Ugh, Where to begin

[abiscow]

So first day on water to test new engine. Couple problems which is to be expected. Motor is a Blueprint 355 marine bloc Chevy. Runs good no issues. Problems now arise w the drive. 2 things. One, my gear oil seems to be disappearing. I used the original drive that came w boat. Maxum 2400SD. Originally had a 5.0 in it. Changed oil in drive, filled up remote reservoir, and after running for about 45min, alarm came on and bottle was 3/4 empty. Thinking I had air, tried again and same thing so leak somewhere in drive. Second problem, overheating intermittently. Ran it for 45 min or so no issues then coming into the dock at idle, temp shot up to 195/200. Shut it down at that point. The history is really unknown other than it sat in the salt for a year w no maintenance. Engine was shot. So new engine, original drive. Thinking its the impeller. Drive was heavily encrusted before I sanded and repainted. So, where should I look for this leak. Nothing obvious I can see. Looked at the shift shaft and is dry. Could this be leaking out the lower? Fix, I also have a brand new 2.0 drive sitting in my garage for my previous boat. 5 hrs on it maybe. Should I change gear ration and just use new drive?

[mmwjr]

You need to drain the lube and perform a pressure test per the manual. Most likely there is a bad seal somewhere. When the temperature went up did the alarm come on? I would confirm the temperature with a IR sensor.

[abiscow]

So im not quite sure if it sounded due to the temp or when the bottle emptied. It all pretty much happened at once. Im going to pull the lower tomorrow and replace seals and impeller. Like I said, when I got the boat the outdrive was heavily encrusted. Ill do the pressure test after that. my question is, do you think its worth using the brand new drive and just change the gears. The new drive has a ratio of 2.0 Is there any playbook where I could use it as is. Im not even sure what ratio is on the boat now. I ck the serial on the bell housing and its older than the boat so must have been changed at some point.

[mmwjr]

The current gear ratio doesn’t really matter since you changed from a 5.0 to a 355 CI engine. You may want to reach out to Mercruiser Help and ask them, they are good with providing help.

[abiscow]

So I spoke w Merc customer service today. Based on the old engines serial they were able to get me the ratio of the existing drive which is 1.62 So as I asked before, does it makes sense to reseal a 20yo drive or just have the brand new one re geared to 1.62 I was quoted 400 to change gears that I provide.

[mmwjr]

Did you ask the Mercruiser help what ratio they recommend for the 355?
I don’t know what it would cost to change the gears vs just finding the right drive and selling what you have.

[Mr Hemi]

Chasing problems like drive oil leak can get tiresome and expensive.
Putting a New drive on that you know is good, would eliminate all the troubleshooting.

Likely your impellor is bad, and possible thermostate, especially since it seems to be worse at low RPM.
Cheap fix, although a bit of labour.