I have a 496 mag ho with closed cooling system on it and noticed when I got it back from having outdrive serviced and impellar replaced that the engine coolant reservoir was almost completely empty.I have not had the boat in the water yet but have ran it off the hose for about 5 minutes.It did not get hot but unfortunatley I didnt notice if it was low before I ran it.Im going to fill it and run it to see if I loose it again and was wondering if there is a particular place to check first. I did check the oil and it is clean and no coolant in the crankcase. Also when the impellar was changed would there have been anyway coolant would have been lost there. I would assume not since that is the sea water side.

Ya, the sea water pump doesn't have anything to do with the coolant reservoir. Do a search on this topic. There is an issue with 496's leaking coolant from the heat exchanger when the boat is subject to large temp swings. Mine does it occasionally but it's only a few drops.

Fill Your Coolant level back to indicator mark and tighten the cap and warm the engine to operating temperature, then turn off the engine and slowly release the cap on the coolant resivroir, you should hear the pressure escaping when the cap releases. If the tank is holding pressure you probably don't have a serious leak if any, however if no pressure builds when the engine is warmed up then the pressure is escaping somewhere and coolant with it.
You can also do this test on a cold engine with a Stant or Snap-On pressure pump with the proper adaptor to screw onto the coolant tank. Take the pressure on the system up to about 14lbs. with the pump and let it sit and see if the pressure drops. If it does, you have a leak somewhere and if you see no evidence of coolant on the outside of the engine, then its leaking into the engine somewhere. when you fill this system remember to use Dex-Cool coolant full strength.
Some years of Merc 496's had coolant leaks from the rubber sealing rings under the coolant tank that seal the coolant tank to the crossover tank support. These leaks are external and should be visable and also drop to the bilge. Mercury had a recall on these for some years and even if yours does not qualify they are relativley easy and inexpensive to change if they are leaking.
good Luck, hope these points help in your missing coolant mystery.

Best Regards,
Ray @ Raylar

If all that checks out ok just keep a eye on it on your next few trips out. Just a Thought maybe the teck that changed the impeller tryed to back flush the cooling system and pulled off a wrong hose for a second or too and for got to refill the coolant tank back up. Look in the bilge area all over to see if you see any signs of old cleaned up coolant he may have have missed a spot.

When this happened to me on our previous boat it turned out that the heat exchanger was leaking internally. The pressure of the closed cooling side was pushing the water through a tiny hole and into the raw water side, therefore slowly causing the coolant level to drop. It had to be replaced (I had Mercury Product Protection so it was covered at the time).

Sometimes the head bolts can leak too. Normally outside the valve covers.

I have two head bolts leaking on my '02 HO. They are oustide of the valve covers - #2 and #4. It takes a lot of running to drop the coolant tank empty (and leaves a mess in the bilge)

Hate to hear it but I thought I was the only one. Drives me absolutely ph_cking nutz!!!

I've had the exhaust off 4 times now trying to get it to stop. Same for me 2 & 4 on port side.

Total, utter

What did you clean the threads in the block with?

My guess is the shop that did the service got it hot at some point and you lost some fluid when it overheated. It sure would have been nice if they would have at least topped it back off for you.