Water Pressure Question

[mercurymang]

I have a 75 merc 650 (3cyl) and have recently replaced the baffle, outer cover, thermostat, impeller, etc. The tell tale comes off of the base before entering the block so if there as a restriction in the block, you would not know it by the tell-tale. I have been advised that on this motor they can build up steam pockets in the upper cylinder jacket and cause overheating. One solution is to plug the factory tell-tale outlet and drill a new one in the block above the top cylinder. This would allow any trapped air/steam to escape and the water to get all the way to the top of the block.
Before I go drilling holes in my block, I thought I would throw this out for comments and see what opionions some of you may have.
Additionally, I would like to remove the factory plug in the block and install either a temp guage or a pressure guage.
At any rate, it seems wierd to me that merc would design a block where air or steam would get trapped at the top of the block causing water to not flow there.
I would appreciate you thoughts on this matter.

 

[CrossCurrent50]

Re: Water Pressure Question

Can you post a picture of the tell tale hose I on my 70 hp have the base hose to the fuel pump then out the rear ports that spit water almost imediatly as it doesn,t come from the engine ports. Then I have the tell tale outlet at the top of the rear cover on the block that after the water fills it up goes out the tell tale. I have removed the rear cover plug and installed a temp sensor and gauge in my dash. My water stream is luke wark but not hot as it should be. Did you use gasket sealer on the exhaust baffle and cover?. There are water ports going from the outer cover and through the baffle that let water circulate into the rear cover. If these ports become restricted and clogged the cool water nessecary to cool the exhaust baffle will not circulate and cause a hot stream and steam out the tell tale. If your engine doen,t have a plug for a fitting for a tell tale It might be a great idea for you to add one there.Top of the rear block

 

[mercurymang]

Re: Water Pressure Question

Your tell-tale setup that comes off the bottom plate and goes through the fuel pump is just like mine. The other one looks like what I'm talking about and maybe a later design which incorperated a port at the top of the block. This means that perhaps this is a valid concern that Mercury attempted to address by putting a tell-tale port at the top of the block.
The port that you have at the top of the block, does it appear to be a factory port or does it look like someone installed it. What year motor do you have and could you possibly post a close up pic of where it goes into the block?

 

[CrossCurrent50]

Re: Water Pressure Question

I removed the nylon fitting and added a brass fitting as I was going to put an adapter for the temp sensor there but I tapped the rear cover for the sensor I bought off ebay to accomadate it 12 mm. This is a 1979 70 HP I can take better photos for you soon Gauge

 

[CrossCurrent50]

Re: Water Pressure Question

Pictures of fittings hoses temp sensor The tan wire is for the temp sensor sender to gauge.

 

[mercurymang]

Re: Water Pressure Question

Let me take a different tack with this thread. I have done alot of work to get this motor running like it should and I'm very concerned about getting it out on the water and burning it up. Probably because I have read so many stories of this happening. The biggest concern I have is that I can't really tell if everything is working properly. My tell-tale lets me know if I have water pressure but that's about it. What if my thermostat stops working. How can I tell. Is putting a temp guage in the block really the only way to know if I am running hot or not? Is there a reason why that would not work?

 

[oldman570]

Re: Water Pressure Question

On the head water jacket near the top of the motor there should be a brass plug that you can remove and put a 1/8" fitting into and cut the hose for the tell tail and place a 45 degree Y fitting into the tell tail hose and this would vent the top of the waterjacket aera at the top cyclender if that is all you want to do. I used a T fitting and nipple to place a temp sender and pressure gauge on my 87 70 HP and it wors just great. The motor should have a overheat alarm on it from the factory that will sound when the motor gets heated to around 180 degrees and this shoul let you know of any overheat is happening. A water pressure gauge would let you know if there is not enought water flow or if the impeller is getting worn bad. A new impeller pump shoud put out close to 4 lbs. pressure at idle and should go close to 20 lbs. at WOT. Anything lower or higher is a inducation that something is wrong. JMO
Oldman570

 

[mercurymang]

Re: Water Pressure Question

Thank you. I don't believe my motor has an overheat alarm as I don't have anything currently that would measure engine temp. I do have the brass fitting and I played around with it yesterday. I found a 1/8 NPT connector with a hose coming out. I started the motor on the muffs and got a stream out of it. I think I would like to use this for a temp gauge (what do you think). Yesterday I was running the motor and just using my fingers on the block to gauge how hot it was getting. It was getting pretty hot but it may have been normal. Without a temp gauge it's hard to tell.

 

[Moody Blue]

Re: Water Pressure Question

With the waterpump operating properly, you should be able to keep your hand on the exhaust port cover for quite a while. It should never get hot, just mildly warm.

 

[mercurymang]

Re: Water Pressure Question

Ok, The outer exhaust cover was fine but the top of the block was fairly hot.