Merc 3.7 coolant overflows to reservoir under load

I just purchased a 1990 IMP Ashanti 180 that has a Mercruiser 3.7L motor in it. I was told the antifreeze was not holding pressure and was leaking into the raw water. I discovered a leak in one of the heat exchanger tubes and so I used a couple threaded plugs to seal of the tube. The cooling system now holds pressure, so I filled up the cooling system with antifreeze and as soon as I started the motor, the coolant shot up about 3 inches into the air and then went back down. I thought maybe there was an air pocket, so let her idle as I monitored the cooling system level and topped off as necessary for about 5-10 minutes.
I put the boat in the lake for a test and all went well util I went to accelerate. Any time the boat is under load, coolant will overfill the plastic reservoir at the bottom of the motor and sometimes even spill over the reservoir pressure cap. I replaced the cap and tried again but things did not change. I brought the boat back to land and tried filling up the cooling system and making sure there were no air pockets but each time I started the boat, I would get a huge surge of coolant from the reservoir/elbow and then I would cap the system and shut it off after only 15 seconds. The system would be under extreme pressure but the coolant and engine were cold to warm (at best). I can run the engine at idle for extended periods of time without any problems and even at higher rpms with muffs on but any time I accelerate rapidly, the coolant will spill out the coolant cap.
I have a feeling that I am somehow getting exhaust pressure into the cooling system from either the manifold or the elbow. Am I correct? The issue is that I don't seem to be losing any coolant. So how do you get exhaust to leak into the cooling system but not coolant into the exhaust? I have worked on many cars but I cannot figure this one out. Advice??? Please???

Re: Merc 3.7 coolant overflows to reservoir under load

More likely you're getting combustion gas into your cooling system through a blown headgasket. There's a procedure in the manual (don't have it handy at the moment) for hooking up some tubing in line with the reservoir and looking for air bubbles...

Re: Merc 3.7 coolant overflows to reservoir under load

Time for a compression check and cooling system pressure test. You can borrow the tools (free) from Autozone. Sounds like you may have a head gasket problem. Those two tests will tell you what is going on. Let us know what you find. Good luck.

Re: Merc 3.7 coolant overflows to reservoir under load

Not sure that either of those tests will show the problem. I wonder if the coolant pressure test would pressurize the system enough to show a leak? And a compression test might read low, or might read just a little low...OP's symptoms only appear under load...

Re: Merc 3.7 coolant overflows to reservoir under load

Compression test might not show a slight leak, but a partial leakdown test will show a blown head gasket for sure. Get each cylinder up on TDC on the COMPRESSION STROKE and pressurize the cylinder with about a 100 lbs of pressure, then with the heat exchanger cap off, watch for bubble in the coolant. Do the test on all cylinders to find which one is causing the problem.

It's a typical test on cars as well as boats. On the 470 engine it's especially helpful due to the open deck design of the block.

Re: Merc 3.7 coolant overflows to reservoir under load

When you do the pressure test you look for an increase in pressure in the cooling system while the engine is running, seems to me like that is what is going on here.

Re: Merc 3.7 coolant overflows to reservoir under load

Don S said:

then with the heat exchanger cap off,

Click to expand...

Heat exchanger or coolant res cap?

Re: Merc 3.7 coolant overflows to reservoir under load

I did the compression test and I found all 4 cylinders at 145-150 psi. When I had all spark plugs out and cranked the engine, there was no "blast" from the coolant reservoir/elbow. So I put a spark plug in each cylinder hole starting with the back and when cylinder #1 had a spark plug in it, the coolant would shoot up about 6 inches from the reservoir neck. Though compression was good in all cylinders, I suspected a blown head gasket on #1 cylinder. I removed the cylinder head and found that the head gasket had disintegrated just in front of #1 cylinder and no paper remained right up to and inside of the metal "compression ring" on the head gasket. With no material behind the metal ring to reinforce and seal, the 150 psi was blowing right into the cooling system and pressurizing the coolant to beyond what the 14 psi reservoir cap could hold while under load.
I have pulled the head and am awaiting parts to replace the head gasket. There were no signs of overheating and/or warpage so I am not going to have the head milled. A new gasket and more frequent coolant changes should suffice.
On another note, once I was under way in the lake, even though coolant was being forces out of the overflow bottle, I wanted to see how she would perform. At one point the engine rev'd pretty high with no feel in acceleration. I thought this might be from exhaust gas escaping around the prop but everything looks good. Any suggestions on what could cause the sudden high rpms under load after plowing through the water for about 3 minutes on full throttle? I quickly throttled down and coasted back to the dock and that was the end of the trip. I just don't want to get her back out there with a fresh head gasket only to find that I have another issue to deal with.

Re: Merc 3.7 coolant overflows to reservoir under load

Do not go with an aftermarket head gasket. Spend the money for a OEM Mercruiser head gasket pn 27-13709. It has extra red silicone beads molded into the gasket that help it to seal. Sounds like you may also have a bad coupler or spun prop hub.