Parts being thrown at it and it still overheats when throttle is given...

[rhillman75]

So here is a follow up since my last thread. Referring back to where I have come from, this is a 1989 Mercruiser 5.7L. On our last boating last year the temperature began to rise when throttle was given. When it was brought back down to idle it cooled off. In the last weekend I have replaced the impeller/housing, water pick up hose from the upper drive to the transom (other hose was cracked in a few spots), and changed thermostats from 160 degrees to 142 degrees. After running on the muffs and getting warmed up, the temperature will still rise when throttle is given up to 1,500 rpm, and the temperature will go down when returned to idle.

Here are my results from my testing.

1. I disconnected the hose from that comes from the cooler assembly to the thermostat housing and the flow is good.
2. I disconnected the lower hose (big) from the pump to the thermostat and the flow is good.
3. I disconnected the hose going from the thermostat housing to both manifolds. I flushed both manifolds independently and had decent flow from the drain ****, but I did notice a small amount of metal (maybe kitty litter sized) attached to a pencil magnet that I inserted through the drain **** while they were flushing.
4. I pulled the thermostat and flushed the engine.
5. Both manifolds and the engine were flushed for about 10 minutes.

Total accumulation of kitty litter sized metal fragments was probably a couple of tablespoons. The boat stayed in the water for 6 months last year with zero issues until the last run. We have never had it in water shallow enough to worry about mud or sand. I am getting opinions from 3 different marinas with different assumptions. 1 marina said it was back pressuring or building pressure resulting in the increase of gradual throttle.

1. Manifolds
2. Risers
3. Both 1 and 2
4. Circulating pump

At this point I do not know what to do since this literally showed itself on the last trip and the boat was used every weekend for hours on end for 6 months in a row without any overheating issue. Do manifolds and risers fail overnight and I wasn?t aware of it? I?m just not sure which way to go or if there is other testing I can do. I?m scheduled to be on the dock in a month and a half so taking it down there with this issue is not an option.

 

[chadpcb]

I am not really savy on I/O but I do have alot of experience in the IB area.

Here is a couple of things to check.

1. First and foremost check your impellers. If they are weak then at higher rpms they get further away from the wall of the pump thus producing less water. They may look good but still could be bad.

I think your setup has 2 raw water pumps. One in the foot and one mounted on the front of the engine.

2. If the lines on the engine pump are reversed it still will cool at lower RPMs but rapidly heat up as more gas is given. Easy way to check this. Disconnect the presumed intake line from water strainer and adapt a short piece of water hose to it and place in a 5 gallon bucket of water. If it sucks out the water then it is right, if it blows bubbles then it is wrong. But if it does suck water then I would look at impeller.

3. Make sure there is no air getting into the raw water line. The air will get worse as the RPMs go up thus causing air lock on the Thermostat.

Check the gaskets on the water strainer and all hose clamps. Also make sure water strainer valve is completely open.



I purchased a 82' Mako 236 that had issues of over heating at higher rpms and my issue was a combination of the above 3 items.

Hope this helps

 

[chadpcb]

Forgot to mention check the Thermostat. Easiest way to do that is suspend it in a pot of cold water. Use a candy thermometer that will attach to the side of the pot. turn on your heat and watch the temp on the thermo. Make sure it opens around 120 degrees. Make sure it fully opens. Partially opening thermostats will cause over heating at higher rpms.

 

[chadpcb]

Something else to check. If you have heat exchanger disconnect all the hoses and remove. Check for any rubber from impeller in the heat exchanger manifold.

 

[Bt Doctur]

Try this, with the therm in place are the manifolds and riser cool? Remove the therm and is the motor and manifolds running cool/cold?

 

[rhillman75]

I checked both the 160 degree t-stat and the 142 degree t-stat this evening and both open fine in a pot of hot water within 2 degrees of actual reading. The previous water impeller in the leg looked new just as the one I just installed over the weekend. I rechecked my lines to the t-stat housing and they are in the same position as they were before this all happened last year. I will pull the t-stat tomorrow to see if the temps at the manifolds change. I am assuming that if the temps at the manifolds do not change and the temp gauge rises after I pull the t-stat, then I am now leaning towards new manifolds and risers. What is the marina referring to when they state the system is building pressure at an rpm increase?

 

[Bondo]

Ayuh,..... The used heated water has to be able to get Out of the system,....

Pull the Risers, 'n See if the exhausted water ports are Plugged,.....

 

[achris]

A few things worth doing.

1. Water pump output test.
2. Water pocket cover leak test.

Both of these can be done at the same time.

The procedure for checking the output is here ->; http://boatinfo.no/lib/bulletins.html#mercruiser-service-1982_04

For the water pocket cover -> http://forums.iboats.com/forum/engi...elp?t=556903&p=5877255&viewfull=1#post5877255

 

[Bt Doctur]

I suspect the engine circ pump has failed. With the therm inplace the water cant circulate thru the block without a circ pump. remove the thermostat and there is now free flow thru the motor just by the pump in the outdrive or the engine driven raw water pump if this is a Bravo.
Serviced one last year with the same condition. All of a sudden it decided to get hot. Replaced the therm and no effect. Thought the owner installed it wrong, so I installed another one, no effect. At idle the temp would come back to normal.

At cruise power motor and manifolds were very hot but the risers were very cold.That showed water leaving thru the risers but not circulating.
With the therm removed the motor ,manifolds, and risers were cold.Installed a new engine circ pump and no more hot problem.

 

[rhillman75]

So now I am not only scratching my head, but I?m starting to get pissed! I started the engine tonight again and let it run at idle on the muffs for 30 minutes with the 142 degree t-stat in place. The starboard side riser that was hot to touch before this whole episode in question started is now cool to touch on the top of the riser. I now have a portside riser that is hot to touch. The entire 30 minute run cycle consisted of a throttle only situation where I increased it to 1,700 rpms and let it stay there for 5 minutes, while I watched the IR gun scanning every possible hose, manifold, and riser. This time around the manifolds were a 10 degree temperature difference from the front of the manifold to the rear. I ended up letting the engine cool off for an hour and taking both hoses off the t-stat housing from the manifold and the riser and flushing them independently for about a minute I started the engine again and the temperature remains the same and does not overheat at 1,700 rpms. Did I flush something out or what??? The only difference is that it is 60 degrees outside so the water from the hose is cooler is all I can come up with. I did pull some small pieces of metal (kitty litter sized) from the starboard manifold side with a magnet from the drain ****. I?m stating to wonder if I have a flow problem now.

 

[achris]

It's quite normal for one elbow to be hotter than the other when there is no back pressure on the system. And all it requires is for the boat to be a degree or so different on the trailer, or the trailer to be in a slightly different position for the 'other side' to be the hot one.... And yes, it can change overnight. Path of least resistance and all that....