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Overheat at full throttle

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switlikbob

Contributing Member
"I am still having issues with

"I am still having issues with my motor getting hot at full throttle. I have a new raw water pump, net thermostat, new suction hose for water induction. The only thing that I haven't changed is the engine water pump (I have a heat exchanger). Is this my problem?"
 
"I assume that you have verifi

"I assume that you have verified that the o-ring between the lower and intermediate units and also the water hose connector and casket are in good condition and that the grids in the outdrive are clear.

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How do you do the first thing

How do you do the first thing you mentioned Pescador? Do you have to take more than the lower unit off?
 
"If you follow the instruction

"If you follow the instructions on the Seloc or Clymer repair manuals, you have to take the whole thing apart. However, by looking at the drawing, it seems possible to just remove the lower gear unit and replace the seals, as long as nothing moves while trying to put everything together again."
 
"Take your intake hose off of

"Take your intake hose off of wherever it's going to in the boat, not off the chrome tube but where it's headed. Now put a garden hose into that hose somehow and clamp it on. Backfeed the system and you should see the water come from the lower vents. Now take something, I use some old rubber from an innertbe, and cover those vents by hand as tight as you can, you're gonna get wet. Look at the drive all the way up from the vents and there should be no water leaking out anywhere but what you can't hold back. I know it's not to technicle but hey it does find some leaks occasionally. This is checking what EL said."
 
"Boat Doc, it works and that&#

"Boat Doc, it works and that's what counts. Someone else suggested recently covering the intake grids with duct tape, and that should also work."
 
"Thanks for the replies...I am

"Thanks for the replies...I am running a 280B drive and GM 350 5.7L motor (carb) with electronic ignition. El, all of the things you asked me to check have been checked and a new o-ring was installed, new neck was also installed. I used to have a clear hose that ran from the water intake to the raw water pump...you could see the water coming in at a good rate and watch its flow increase with RPM. The clear hose was actually collapsing due to the suction force of the raw water pump, so I purchased a reinforced suction hose, which helped a good bit. Is it possible that my engine water pump is rotted out and is cavatating at higher RPM?"
 
"At this point, I would start

"At this point, I would start looking at the exhaust elbows and then the mani's. If you can't get the hot water out, you can't get the cold in."
 
The exhaust manifolds and rise

The exhaust manifolds and risers are brand new and run in fresh water. What could be wrong with the elbows? They shoot water out at a nice rate...the manifold are actually not even too hot when the RPM's are up...it's just the motor that is getting hot...
 
"Boy, you have a tiger by the

"Boy, you have a tiger by the tail. After all you have done and replaced maybe the circ pump is bad or possibly the wrong direction. I allways have a good used one laying around my shop so I would just stick it on, without buying a new one,and try it but with all you have done what else could it be. Sorry for no better answer."
 
"Bob, by elbows...are you refe

"Bob, by elbows...are you referring to the risers?

Also, since your engine runs on a heat exchanger (mine does, too), are you also running fresh water through the manifolds? If so, you need a larger raw water pump than the one originally supplied with the engine, and in some cases a heat exchanger of higher capacity. In my case, because of those requirements, I decided to keep my engine manifolds and risers raw water cooled and use the fresh water cooling only for the engine block and heads."
 
"By elbows, I am referring to

"By elbows, I am referring to the rubber elbows that stick out of the transom that release exhaust and water.

I have a very large raw water pump that I added new a few months ago, so I doubt that is my problem. The heat exchanger that I have is also quite large (takes at least 2 gallons of antifreeze, but I can't remember how much exactly). The exhaust and risers are raw water cooled as well, and never seem to heat up. It seems that I am running the same setup as you are..."
 
"Good thinking El!! Had'nt

"Good thinking El!! Had'nt thought of that yet, but his reply kind of kills that theory with the raw water cooled exhaust system. Wish we could get this figuered out for him. What do you think of the circ pump idea? Is it known GOOD and the right direction? I think most all MARINE pumps are bi-directional but not sure."
 
"Bob, if I read your post corr

"Bob, if I read your post correctly you are running a through-hull exhaust instead of running the exhaust and water through the outdrive. Is that correct?

If the manifolds run cool, that is an indication of a good raw water flow; in which case, it seems to me there is not sufficient heat transfer taking place at the heat exchanger. Possible causes that I can imagine include a fouled heat exchanger, a thermostat of the wrong heat range, or not sufficient coolant circulation due to a defective circulation pump or to the belt slipping on the pulleys."
 
"How about a block that has a

"How about a block that has a lot of sand and or other crap partially filling it up and not getting enough heat transfer. I've come accross that working on river jet boats. Popped the freeze plugs (casting plugs) and low and behold, filled with sand. Just a thought!!"
 
"Thanks for the replies...El,

"Thanks for the replies...El, I am running my exhuast through my 280 drive...the exhaust elbows are for the exhaust bypass I think...you know, the little 4 inch curved rubber elbows that stick out on either side of the outdrive.

The heat exchanger was taken apart and inspected before I installed the new engine, and it was fine. I am running a 165 deg thermostat. I might pull the thermostat completely and see if that helps with the high temp situation.

BoatDoc, I am not sure of the condition of the water/antifreeze circulation pump. I transferred it from the old motor. I am planning on changing it, and I am pretty sure that it is an automotive water pump. However, that should not matter much as it is running antifreeze, not raw water."
 
"Bob, so by 'elbows' y

"Bob, so by 'elbows' you are referring to the double bypass rubber outlet hoses. Aaaaah, got it now!! Yes, I think you are correct and both of us have the same setup. I am also running either a 160º or 165º thermostat (cannot remember now) and the coolant temperature is constant (about 180º) whether the engine is idling or at WOT. How "hot" is "hot" in your case?"
 
"Boat Doc, I too am under the

"Boat Doc, I too am under the impression that circulation pumps for standard and reverse rotation GM V8 engines are identical, but I could be wrong. Also, even if a marine circulation pump has bronze impeller and stainless shaft, an automotive pump can be safely used in fresh water cooled blocks like both Bob and I have. Like Bob's, the circulation pump on my 350 is automotive as well."
 
"El, my engine gets to about 1

"El, my engine gets to about 160 when warmed up and runs at 160 all the way up to about 3400 RPM. At WOT, which is about 4100 for me now (a prop change is in the works), it has gotten up to 200 deg. I always throttle down at that point to allow the engine to cool back down...the engine has never actually "overheated", just gotten hotter than I like it to. I always have my eye on the water temp gauge. My wife was driving the boat this past weekend and I decided to look over the stern...I saw a considerable amount of steam coming up from the outdrive...the engine was at normal operating temperature...is that normal?"
 
"Bob, if the prop is too big y

"Bob, if the prop is too big you may be overloading the engine. Also, what you see may not be steam but exhaust gas mixed with water, which would be normal."
 
"I am running a 15x16 prop rig

"I am running a 15x16 prop right now. I really don't think it is putting too much stress on the engine. This has to be a cooling issue, not an overload issue. The next time I go out, I will pull the thermostat to see what kind of water temps I get...I'll post results...this issue may be as simple as putting a 140 T-stat in..."
 
"Gas engines produce more powe

"Gas engines produce more power and better fuel economy at higher temps. Changing to a low temp thermostat is not beneficial to your engine as it tends to defeat an important purpose of fresh water systems. With heat exchangers, the engine thermostat can be, and should be, 20-30% hotter than a salt water cooled engine.

.It sounds like nothing is wrong here but as a process of elimination it would give you some piece of mind to replace the circulation pump. You have a large engine which produces a lot of heat. If the exchanger does truly hold 2 gallons(3.5-4 gallons total) it should be adequate. Just make sure the antifreeze ratio does not exceed 50%. If it does the engine will run hotter than normal.

As an aside; most commonly, the rubber bypass valves on the outdrive are called "flutter valves" or "duckbill" valves. I'm not sure the boating community uses these terms, probably not"
 
Did you ever solve the problem cause I'm having the same issue since I put a set of aluminum heads on. I changed raw water and circulation pump and hoses n still can't find problem. It ran normal below 180 at 3800 rpm but 4000 to 5000 temperature rising up to 200 degrees n the longer you hold it goes upto 207
 
switlikbob said:
"Thanks for the replies...I am

"Thanks for the replies...I am running a 280B drive and GM 350 5.7L motor (carb) with electronic ignition. El, all of the things you asked me to check have been checked and a new o-ring was installed, new neck was also installed. I used to have a clear hose that ran from the water intake to the raw water pump...you could see the water coming in at a good rate and watch its flow increase with RPM. The clear hose was actually collapsing due to the suction force of the raw water pump, so I purchased a reinforced suction hose, which helped a good bit. Is it possible that my engine water pump is rotted out and is cavatating at higher RPM?"
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Is this a salt boat. You could have the water pump impeller losing its bends that throw the water. Or rusted off. and i assume you changed the rubber circulator at crank shaft. water pump is easy enough to change or take off and inspect.
 
I think the issue is more than water pumps cause I change two raw water pump and circulation pump. So either raw water pump suck air from somewhere or something obstructing the thermostat from open fully.
 
Found in my raw water pump on front of engine that the rubber vanes had 3 rolling backwards probably due to checking timing marks and rolling engine backwards slightly.
 
Ken P said:
Found in my raw water pump on front of engine that the rubber vanes had 3 rolling backwards probably due to checking timing marks and rolling engine backwards slightly.
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I assume you are talking about the rubber impeller. I thought about that on my fresh build after my motor was in and i have to rotate the engine to get top dead center to drop distributer. I had to take that off so i could rotate the motor and check it to see if those things pop back and forth and they did. Did you use a new one with the lube they send with it . I think it's the lube that makes it happen. Sorry for the long post but im glad you figured it out.
 
markusbr549 said:
I assume you are talking about the rubber impeller. I thought about that on my fresh build after my motor was in and i have to rotate the engine to get top dead center to drop distributer. I had to take that off so i could rotate the motor and check it to see if those things pop back and forth and they did. Did you use a new one with the lube they send with it . I think it's the lube that makes it happen. Sorry for the long post but im glad you figured it out.
Click to expand...

The lube is Glycerine and shouldn't make a difference WRT reverse direction of the vanes, but the easy way to test this is by removing the impeller, cleaning it and reinstalling it with lube AFTER the districutor has been positioned correctly. Theoretically, the water pressure should move the vanes into the correct position once the Glycerine is washed away.
 
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