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Cooling Question

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Troy leger

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Question is on a 1997 raw wate

Question is on a 1997 raw water cooled mercruiser does the water flow through the risers first or the engine block.
 
"Raw water comes in from the r

"Raw water comes in from the raw water pump up to the circulating pump then through the block, though the thermostat and out to the exhaust manifolds, up through the risers and out the exhaust."
 
So if the thermostat was stick

So if the thermostat was sticking the risers could possibly overheat?
 
"The risers are the only thing

"The risers are the only thing that won't overheat. Depending on the bypass routing, the manifolds could also be fine.

I've seen two configurations. When my 5.7L was raw water cooled, the bypass ran to the bottom of the exhaust manifolds, up the spacers and out the risers so they would always get cooling. The Merc manual also shows the bypass being routed to the risers while the block and exhaust would have little circulation until the tstat opened.

I've also heard of many times where one riser gets a whole lot less cooling than the other, especially when the tstat is stuck shut because the bypass doesn't distribute flow evenly between port and starboard sides of the motor."
 
Yes I think mine goes through

Yes I think mine goes through the block first then out the risers because my risers felt real hot to the touch but the gauge was showing normal temp.
 
"You've got some blockage

"You've got some blockage going on. The t-stat probably isn't the problem, if it was stuck shut, the engine temperature would be hot and the risers would get more raw water cooling flow, therefore would cooler.
If it was stuck open, the engine would run cooler but there'd be no reason for the risers to get real hot, warmer than usual, maybe.

Once the risers start to rust, chunks flake off and start blocking the water passages. From that point, the water flow tends to 'channel' through the path of least resistance, the other parts get less flow and get even hotter, and rust even faster... sounds like it's time for new risers and manifolds."
 
So where is the temperature se

So where is the temperature sensor located on the engine. There are two wires connected to the thermostat housing is that it?
 
Yup.
One wire is for to the t

Yup.
One wire is for to the temperature gage sending unit. The other is for the temperature alarm sending unit. (Tan and tan w/blue stripe.)
One very useful tool in troubleshooting cooling system problems is a hand held infra-red thermometer. They're not that expensive ($50 for a decent enough model) and it'll give you a much better idea of what's going on and where.
 
So the risers should always ha

So the risers should always have constant cooling water if there is no obstruction. The thermostats function is to open and let water into the block correct.
 
"Correct... for the most part.

"Correct... for the most part.
The block is full of water. There is a semi-closed ciruit which circulates water through the block continuously. When the engine starts to heat up, the thermostat opens to allow the hotter water to circulate out of this circuit. In your case (no Hx or radiator like a car) the hot water mixes with some of the raw water heading to the risers.
The other part of it is that as the hot water leaves the block, raw water enters.
How much raw water goes to the engine depends on how far open the thermostat is. Raw water that is diverted to the engine and not directly to the risers is replaced by the water coming from the engine. So your first statement is absolutely correct... the risers should always have cooling water flow.
You may be right on the fine line of there being just enough flow to keep the engine cool, but not enough to keep the engine and the risers cool... it won't last like that for very long."
 
Ok my engine never overheated

Ok my engine never overheated just the risers felt real hot I mean you couldn't hardly touch them they were so hot. So I flushed my system out and alot of rust and sand came out. I changed the water pump impeller and the thermostat. I have not test run it in the water but at home with the paddles it does not overheat.
 
"Well, based on that... you sh

"Well, based on that... you should be a whole lot better off.
Keep an eye on those risers. This is where the ifra-red thermometer comes in handy. You can keep track of the trend. It takes not even a second, once the engine is up to normal temp to pop the engine cover up and get the riser temps. A notable step change or rapid rise in observed temps are good indications of pending doom.

And keep the outdrive out of the sand!"
 
Ok one more question if the bl

Ok one more question if the block is getting plenty of cooling that means the risers are not clogged correct?
 
"That means there is enough fl

"That means there is enough flow to keep the engine cool.
If you're trying to decide weather or not to replace the manifolds and risers, about the only thing I can say from here is that the risers (and manifolds on raw water cooled engines) are consumable items... especially in salt water. In which case, if you are religious about flushing the engine with fresh water after everytime you use it, they'll last 4-5 years.
In fresh water, they'll last considerably longer.
It sounds like you should be OK for now.
Keep one more thing in mind:
Expect what you inspect. If you haven't had it in your hands and looked at it, you really don't know what the condition is.
Your impeller is good, you flushed the cooling system, got a lot crap out, and ran the engine. If it were me, the next step would be to float it and run the heck out of it and see what you have from there."
 
Add the following to the first

Add the following to the first sentance of the last post:
"in the driveway under no load."
 
"Troy Leger,
Were both of you

"Troy Leger,
Were both of your risers hot or just one?

(I'm asking cuz my starboard is aways hot and my port riser is always cool.)

-Ed"
 
I guess I'm a little confu

I guess I'm a little confused. So how am I to keep both risers and elbows cooled evenly or in some cases do they just run hot on the starboard side. If I use the infrared gun on them what temp should they be?
 
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