Head gasket or Cracked block??? 1987 4.3 175/v6

[I buy lemons :(]

I recently purchased a Thundercraft Magnum 290 with twin 4.3 Mercruiser engines. This is our first boat. We took the boat out fir the first time and while we were maintaining the no wake @ 5mph my starboard engine overheated badly( Smoking, burned paint and burned holes in trim lines). I shut down the motor and crutched the rest of the day on the working engine. I I took the boat home tore apart the lower unit and replaced the impeller.( the old impeller came out in chunks) I ran the boat on muffs in my driveway for 15 minutes or so and no issuez, no leaking oil, no overheating, no smoke etc.
I took the boat and launched it today and engine was fine until i hit the lake. Got to the lake aprox 4 miles from launch and sped the boat up and started to get too hot so i backed off of it and it cooled itself down at low speed. I ran at low speed the rest of the afternoon and never overheated. I check my oil cap when we docked and found a bunch of mayo(oil and water mix) and a small amlunt of oil in my bilge area.
So my question is how do i know if it is a headgasket or a cracked block. Im a parts changer at best. I just do not want to throw money at a lost cause. I will absolutely try changing the head gasket myself. Couple things if i attempt it. I will send the heads out to a machine shop, but do i need to know how to do timing because of taking off intake and distributer? What technacal knowledge would i need to do the work properly? Also how do i tell the difference between the head gasket or a cracked block? Sorry for the long story just hoping it helps with a diagnosis.

 

[dubs283]

Wouldn't worry much about the milky blow off at this point. You've inspected the first step of new to you boat

As another regular poster on this forum would say, you need to complete the regular maintenance the person who sold you the boat didn't perform

Show us pictures of your boat!

 

[I buy lemons :(]

Thats all i have at the moment for pictures of it.

 

[dubs283]

Nice!

I'd invest in twin 5.7 v-8's or full transom re-fit with a single bbc and bravo 3

The 4.3 liter engines you own rn are worth decent money, nice offset for a reasonable upgrade

 

[tpenfield]

Welcome to iBoats @I buy lemons :( . . . . sorry to hear about the issues.

My comments . . .

Since the impeller was destroyed, you should look for & remove any remnants of the impeller further along the cooling system.

Once you discovered the mayonnaise, did you happen to notice the oil level on the dipstick, and if it had gone up?

You could do a cylinder leak test to determine if you have a blown head gasket . . . . or . . . A blown head gasket will put exhaust into the cooling system, particularly at moderate and higher RPM. Temporarily swapping to some clear hoses (the ones that go to the exhaust manifolds) can make it easier to see if you are getting exhaust gasses in the cooling system.

If you are getting a lot of water in the oil, it is more likely a cracked engine block. A cooling system pressure test would be able to verify. Assuming that you have raw water cooling, you'll have to figure out a way to block off the hoses to the exhaust and apply pressure to the engine's cooling system.

If you can post the engine serial numbers and maybe a few pictures of the engines, that can help with responses.

Aren't boats fun !?!?

 

[Scott Danforth]

Welcome aboard

And yes, prior to dropping the boat in the water in the excitement of new to you boat ownership, the drives should have been pulled, the alignment checked, gimbal bearing and u-joint inspection, new bellows and the drives should have been split and new pumps installed

(All the maintenance the prior owner didn't do)

Since that didn't happen and the Starboard motor overheated to the point of burning the paint (over 250 degrees). You burned the flappers, rubber exhaust parts and everything plastic down stream including the items in the exhaust cavity of the drive

Pull the drives, pull the cooling system apart to find impeller bits.

Because of the overheat and potential cracked block(s). Do a cooling water jacket pressure test. Do a compression and leak down test.

Do the trouble shooting and testing. Determine what is wrong. Then and only then do you get into parts acquisition and changing them