Overheating mercruiser 3.7 ?

I have a question I hope someone can help me clarify an overheating issue with my 1987 Wellcraft with a Merc. 3.7l closed cooling system engine. Went out last week after the boat was stored for 2 years, ran good for 2 hours then started to lose power at higher RPMs until the engine stopped and locked up. My temp gage never seemed to work correct since I bought it new but didn't show an overheat but this is what I suspect happened. Got towed out of the water as the starter just clicked and could not turn the engine over. Next day with the muffs on the engine started fine in the driveway. The coolant level in the engine is ok and I see no leaks. I took the lower unit apart assuming is was a sea water water pump issue and the pump I took out was in good shape but I replaced it anyhow. I noticed the the hose for the pump that is in the bellows area was kinked and I have Unkinked it with a fat Wire tie to hold it open and hoping this works as I see no way to change this hose without taking the boots apart. My ? is if I disconnect the hose from the sea water pump on the inside of the transom and run the engine with muffs how much water and pressure should I have at idle. When I checked this before it was pumping but not very hard, I feel it should be like a garden hose spray or an I wrong? The heat exchanger and the thermostat were replaced around 2 years ago so I feel they are ok. Any help would be appreciated as now after the repair I'm afraid to break down again.

Thanks,
Gman

Re: Overheating mercruiser 3.7 ?

change the thomostat , 2 yrs is a long time for a marine thomostat .

Re: Overheating mercruiser 3.7 ?

i had almost this same problem with a boat i bought recently, turned out it was a combination of a few things, i had a bad sea water pump but you said yours was ok, if i were you i would replace that thermostat, i agree 2 years sitting without use is a while, also i found that my boat had a leak in the back of the heat exchanger. seemed like it wasnt stored correctly and froze which bent the rear cover enough for the sea water to pour out and seeing the previous owner had an auto bilge he never notice, he wasnt to smart, after i adjusted the exchanger cover it seemed to run perfect. just a few thoughts...

Re: Overheating mercruiser 3.7 ?

seems odd that a thermostat would change or go bad after 2 hours I would think it would have opened within the first 5-10 min then stayed open maybe? or not opened at all. If it overheated . . . did it smell hot like burning paint? I would all so run a compression check to compair all cylinders to norm. I would also check for any blockage in the intake. I have a closed cooling system and my water intake for the SWP is under the hull not via the out drive if yours is the same pull all hose on the pump and check for junk. assuming the engine over heated the pistons would have swollen which might have galled a cylinder which will casue low compression, then after it cools the piston shrinks and it runs again.

Re: Overheating mercruiser 3.7 ?

gman653 said:

.......My ? is if I disconnect the hose from the sea water pump on the inside of the transom and run the engine with muffs how much water and pressure should I have at idle. When I checked this before it was pumping but not very hard.....

Thanks,
Gman

Click to expand...

I have the same boat and engine. I too was in doubt as to the quanity of water being sent to the cooler from the sea water pump. I replaced the hose from the transom to the cooler inlet with a section of nylon reinforced clear tubing to keep an eye on things. To kinda answer your question, when on muffs and the garden hose (3/4") wide open, I can see air bubbles in the clear hose any time the engine speed approaches 1000 rpms. The garden hose CAN NOT keep up with the sea water pump's demand. On the other hand, set the leg in a sawed-off plastic barrel of water (or underway) and I NEVER see bubbles. That sea water pump can move a ton of water !!!

My overheating problem was caused by an exhaust flapper remnant lodged in the lower unit just in front of the prop. As a precaution, I up-graded my cooler to the 4" version and things have been 'cool' since.

FWIW....Ed in 'ol Virginny

Re: Overheating mercruiser 3.7 ?

I installed a T in my seawater intake hose before the pump with a ball valve between the intake and the T. Using a garden hose, the pump pulled so hard the the garden hose collapsed from suction so I made a 10gal plastic water tank held about 8 feet up then used a 1 inch i.d. ribbed hose which I got at home depot that was made for fish pond pumps. then connected the tank to the T via the ribbed hose and let gravity feed the intake. At idle it will empty the 10 gal tank in about 20-30 seconds.

Re: Overheating mercruiser 3.7 ?

I also use it to wash out saltwater with salt elimnator

Re: Overheating mercruiser 3.7 ?

Oh.....one more thing !!!!

Get that temperature guage fixed !!!!! It's the pulse monitor for your engine !!

Ed in 'ol Virginny

Re: Overheating mercruiser 3.7 ?

Thanks for the quich replies, I just got done putting the outdrive back together and testing the pump and shifter after having it apart. I ran it and seem to have better flow from inside the transom than before but not sure if the overheating problem is solved as I need to have it at higher RPMs than I can do in the driveway. I didn't smell anything burning as was asked in a post and the engine seems to run fine. I would like to ask a few ?s to make sure I got the lower half on correctly as this was the first time I took this apart. I read about making sure sure I was in forward gear and held the prop counterclockwise as instructed by the manual. I lined up the shifter splines and the water tube and got it together with no problem .It shifts ok with it running into forward and reverse and neutral. My ? is with the motor off and the shifter in forward the prop locks counterclockwise but spins clockwise, In reverse it doesnt fully lock but clicks when turning but with the engine running It spins correctly with no grinding , is this normal or am I off a tooth? One other ? with the muffs on and the engine running I get good water flow through the transom but I noticed water coming from a hole right below the shifter pin and sleeve that must line up when putting the outdrive on, this hole is in the lower unit. Im not sure if this is normal as I didn't notice this before I took things apart.there is also a small hole located near the upper vent plug that is removed to fill the outdrive with oil, is water supposed to come out of there too. Mine is not. I'm hoping I don't have to drop this down again. Thanks again for any help.

Gman

Re: Overheating mercruiser 3.7 ?

i notice something you said something about "upper hole to fill the oil" . . . Im not sure about this but I read that you fill the out drive from the bottom drain hole with a pump that pushes the oil in from the bottom to prevent air bubbles ??

Re: Overheating mercruiser 3.7 ?

Yes I did fill from the from the bottom, this is the upper vent hole that is left out when filling from the bottom until it comes out the upper vent. The smaller hole looks like it's about 1/8 inch in diameter and located beside the vent, not sure what its for .
Thanks again,

Gman