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AQ145 overheating - need help

n36511

n36511

Regular Contributor
Ok, so now that I have my points corrected and timing done, the engine runs great. However it now overheats if I rev over 2500 rpms while underway. I have changed the impeller, thermostat, cleared old impeller parts from strainer and oil cooler and still have the same results. The engine, an AQ145A has both the closed cooling and raw water cooling. From the closed cooling system cap, while running there is a slight drip that comes from the cap. I have replaced the cap a few times and that did not help. Also, looking down in there, while the engine is running, the water just appears to be sitting still (was expecting to see some flow of water). Here is what I am thinking of doing next and would like some feedback from you guys:

1. remove and inspect circulating pump (i already have a spare so this seemed like a logical step)
2. remove and inspect exhaust manifold
3. check raw water line from outdrive to transom plate (I get a good flow of water to the oil cooler so I am not sure that this is raw water side related unless it is sucking air while moving forward as apposed to sitting in the slip)

The boat is a 44ft houseboat so removing it from the water is a 500$ affair each time so any outdrive work will be last unless it can be done in the water :) .

Thanks for the help
Scott
www.boaterstoolbox.com
 
Hi Scott, I had this on my AQ145a, very much the same with that if I slowed down the engine cooled down again. It turned out to be the inlet pipe on the transom near the outdrive. When you go over a certain speed a "well" of water is created at the back of the boat and the pipe sucks air. When you slow down the "well" ceases and the pipe now sucks water again and the engine cools down. I just replaced the pipe and the problem went away. Hope this helps

Nick
 
No sorry Scott the pipe is up behind the outdrive helmet so I would say it has to be done on the hardstanding, but I could stand to be corrected.
 
ok so I have an update and need some more help. I was able to run the engine this weekend and the problem appears that enough of the coolant is leaking out and that is causing the overheating issue. It took about 1/4gal to top it back off this morning. I cranked the engine and before it even reached normal operating temp the coolant was leaking from the cap. I removed the cap, and the water level would just slowly rise and then start overflowing. There are no bubbles in the water nor is there any oily film in the water. Any ideas on where to go from here???
 
One of the most frequent causes of overheating while underway is what Nick said. It is common for the sea water pickup snorkel to develop a leak. Less common problems include heat exchanger blockage, failed thermostat (take it out and run the engine as before) and worn out exhaust manifold. The manifolds don't last forever and are suspect after about 5 years. fwiw. Tom
 
The circulation of the water in the raw (sea) water sytem and the circulation in the closed (engine) cooling system has nothing to do with one another, save for exchanging heat in the heat exchanger. Therefore, if you do not have circulation in the closed(engine) cooling sytem, look for the problem in that system and not the raw water system.
It sounds like you have a faulty thermostat. Take it out and run the motor again to check if the water now circulates. If it does, replace the thermostat with a new one.
It is highly unlikely that the impeller of the engine water pump (NOT the raw water pump) is faulty, but not impossible. If you get no circulation with the thermostat out, this could be the problem. Anyway, make sure the normal working of the engine waterpump is not impaired. You do say there is no bubbling etc, so one could rule out cavitation of the engine waterpump due to air in the cooling sytem. (Cavitation of the impeller would also impair water circulation.
In all likelyhood you have a defective thermostat.
Schalk
 
Thanks for the replies... I have an update that may help. I think I have been hunting down multiple issues. I removed thermostat and no change. I replaced the radiator cap with another one and that seemed to help reduce the leak. I then went back to the basics and pulled the raw water impeller again and it was toast. I replaced it and fired it up. I removed the raw water strainer cap and noticed bubbles coming from it. So I am thinking that the coolant coming from the radiatior was due to overheating in the expansion tank and a faulty radiator cap. And I think the raw water impeller failed again due to air entering the line somewhere. Due to the size of the boat I cant pull it without professional help and $400. I cranked the engine, got in the water with a syringe filled with a die and a mask on and injected the die around the water intake and didnt notice any being sucked in....which is telling me that it is more than likely the tube between the foot and the housing. Any thoughts on my theory? There should be zero bubbles in the raw water line right?
 
I think you are confusing the raw water system with the engine cooling water system. I you do not have circulation through the raw water system then, of course the engine cooling water will start to overheat. Much like the water in the car radiator not getting cooled by air passing through it, your boat engine is not getting cooled because no raw water is passing through the heat exchanger.
However, if the engine water pump is working the water should still circulate in the engine even if there is no flow through the raw water system. Similarly, the water in a car radiator will circulate even if there is no airflow through the radiator.
The fact that your raw water impeller is toast confirms that the raw water system could not operate, therefore the water in that system will remain static if you take off the cap to inspect it. The only explanation can be that you have confused raw water with engine cooling water. Hope you catch the spook.
 
Scott, I believe what Nick is referring to would be the "S" hose connects to the Water Neck fitting. These Water Neck fittings need routine replacement, as corrosion gets to them in short order.
This part does come up and out of the water when on plane, and that is usually when the over-heating issue comes up.

The upper pivot tube bushing in the main suspension fork is also important. If this bushing allows for any slop, the water neck fitting special "Beaded Gasket" will not form a suction tight seal.... especially when this gasket "bead" looses it's resiliency.

There is also an O-ring at the lower-most portion of the pivot tube that can occasionally cause a suction breach issue.

I'll suggest that the sea water system can have an effect on the closed cooling side, when the sea water side cannot dump enough heat from the Ethylene Glycol side.
But I do agree, that both sides must be looked at independently.

The cap that you mention water overflows from, sounds like the sea water strainer cap.
This would be normal to see water here.

The Heat Exchanger "coolant" cap should never be removed when the engine is warm/hot.

Is your E/G coolant recovery reservior working correctly?

.
 
Problem fixed!!!!!!!!!! Ok so it was multiple problems. First problem, closed cooling radiator cap was faulty. Replaced. Second issue was that new impeller was toast. Replaced. Opened the raw water cap (the one beside the closed cooling cap) and noticed a lot of bubbles coming out of it. Bought some clear tubing and routed it from the copper line to the raw water strainer (this is the strainer before the sea water pump). Cranked the engine and observed large amounts of bubbles flowing thru the clear tubing...... I then use the clear tubing to bypass the strainer (connected directly from the copper tubing from the sea water pump to the tube that comes out of the bellhousing in the engine compartment). The bubbles disappeared. I took the boat out and ran it at WOT and it has never run this normal!!!!! It appears that the plastic sea water strainer has an air leak that was hard to detect due to it being on the pull side of the water flow instead of on the pressure side of the water flow!!! Thanks to everyone that assisted in the troubleshooting! I am going to have a beer now! - Scott
 
Must have been lots of air bubbles....:) These impellers are water lubricated and will fail rather quickly without any water, damhikt. But I have had them survive with less than optimum water flow, a few bubbles...Good that it's fixed!
 
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