Cracked Block 3.0 Mercruiser

[Robby662]

Just replaced the drive unit to find I had a cracked block (1985 3.0 Merc 140 I/O). I had to unfreeze the steering also which most of my advice came from reading post right here. You guys are great. I went to two different welders today and neither was too enthused about welding cast iron. One - plain would not do it and the other said he would if I pulled the block out. I gave up on getting it welded right and went the JBweld route (fingers crossed). The one thing I have wondered all along is - Are these cooling systems as pressurized as a car engine? Would leaving the thermostat out help? Hurt? What would be worst case scenario. I am already making sure the bilge pump works and I don't plan using boat on open water just smaller lakes. Thanks for any help

 

[Bilge Monkey]

Re: Cracked Block 3.0 Mercruiser

the pressure shouldn't get over about 7psi.

 

[Reel Poor]

Re: Cracked Block 3.0 Mercruiser

My 5.7's run 12 to 15 psi of water pressure in the intake manifold at 3600 rpms.

Running without a t-stat will create hot spot in the block.

 

[Haut Medoc]

Re: Cracked Block 3.0 Mercruiser

You might get away with a JB fix on an external crack if you prepare the surface properly......
Worth a shot, IMHO.......
If you have a raw water system, I would think you have a better chance of sucess....
Leave the thermostat in!.........

 

[Robby662]

Re: Cracked Block 3.0 Mercruiser

Thanks for the advice. I knew the performance would suffer w/o a t-stat. I really was not sure about it actually doing even more harm. I thought it would just run cooler with less pressure.

 

[Reel Poor]

Re: Cracked Block 3.0 Mercruiser

BTW....My 5.7's are RWC engines running the pressure I posted above. They're new engines and I have not run them over 3600rpm yet or I would post the WOT pressure, which I'm expecting to increase with rpms.

 

[Bondo]

Re: Cracked Block 3.0 Mercruiser

If your Block is Cracked, Only on the Outside,+ It's Not dumping Water into the Oil,.........

The Epoxy Repair method is Better than Welding.......

Welding tends to Cause more Cracks than it Fixes,+ a Good Weld Job should be Over-lain with Epoxy to take care of any Weepers......

But like I said,...... Epoxy Alone is Just Fine......
I've done Alot of Block Welding,...... Never Again.......

Now,..... With that all said,.......
Have you Inspected the Manifold,..??..??......

 

[Robby662]

Re: Cracked Block 3.0 Mercruiser

I had to take the manifold off to get to the crack. I thought it was strange that one guy wanted the block out to weld it, you can walk up almost w/o getting in the boat and put your finger on it. From reading the post it seems its in the typical spot right behind the manifold. I am going right now to put it back on. Bad thing is I did not read your post soon (bond-o) enough about using a dremel and all to properly prepare it. I used a wire brush and WD-40. I was surprised to see so many people using that method and it working, sometimes for years. This was my brothers boat, I was supposedly just needed to tune up and put a new belt on. HAHA

 

[Bondo]

Re: Cracked Block 3.0 Mercruiser

Hopefully the residue from the WD-40 Won't affect the bonding of the Epoxy,........

Good Luck........

 

[omgitzdave]

Re: Cracked Block 3.0 Mercruiser

Just curious, how did you end up with a cracked block? Did you winterize correctly?

All this talk of cracked blocks has got me nervous! (even though I used an anti-freeze "kit" to winterize, and then I drained the anti-freeze).

 

[whywhyzed]

Re: Cracked Block 3.0 Mercruiser

He doesn't want to weld in the bilge for safety reasons - gas tank on board etc...
I successfully repaired a 3.0 with JB a few years back. It held for the 2 years I used it like that. Weeped a tiny bit from another crack further down the side. I never worried about it. I bought another block in case it ever got worse, but it never ended up getting any worse.
I think you'll be fine.

 

[Robby662]

Re: Cracked Block 3.0 Mercruiser

The stuff I used to seal the crack was really called "magnum steel" and so far so good. I took it out today (1 hour) after getting the manifold back on, she did not leak a bit. I put a new thermostat on and everything seems fine. I got the instructions for putting the drive unit back on and unfreezing the steering off this website. This site is/was so valuable for someone like me that has little experience and has little choice but to do their own work. Thanks again to everyone who takes time to contribute. There is a wealth of information here.

 

[Robby662]

Re: Cracked Block 3.0 Mercruiser

The plugs were open when I got this boat. The cracked block surprised me because he said he had drained it. I checked the oil and let it run on the muffs but the drive was so bad, the impeller must have been bad too. I sure would not have put a new drive on cracked block but thats exactly what I did. I still do not know of a way to get water to the engine outside of using muffs, maybe I could have checked it with the drive off? The motor did not even smoke and still runs smooth so I had no reason not to believe my brother. Does the freeze plugs on these things ever really work? Maybe merc ought to design the whole block out of freeze plugs.

 

[Reel Poor]

Re: Cracked Block 3.0 Mercruiser

Glad to hear you got it repaird and everything worked out.

"Freeze Plug(s)" is a misnomer. They are core plugs. The holes are in the blocks so the casting foundaries can get the casting sand out of the block.

 

[180shabah]

Re: Cracked Block 3.0 Mercruiser

That magnum steel is pretty good stuff, I used it to make an exhaust repair on my car (2 days before inspection) and figured I would replace when the weather got warmer. Well it was still holding three years later when I got rid of the car.

 

[patbras]

Re: Cracked Block 3.0 Mercruiser

Just replaced the drive unit to find I had a cracked block (1985 3.0 Merc 140 I/O). I had to unfreeze the steering also which most of my advice came from reading post right here. You guys are great. I went to two different welders today and neither was too enthused about welding cast iron. One - plain would not do it and the other said he would if I pulled the block out. I gave up on getting it welded right and went the JBweld route (fingers crossed). The one thing I have wondered all along is - Are these cooling systems as pressurized as a car engine? Would leaving the thermostat out help? Hurt? What would be worst case scenario. I am already making sure the bilge pump works and I don't plan using boat on open water just smaller lakes. Thanks for any help

Hi Rob,

can you tell me how your experience with the jb was and how it held or is holding up?

Thanks for your reply Patrick

 

[whywhyzed]

Re: Cracked Block 3.0 Mercruiser

Patbras, you are replying to a thread that is about 1 year old. The original poster of this thread hasn't visited iboats in almost a year. Most of us can't recall what we had for lunch. Best to start a new thread with questions.