down again and I immediately realize that I was out of oil. I filled up the reservoir and off I go. About 30 minutes in (give or take) the motor shut down again. I tried to start it but it knocked a few times than shut down again. As this point it won't even crank over. Did I blow up my engine. Thanks and any advice is appreciated!!!

 

[ssbob]

I forgot to mention that I changed the lower unit oil before the trip. It was the first I change the lower oil since I owned the boat. The oil looked good. The first little bit was grey and the rest was fine. I don't think they are related but I can not be sure..... again thanks!

 

[91Astro]

Ssbob,

I am no expert but I can tell you what happened to my '91 Mariner 150 V6....

The oil injection system failed before I bought the boat. Being a newb, I bought the boat after seeing the motor run in a tub of water. I know now..., Anyway, as I ran it, the motor kept bogging down and finally quit. Long story short, I had 50-70 compression in the scored cylinders and was fortunate that the motor was still a "virgin" and could be rebuilt. My tech said he sees this happen 3 to 4 times per year. He disabled my oil injection system so that now I premix (50:1) my fuel and never worry about the oil injection. He removed the oil resevoir under the deck as well, giving me lots of storage room. I have all the oil injection parts in a box should I ever sell to someone that wants the oil injection system back. Motor runs great after rebuld two years ago, however, I did just have to decarb with Seafoam.

My rebuild ran about $3000. I hope yours is a simpler and less costly fix.

Best regards,
Dan

 

[ssbob]

Thanks Dan.... I'm a relative newbie as well. I had this boat for five years and this is the first real issue I had. Anyway I wanted to update you since you did reach out. I was able to do a cold compression test and my reading were ridiculously low 15,on the the bottom and middle cylinders, and 25 on the top cylinder. I had to do the cold test bc I could not get this thing started. Anyway, today I changed the battery, soak the spark plug hole w seafoam, cleaned the carbs, and used starting fluid to finally get it running again. Unfortunately I heard this rattling/knocking noise coming from the motor. I immediately turned it off. I did a second compression test right after I had it running and my reading cam up. I had 70 psi on bottom, 85 on the middle and 100 on the top. I did the test but not sure what it means. From what I read here is the should be much closer read to each other but I don't know what to do since there are not that cloIs this thing fixable or worth it to fix.

 

[ssbob]

sorry for the bottom sentence on above post... I sent it bf I was ready..... Anyway, the bottom cylinder is at 70 psi, middle at 85 and the top at 100. I did the compression test but not sure what the readingds mean. I read here that the psi should be within 10-15% between each cylinder but again not sure why or what it means. Again thanks!!!

 

[91Astro]

My tech charged me $35 to come to my house and determine if the motor was able to be rebuilt.....perhaps you can find someone to do the same? Your readings would indicate to me, based on what i have learned in the last two years, that your powerhead needs be rebuilt. I had a tough time agreeing to a $3000 rebuild, but I ended up with a 150 V6 that should last a long time...much cheaper than a new motor. Hopefully, an expert will weigh in on this for you.

Good luck....
Dan