Slow Anti-freeze leak on a Merc 7.4L FWC

J

johnsross3

Member
Sep 3, 2007
95
Newark, DE
Boat Info
1995 Sea Ray 370 Sedan Bridge
Engines
Twin 454 Mercruisers
One of my Fresh Water Cooled 7.4L Mercs has developed a slow anti-freeze leak (as the overflow needs to be refilled after about 10-15 hours). I suspect it is internal as I see no evidence of anti-freeze in the bilge. Since the leak must be slight, I'm wondering whether I could try a automative radiator stop leak solution. Has anyone tried this, or is this ill-advised?
 
You really need to find out where it is going. Leak solutions by there very nature will clog the heat exchanger creating a bigger, more expensive problem for you. There are several things you can do to track down what is going on.

The most probable leak areas are:

1) loose hose clamps. Start there.

2) riser gaskets which leak coolant into the exhaust manifold and will eventually cause the engine to hydrolock. When was the last time these were changed? If it is more than three years, change them. White smoke and a sweet smell from the exhaust will oftentimes be a clue.

3) leaking head gasket caused by a failing gasket or crack.

-A automotive pressure tester will tell you the rate of pressure loss over time and may reveal the leak.

-Antifreeze dyes and a UV light are also useful in tracking down where it is going.

John
 
Pressurize the cooling system with some dye and see if you can trace down the leak that way. If you can't find the leak on the exterior of the engine but in fact the gauge pressure is decreasing, I would suspect leaky manifolds/risers, head gasket, or heat exchanger.

Doug
 
Do you have a hydronic heater?
How long has it been since your boat has been serviced regarding the coolant?

I have put up to 1/2 gallon after refilling the coolant working on the heater.
 
What John said.
I'd also rule out a defective cap- replace or swap with the cap from the other engine and see if it solves the problem.
 
One other suggestion is to get the oil tested. I doubt it is finding it's way into the crankcase but an oil test will tell you if it is. My money is on the riser gaskets if they haven't been changed in a while.

-John
 
And pull the end caps off the heat exchanger and see if coolant is there. If so, you know you have a leak in the heat exchanger
 
I had same problem last year. small leak in heat exchanger. removed exchanger went to radiator shop. He closed off 3 tubes cleaned out in acid bath, painted new cap. cost $185 No more leak and motor runs cooler by about 10°
 
johnsross3 said:
One of my Fresh Water Cooled 7.4L Mercs has developed a slow anti-freeze leak (as the overflow needs to be refilled after about 10-15 hours). I suspect it is internal as I see no evidence of anti-freeze in the bilge. Since the leak must be slight, I'm wondering whether I could try a automative radiator stop leak solution. Has anyone tried this, or is this ill-advised?
Click to expand...

I mucked around with this for almost a year on ours. Turned out to be a bad exhaust manifold (and they were new).
 
BTW, NAPA makes a kit you can use to test to see if you are getting exhaust in your cooling system. It was positive for ours, which pointed us to the manifold. I forget what's it's called, something like an NAPA Combustion Leak Block Tester. I think it costs around $40.
 
I had the same problem earlier this season... we were swimming off the back of the boat one day and I saw these shiny spots on the top of the water that were the size of a dime - not a lot but here and there. Didn't think anything of it at the time. About a week later I looked in the engine compartment and saw the starboard coolant overflow jug was nearly empty. After putting 2 and 2 together I realized I had an antifreeze leak in the Heat Exchanger.

As a band-aid fix until the end of the season I put a couple of ounces of radiator stop leak in the heat exchanger fill cap and refilled the resevoir. Since then (30 hours) I've lost about 8-oz of antifreeze - so it definitely slowed the leak rate.

In the off season I plan to get the heat exchanger fixed at a radiator shop like John did above.

Hope this helps.
 
I cannot recall, it came from Jasper with the long blocks. It was replaced under warranty.
 
I know people like to debate this subject. However, if you plan on keeping your boat, the premium of buying OEM exhaust parts is 30-40%. Companies like perfprotech.com run sales which close that gap even further.

The casting process is critical to the longetivity of the product. I'm not an OEM fanatic about most things except with exhaust manifolds. I just see people make decisions to save money and end up paying a lot more as a direct result.

I like Jasper. They do a good job of standing behind their product.

-John
 
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