Help needed on cooling system

"ok, first off it is a 170 3.7

"ok, first off it is a 170 3.7 mercruiser with closed cooling system. i am getting ALOT of pressure in my cooling system. it blew a riser gasket so i bought a new riser/coolant tank and put that on to have another riser gasket start leaking on the new riser. so meanwhile my coolant is slowly leaking out of the riser and coming out of the reserve tank for the coolant from pressure. i am also getting what look's like identical to sand in my coolant from who know's where, the motor is entirely rebuilt cleaned etc. so nothing should be like that in the motor or any part of it. the heat exchanger was cleaned and pressure tested and it check's out ok. would the riser gasket leaking again that my just have been a bad seal cause this pressure in my cooling system and somehow let the sand/mud stuff in. i haven't been in any sand though only clean deep lake water and that should stay in it's own system anyway. i don't understand it, also would regular antifreeze be eating the aluminum up causing the sand stuff, any help is great, i just hope it is something simple and maybe the exhaust in the manifold is leaking in by the blowm riser gasket causing the pressure, dunno....anyone help me please"

"What temp T'stat are you

"What temp T'stat are you running? 170 F is what you want. A hot T'stat like in a car will run too hot and create too much pressure on the system.
The sandy stuff you describe may be a decomposition product of the anti-freeze. You should use only marine rated permanent anti-freeze, and use only de-mineralized bottled water to mix it with.
Rod"

"i were running a factory spec

"i were running a factory spec t stat but i think it is a 140 cause that is what the boat normally runs at. the sand is gritty, some big as a sixteenth of an inch or slightly larger, like little rocks in the sand. i took the t stat out today and put a torch close to it and had to wait a long time for it to open. the sand stuff were around the tstat too. i left it out put it back together and still when you take the cap off it has a ton of pressure built up so that when you crack the top it shoot's it out of the reserve thank that is two feet away. what about running it for a test run on just plain water and no tstat.. do i need the tstat.cause it's not like a car that you need it to get heat from the heater or sensor's for the computer. this motor has no sensors, computer, etc. just a plain old carb. can i just leave the t'stat out and run the antifreeze and mix that you recommend. it should get plenty of circulation that way. also if i leave the cap off the coolant tank on the riser, the thing seem's to run fine but will push water out the top alot when you first crank it. the cap is a 13 lb. cap which may be too much but it came with the new riser last week."

"You're fine without the T

"You're fine without the T'stat for test purposes, but you should put it back in for continuous operation.
If what you are finding is definitely sand, it must be coming from somewhere. Is it possible that the engine cooling jackets or heads got sand in them somehow during the rebuild operation?
Rod"

"nope, no where should be havi

"nope, no where should be having this in it. my only guess is that it could have been some built up in the intake and exhaust manifolds and is now breaking up.. i will keep flushing the engine some in the future and hopefully it will go away...but the pressure thing still bother's me. i am going to replace the riser gasket again tomorrow and use some sealant on it this time. hopefully it will all be ok then. is the cap at to great a pressure??? also what would it hurt to just run it without the t'stat... i can't seem to think it would hurt anything, it would only ensure that the engine runs at a cooler temperature."

"Running an engine too cold pr

"Running an engine too cold prevents the oil from gaining a high enough temp to vaporize off the moisture and fuel contaminents. So you need to run a T'stat, and if your engine is designed for a 140, that is what you should run. No problem in testing it without a T'stat tho for a short while.
I don't really see how sand could ever get into any part of the cooling system unless the boat has been run in very sand laden water at some time, or the parts were sitting out in the weather for a while and debris blew into them.
Rod"

"David:

Is the pressure c

"David:

Is the pressure cap releasing as designed? The gaskets blow if it isn't working since I assume you have all new hoses with the engine rebuild. Coolant heats up and pressure builds, cap doesn't release and pop go the gaskets.

"motor is entirely rebuilt cleaned etc. so nothing should be like that in the motor" I would think that if they sand blasted the block or heads to clean them that they would have removed all the sand. Sadly, it's not always the case. You may need to pull the freeze plugs, drain cocks and waterpump to flush the block. Also backflush the heat exchanger.

Guy"

"Have the cap tested or replac

"Have the cap tested or replace it. If that doesn't fix it you may want to run a compression test and leak down test. I know you say it's a fresh rebuild, but I have seen head gaskets blow and cause the same things you are explaining with the pressure.
Jack"

"the cap is good and the oil i

"the cap is good and the oil is clean as can be so i think it's not head gasket. the motor is fresh with only two or three hours on it. i seem to think it is the riser gasket causing this problem. i am going to put a new one on tomorrow and use some good sealant to get it all sealed up. then it is going to sit for a few days and recheck torque and fill with fluids. maybe i am filling it too full to start with. i filled the resivoir up to top and coolant tank up to top, figuring when the t'stat opened it would go down but it could have created a big air pressure pocket and then blew the riser gasket and forced coolant out through the working cap and through the overflow line into the coolant resivoir thus forcing my coolant to blow the cap off of that since it is only a little plastic cap. i hope that is the culprit. should i run it to warm with cap off and slowly filling with coolant??? and do i need to leave it a little shy of full in the resivoir and a little shy in the coolant tank????"