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Omc colling question

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andrewmaeyaert

Contributing Member
"hi. i rebuilt a 351W for a 78

"hi. i rebuilt a 351W for a 78 cobalt th 18. i put the engine in, and fires it up, but it overheated. i am assuming the impeller is good because there was plenty if water in the engine. when i started it, there not a lot of water coming out of the prop, and the exhaust manifolds got hot. the temp gauge read about 190-210 when i shut her down. any ideas?"
 
"also, to diagnose my problem,

"also, to diagnose my problem, i need to better understand the omc 800 cooling system. the engine is a 5.8. i have no idea how water gets into the engine, and i have no idea how the thermoustat is designed to work. does anyone have any advice or diagrams."
 
"the impeller doesn't just

"the impeller doesn't just put "water in the engine". It pumps many gallons per minute through the engine and out the exhaust taking the engine's heat away with it.

If the exhaust manifolds got hot, then the impeller is not moving the water. Probably because the rubber vanes are aged and have failed, or because rust and scale have plugged up the exhaust manifolds to the point where water can no longer flow through them.

Thermostat has nothing to do with water to the manifolds.
T-stat only controls water to the block&heads.

this is Merc, but similar:
http://www.perfprotech.com/store/App_Themes/PPT/images/tech-specs/rwc-half-flow. jpg"
 
"Ok the drive pulls the water

"Ok the drive pulls the water thru the intakes located on the lower unit, pulls it up bye the use of a 5 vane impeller, and thru a long hose inside the engine bay that is connected to the intermediate housing.

The long hose goes to the t-stat, then to the engine mounted water pump, that is driven bye a belt, thru the engine, and to the exhaust manifolds thru the risers and out the drive.

First thing you must do is check the t-stat, i would just go ahead and buy a new one and install it.

Then i would replace the impeller.Heres how.Not hard to do at all. I did it myself. And so can you. Have a chair or small table ready when you drop the lower. Drain the upper unit.

No need to remove cable. Lift drive up about 10 degrees. Remove top bolts.

Drop the exhaust and lower together. Place on chair right next to drive with cable. Careful don't pull the cable expensive. I tied the lower to my swim platform.

Pull the 4 bolts that hold the water pump and shafts. It comes out in all one unit.

Might be tough to get it out. I tapped mine out. Gently!

Do you have a manual?

If you want to call me on the phone or send me a e-mail and i will talk you thru it."
 
"thanks cheifalan. i think i h

"thanks cheifalan. i think i have a serious problem then because when i remove all of the hoses off the t-stat housing, and turn on the garden hose, water comes from the water pump, not from the intermediate housing. i think i will try replacing the manifolds, (because they are cracked. freeze damage), then the t-stat. if that doesn't work, then the lower unit water pump. if that doesn't work, then i will look at the circulating pump on the engine, and make sure the water is being routed through the engine right. if still no luck, i will do some more research. i will check back in tonight and give an update.

p.s. cheifalan, i may need to take you up on your offer to talk me through the water pump replacement. thanks"
 
"'The long hose goes to th

"'The long hose goes to the t-stat, then to the engine mounted water pump, that is driven bye a belt, thru the engine, and to the exhaust manifolds thru the risers and out the drive. '

With a small add on. If the thermostat is closed, the water will exit directly to the manifolds through the two thinner hoses. Depending on blockage in the manifolds and exhaust back pressure, the amount going to the manifolds should mainly be the same, and the manifolds should have even temp. If the thermostat does not close properly, it might be some disturbances, and even if the outlet to the manifolds does not hold the same flow (corrosion).
Running the engine at about 1200rpm unloaded, the thermostat will not open.This means manly cold water to the manifold. Then feel by hand or with a heat gun if any difference in manifold temps. If a difference, one manifold might be blocked, or one outlet from the thermostat housing.
If manifolds more than 5 years, change both, but keep the good one as 'spare'."
 
"The long hose coming from the

"The long hose coming from the intermediate that connects to the t-stat disconnect from the t-stat and leave connected to the intermediate, put water to the drive with the garden hose and start the motor point the hose that comes from the transom straight up how high it the water shooting you should get about 3 feet maybe more, a good stream.

You need do this for only maybe 3-5 seconds and shut off the motor, won't hurt anything.

If a good stream of water is coming from the hose then the problem is down stream of the hose, after the t-stat or the t-stat. Understand?

One thing you can check with all the hoses connected and water going to the drive and the motor running looking down on the ball gears / finger gears, there should be a good stream of water shooting on the gears. You can see it between the upper, and intermediate drive. If a good stream of water is present and a good stream of water is coming out the long hose disconnected from the t-stat then for sure we will have to track down where the blockage is, be patient we will track it down.

One last thing the drive must always be fully down and water going to the drive when ever you run the motor, always, there is no trim on the drive with a stringer. Some motors and stringer drives have a trim on the motor but no stringers have a trim on the drive.

Hope i was clear with the last paragraph."
 
"thanks cheifalan. it is rain

"thanks cheifalan. it is rain up here in fargo right now, so i can go out and work on things. i think i am going to run to the local junk yard, and pick up a set of manifolds to replace my cracked ones. i will keep you posted"
 
Before you install any manifol

Before you install any manifolds and rises make sure you check to see if the flappers/ shutters broke loose and is blocking the exhaust ! Very important!
 
"If its a 351w it should have

"If its a 351w it should have a heat exchanger on it. https://www.skidim.com/prodinfo.asp?number=RK025009D Water comes in the drive/foot and thru the raw water pump(sheerwood) INTO the H/E and out thru the riser. May be some blockage going on. Use Barnacle Buster to clean the H/E. Mine was full of mud. Also check the zinc pencil on the H/E. My 2cent.
282537.jpg
"
 
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