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74 Mercruiser overheating

marky1212

marky1212

Member
"I have a 87 7.4 alpha 1 sea w

"I have a 87 7.4 alpha 1 sea water cooled that slowly runs up to 200 degrees with water ears on or in the water. I have replaced sea water pump,motor water pump,thermostat (140 degrees), temp guage and sender. I have taken the risers off and checked them and they look ok. The riser on one side does run about 30 degrees warmer than the other 130 verses 100 on the other side. The hose coming out of the bottom of the exhaust manifold on one side is about 190 and 160 on the cooler side. Water coming out the transom is about 130 with my infrared gun, yet the boat guage says 200. Im stumped! PLEASE HELP!!! THANKS."
 
"T'stat installed upside d

"T'stat installed upside down? Rust & scale buildup in the T'stat hsg.? Water is not getting out of the engine fast enough as evidenced by the 190 deg. water going to the bottom of the exhaust manifold. difference in temps is ok but not the high temps. Check your T'stat. installation. There may be a plug on the bottom to inspect the inside for crap. I would just replace the hsg. assy. if it is all crudded up.

Is this picture your T'stat hsg? Item #13 is a double check ball valve sending water to the upper exhaust. See if the balls & spring are working.

http://www.mercruiserparts.com/Show_Pictures3.asp?dnbr=%2014296&ivar=images/CRUI SER/14296/25.png&inbr=938&bnbr=140&bdesc=THERMOSTAT+HOUSING%28OLD+DESIGN%29"
 
"Thanks Guy, Yes, I belive tha

"Thanks Guy, Yes, I belive that is mine. I know it is the older style they used then. Therm. is in correct and there is no crud in housing. Even tried without a thermostat one time and did the same. The spring loaded ball valves seem to be OK also. Let me ask you, on both hoses going to riser and bottom of manifold is the flow coming from engine on both hoses?"
 
Cool water comes out of the to

Cool water comes out of the top cover to the elbows while mixed water goes to the bottom of the exhaust manifolds. Have you lasered the temp of the intake manifold near the base of the T'stat tower? If it is near 200 deg. then maybe you have a bad T'stat or the wrong temp T'stat like a 195 deg. car type.
 
"Yes I did laser that area and

"Yes I did laser that area and it was close to 200 degrees. I bought thermostat from a marine store, its a quicksilver,merc. 140. I even did the stove top water test and it opened at 140.
CRAZY!"
 
"Was the impeller you removed

"Was the impeller you removed in one piece? If not then a chuck of it is blocking sea water going into the T'stat hsg. W/a cool engine, remove the water inlet hose from the T'stat hsg. and start the engine to see what kind of impeller pushed water flow there is; try to fill a gallon bucket in under 30 seconds."
 
"I just recently bought this b

"I just recently bought this boat and it had this problem when I got it. The previous owner did give me paperwork on a recently rebuilt lower unit so who knows. The impeller I just took out was all there, but I wil check that water inlet hose tommorow. Thanks again."
 
RE: " slowly runs up to 20

RE: " slowly runs up to 200 degrees with water ears on"

My 5.7L sucks more water at idle than a garden hose can supply so I only run it quite briefly on a hose. You still have the "in the water" issue however.
 
"Well I only had the boat in t

"Well I only had the boat in the water once and it was for a very short period as a test. The temp went up slowly, close to 170 and then I broke water pump belt and of course it did get hot and I imeadiatly shut it down. It happend right at dock, so I am just guessing it does the same thing in water. I just sold a 1989 350 merc. and I could run that thing all day with the ears on and it would stay at 145. Maybe this big block isn't drawing enough water through ears. I did the test today that Guy mentioned in the last post and I am getting over a gallon of water in less than 30 seconds form the sea water pump at engine. Have you or anyone heard of clogged water passages in the intake manifold? Thanks"
 
Mark:

If the water pump is

Mark:

If the water pump is pushing that much water into the T'stat for distributon then I would suspect a blockage in the T'stat hsg. assy. or the exhaust system. The water is not exiting the engine fast enough.

Have you ever had the exhaust manifolds and elbows cleaned at a radiator shop? How old are they? Looking good and doing the job are two different things. Is the circulating pump hose or other hoses collapsing? What about the water inlet hoses?
 
"Well, after everything I repl

"Well, after everything I replaced, i'm pretty sure it was just a matter of water pressure at the water ears. The boat I just sold with a 350 merc always ran cool with just about half of the water turned on, so I figured the same with this boat being the same outdrive and only a bigger engine. Then I read Roberts post and figured I would try wide open water at the tap and also made sure the ears were very tight with no leakage, sure enough it only got up to 160 tops. That big block must SUCK SOME WATER! Thats really weird. I am going to try and put it in water tommorow for the real test. As I mentioned before the only other time I had it out it seemed to start running warm and then tossed a waterpump belt. Now all belts and pullys are new. Thanks for all your replys to this post. I almost feel kind of stupid
uhoh.gif
. I will let ya know how it does in the water. Thanks again."
 
"I don't remember the exac

"I don't remember the exact numbers, but they are buried in the MERC shop manual (avail on line thru a link on this forum, THANX!!), but for the 5.7L engine w/FWC you are looking at 10 to 15 gallons per min per 1000 RPM...The 7.4 would need more."
 
Took the boat out Saturday and

Took the boat out Saturday and it ran 165 degrees at low RPM to 2500 and 175 degrees up to 4000 RPM. Does that sound OK for a 140 degree thermostat?
 
Not in my opinion. My 143 deg

Not in my opinion. My 143 deg.T'stat stays on the number at all RPMs. I still think you need to have the exhaust system cleaned or you have air entering the water poscket in the OD. Air in water won't transfer heat.
 
"Maybe your right. A person on

"Maybe your right. A person once told me that you can take water line off the bottom of manifold and run it in bilge. If it still runs warm, its not manifold. When I put the thermostat in, I could blow through the hose to manifold. Does that mean they are not clogged? Also when all this overheating crap started I took the risers off, inspected all passages and surfaces and all was good and free flowing."
 
"Another thing I forgot to men

"Another thing I forgot to mention is whe it did get to 175 at high RPM it would stay there when I backed off throttle to low speed, and then when I nailed it, it almost instantly droped to 160 then slowly back up"
 
Corrosion buildup in the passa

Corrosion buildup in the passageways of the exhaust manifolds and riser/elbows can act as insulation preventing the full transfer of heat to the cooling water. At the same time there seems to be sufficient room for water flow. HOW OLD are these parts?
 
I just bought the boat and don

I just bought the boat and dont know. I do know the previous owner had it for 2 years and did not put manifolds on it. I'm sure they are over 5 years old. When I was putting spark plugs in I noticed a couple of the bolts that secure the exhaust manifold to head were almost rounded off from rust (dread the day I have to change them) so i'm sure they are pretty old. For some reason though the risers look newer than manifold.
 
"head were almost rounded

"head were almost rounded off from rust (dread the day I have to change them)"

Grind or cut the rounded bolt heads off and slide off the manifold; reinstall w/new bolts after cleaning or replacement.
 
"I will try that but their pre

"I will try that but their pretty recessed next to the exhaust tubes on the big block. The small blocks are more out in the open. So whats your opinion for replacements? GLM,Osco......?"
 
I prefer Merc. OE parts. Look

I prefer Merc. OE parts. Look on eBay for Merc. new "take offs". They come from new "drop-in" engines when repowering.
 
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