1988 Mercruiser 470 loosing antifreeze

I have a 1988 Mercruiser 470 (165HP). I had the engine rebuilt over the winter as the head gasket on the port engine was bad. Both the port and starboard engines are loosing antifreeze when I take them out and run the boat. Sitting at the dock, they are fine. The port side engine is losing about 1 quart of antifreeze and the starboard about 2 quarts, this is on a fairly short ride of less than 30 minutes. The engines are both running good and temperatures are perfect. I rented a pressure tester and pumped the pressure up to around 18 and found no leaks. I did find that I had 16 pound radiator caps on both and from what i have found they should be around 12 so i have replaced them. There is no antifreeze in the oil. There was antifreeze in the bilge but I believe that is because one of the coolant overflow bottles was too full. I corrected that issue. I also had the heat exchanger pressure tested at a radiator shop and they found no problems with it. New gasket between the riser and exhaust manifold. Oh, i also had both outdrives rebuilt with new water impeller, bellow and water lines.

Any ideas? I'm at a loss since I have basically rebuilt 2 engines, 2 outdrives and replaced all of the trim cylinders.

Re: 1988 Mercruiser 470 loosing antifreeze

Pull your spark plugs and see if the insulator on any of them is white rather than brown or tan. If white then you are probably leaking coolant into the cylinders. Check the weep holes on the lower starboard of the engine waterpump, while running and up to temperature to see if it is leaking. Make sure your hoses to the expansion tanks are in good shape.

Re: 1988 Mercruiser 470 loosing antifreeze

Losing that much coolant should be apparent on a pressure test. You did do it with the system empty, right?

Re: 1988 Mercruiser 470 loosing antifreeze

Took the boat out last night for more testing with 13psi caps. Port engine leaked antifreeze out of the cap when i took the boat off of plane, switched it back to a 16 and didn't have that problem again. Also, this engine seemed to have lost only the antifreeze that came out of the cap. Starboard engine didn't leak out of the cap at all, but the overflow bottle, which started at full, was full to the top and antifreeze was under that engine. This engine had a 13psi cap on it. We are thinking it might need a 16 so I have to get one and try it. The guy that fixed the engines says none of this makes sense.

I am going to check the spark plugs and post back after.

I will get this thing figured out eventually. Boat has never ran this good even with this minor issue.

Re: 1988 Mercruiser 470 loosing antifreeze

Your coolant recovery bottles are not supposed to be "full". They should start out on a cold engine at half full. Check the hoses that run from the filler necks to these recovery bottles as well. Are they free and clear including the nipple on the filler neck under your caps?

Dennis

Re: 1988 Mercruiser 470 loosing antifreeze

leaking head gaskets blowing hot exaust into the cooling side, coolant goes into the res bottles and out the overflows. overflow bottles are never full ,

Re: 1988 Mercruiser 470 loosing antifreeze

There should be a fill line on the expansion tank. The antifreeze should be at that level when the engine is cold. The antifreeze is supposed to flow into the tank when the engine gets to operating temperature and the antifreeze expands (it flows back when the engine cools off). If enough goes into the tank to cause it to overflow, then something beside normal expansion is pushing it.