I am a new member and I am hoping someone might be able to provide some advice about a cooling problem. I own a 1995 Bryant 209 with a Mercruiser 7.4 Bravo One. Two weeks ago I pulled the boat out of storage and started it on the trailer using a garden hose. After a few minutes the boat over heated. Thinking the thermostat might be stuck, I pulled it out and tested it. Although it opened a little late (about 170 degrees), it did open fully by 180 degrees. I then checked the raw water pump and found the fins on the impeller were completely gone. I installed a new raw water pump making sure the inlet and outlet hoses were connected properly. Before hooking up the hoses, I also placed a shop vac on the lines to suck out any possible impeller fins. After putting the boat back together I fired it again. Initially all appeared well. Water was flowing out of the outdrive port and as the temperature gauge passed 160 degrees, it came down slightly only to rise to about 185 degrees which is when the horn sounded. After shutting off the engine I went to town looking for possible blockage. I pulled the oil cooler, thermostat housing, circulating pump and exhaust manifold cooling hoses. I also ran a shop vac through the intake manifold and circulating pump ports on the engine block. I never found any impeller fins nor any blockage. As far as I can tell the new raw water pump is functioning properly. However I am lost as to why the engine is overheating. I plan to replace the thermostat. However I can’t image the thermostat is my problem since it appears to function when I test it on my stove. I certainly would appreciate any advice.

Thanks,

Jim

Instead of using a shop vac try back flushing with a water hose. This will push any blockage out and show you how much water is flowing at the same time.

Rick

Jim, yes try and back flush with water as RV said......the other thing is most garden hoses can not supply enough water to cool the engines the same way as when you boat is on the water.... I work for a company that makes chemical transfer pums. A few years back we were looking into making a rubber vein type pump like the merc one. I had an extra one at home so i brought it in for testing. You would be absolutly floored at what these pumps can put out!!! I dont remember the specs off hand and i know i posted them some time ago but this thing can pump water up and over 116' of head (height) at 3500rpm!!! So i woudl suggest to take the boat on the lake and see if still happens.
And lastly.....i would go and double check that you have the right thermostat in the engine. Most marine engines have 140deg stats and not 180 like cars do!! About every boat ive been in runs at about 150deg.......that is unless you have a closed cooling system.....

If your water system can't supply the volume necessary , I've seem the Merc pumps collapse a garden hose and almost totally shut it off .
Also if your have dual water pickups and don't close off the ones one the nose you can suck air in and that doesn't work either
Give it a try in the water

Thanks guys I really appreciate the advice. I will try back flushing the engine and taking it to the lake. Is there a particular hose that works the best? Also just to be sure, should I have my output hose (one going to oil cooler) connected to the lower fitting on my raw water pump. The lower one is marked "LH Out" and I believe the pump is rotating in a left hand direction.

Thanks again for the advice.

Jim

If you want water pressure, check out this home made contraption on CBPBA. Pretty cool.

http://www.cbpba.com/forums/showthre...&threadid=1728

On my Merc 454 Mag MPI, the oil cooler has honeycombs to stop most debris. This is what I flush to remove my impeller flaps.

7.4, 454 and 502 MPI's use 160. Carbed engines use 143, except for the 500 efi as you noted

What t-stat does the 496 Mag have in it?

Clamp your garden hose into the output of the pump and remove the hose that dumps into the thermostat hsg. If full gush of water does not come out the t/s hose the heat exchangers are clogged. check the input of the oil coolers.
If this discharge looks normal then the input is blocked.
Remove the plastic adapter where the 1-1/4 hose connects to the transom and look to see if the end of the transom to bell hose is crushed by corrosion if this is your problem, a permanite fix is available. Check out im654.

99% chance you got rubber stuck in the power steering or oil cooler. Backflush with hose.

Input/output. The rubber impeller pump pulls water IN, and then carries it 3/4 rotation around the pump to the output. Direction of rotation reverses the input and output.

Looking at the pump with the pulley facing you and the input/output facing away from you with the pump rotated so that the in/out fittings are on the bottom half....

Clockwise rotation means the input will be the one on the bottom left, and the output will be the bottom right.

Counter clockwise rotation means the input will be the one on the bottom right and the output will be the one on the bottom left.

Thermostat must have air bleed hole to purge air pockets. Some auto stats do not.