Well the results are in..




Looks like I will be pulling the heads. Now I just have to decide if it is worth it to pull the motor or just do the work in the boat. Neither really sounds all that fun.....

Sorry for your trouble. As I understand it, Merc specs compression to be a minimum of 100 PSI with a max difference of 70% between the lowest and highest readings. It looks like you're in their spec for what that's worth. It may still be worth digging into it, I just wanted to throw that out there.

How did the impeller look when you replaced it? Did you replace both impellers and how did they compare? If it's a new to you boat, it's probably worthwhile to change both impellers so you have a baseline. Basic, simple stuff but just wanted to ask.

Post back what you find out as the history of these threads provide great information for those of us looking into issues down the road.

I would do a leakdown before ripping anything out of the boat. A compression test will get you in the ballpark, but with a leakdown you can pinpoint the issue(s) a lot closer before digging into anything.

The sound of leaking air can tell you a lot of the story.

What he said ^^^. Still keep checking the easy stuff like thermostat, back flushing, etc. Pulling heads is not fun.

Well last night I went ahead and pulled the heads off the engine. Really wasn't to bad of a job. Just glad I have small blocks (never thought I would say that! haha) so there is a bit more room to work back there. I hear horror stories from people who own/work on the big block 272's. Start to finish was about 2.5 hours and that was not being in a huge hurry. I found exactly what I was expecting/hoping and that was obviously burned & leaking head gaskets between cylinder 3 & 5 as well as a burn mark on cylinder 6. I will try to get some pictures up of the gaskets. I'm dropping the heads off this morning to get checked out for cracks & warpage. Should have the heads back by Monday at the latest which means I should be back on the water next weekend if all goes well.

We had that same temp gauge issue with a 402 in one of our drag cars. It wasn't a head gasket, but actually a crack in the #2 cylinder water jacket. I have seen a head gasket react the same way.

It's been a few weeks since I updated this thread so I want to make sure and post the results in case someone else has as similar issue. The issue was a blown head gasket between cylinders 3 & 5.and possibly #6 as well ( Need to do another compression check to see if it was leaking as well but the gasket sure looked like it was). I had the heads checked for cracks and re-surfaced as a precaution and then re-assembled everything the following week and was back on the water that Saturday. Motor is running great again and the temperature is rock steady at 155 degrees without the crazy fluctuations I was seeing. Thanks everyone for the help!