I got my 78 mustang 17' out of storage and spent last Saturday am getting de-winterized.  At about 1pm, I was ready to fire it up and within a few tries she was running fine. Ran for about 15-20 minutes, everything was looking GOOD. I shut her down for a few minutes and started again to double-check. This time, I notice that water is no longer coming out in the exhaust. Temp gauge gets up to ~160, so I shut her down. 

[1] first attempt, ran fine and temp gauge reading close to 140.

[2] second attempt, overheating - no water in exhaust.  Quite surprising that I went from all good to overheating within a few minutes without changing anything ???

I have my water intake set up to suck water from a trash can (thru a short hose). Hose is clear, no problem there.

- I was thinking to check for suction at the end of the hose; should I feel a constant suction or only when thermostat opens?

Checked the impeller, looks fine (probably 2-3 years old).  Thought maybe the thermostat got stuck (seems unlikely since it was fine a few minutes prior).  Had a spare, so I changed out the thermostat.  Now, I'm getting a water leak at the thermostat housing. So, I feel like there is water and water pressure at the thermostat.  After a bit of playing (gasket, gasket seal, let it dry), try again today. 

No water leak, but temp gauge gets up to 160, So I shut her down. No water in the hose connected into the thermostat housing. 

[a] run without thermostat, to eliminate any potential issue there.  I expect to see water out the exhaust right away.

You should always have water out the aft even if the T stat is closed. 

You should be feeling some suction at the end of your clear vinyl tube.

To me, it sounds like you may be sucking air in on the suction side of the pump. Check the hoses and connections. 

From the raw water pump it goes up the discharge hose to the thermostat housing, 

From the housing it goes to the circulating pump and into the engine, but with the thermostat closed there's no real flow thru the engine till it opens.

From the housing it also goes to the exhaust manifolds all the time the engine is running and turning the raw water pump. This  keeps the manifolds and rubber exhaust hoses cool.

When the thermostat opens, you now have water flowing through the engine and exiting through the thermostat into the housing where it mixes with the water going to the exhaust manifolds.

The housing is kinda a "magic" mixing chamber where the cold inlet water and the warm engine exit water mix together and through the thermostat performing it's "magic" the temperature is maintained near it's setpoint with some warm water going back through the emgine and all of the water eventually needs someplace to go so it goes out to the exhaust manifolds and overboard so that you don't melt your rubber hoses in the exhaust system.

So your question about pump suction, there will always be suction as long as the pump is turning, unless you have an air leak on the suction side big enough to prevent it from sucking.

Since the suction is under a vacuum when the pump is running, you won't see any water leaking out and since you can't see air, you won't "see" the air getting into the system. 

Like you said, you can run water directly to the raw water pump with a hose and see if it cools right that way.

People don't recommend it because it can mask a problem like the suction line leakage just mentioned, so if you do that and it cools, you probably have a good sized air leak on the suction hose, transmission cooler or raw water strainer if you have one or you have blockage somewhere in that same path

If you have a strainer check it's O ring as a likely leakage spot.

Like you asked, you can run water backwards through the suction side to see if there's blockage anywhere and if there's a leak anywhere in that path, you may see water coming out.

If you're idling the engine and getting no suction from your trash can, rev it up to about 1500 to 2000 RPM for a few seconds and see if it starts sucking down the water level. If it's now working, it means you have an air leak that prevents the pump from sucking at idle speed or your impeller "ain't too good". I'd try this before doing the direct hose hook up 

More RPMs mean more suction to overcome a small air leak.

[1] simple check for blockage: run water backward through the suction section.

[2] check hose connections, or cracks/leaks.

[3] then try again, check at higher RPM (and lower RMP); if this makes a difference continue to look for air leaks.

Not to start a debate... but in all the time I've owned this boat (since 88), I have always observed that water does not discharge in the exhaust until engine warms up. before that, pump is not sucking water; no water in and no water out. my understanding is that water was just circulating until the thermostat opens. had never thought through how the water circulation is actually working but... that is what I have noticed.   

Anyway, will try again this weekend...

Gary,

Again, the RWP is always pumping. What you may be noticing is when first started, it may take 15 to 30 seconds for the water to make it out the aft. When not in use, the water has drained out of some of the cooling system so it takes the time to fill before exiting the boat. 

I'm plumbing in a new hose bib in my garage, off a 3/4 inch line. Prior, I was running a hose about 100 feet from the bib on the opposite side of the house to the boat. I could run the boat about 5 mins at best before the ECU did it's thing. Hopefully this fixes that issue. The joys of being on a well in the 21st century ;-)

Ultra...

* Very easy to determine if pump is working correctly.  As soon as you start the engine (whether with a hose or in the water), put your hand on the backside of the pump housing.  If it is pumping water, it will be cool or cold to the touch.  If it's not pulling water, it will get hot VERY quickly (rubber on metal) and if hot enough it is starting to destroy the impeller.  This is a very good test when you first replace the impeller and put it in the water since it now has to suck from the hull all the way to the pump.  It should never be warm when running (except for a few seconds (5 seconds or so)) when it is first pulling in water through empty or near empty hoses/strainer.

* If you plumb a T with a garden hose fitting to your inlet line, the engine will take what it needs, and the rest flows out the hull fitting.  This doesn't check suction, but it does get water to the intake side of the pump without potentially damaging the pump by high water pressure (my outside faucets are around 70-80PSI).  As an added benefit, whenever I use a hose, it automatically cleans out my raw water strainer (which is between the hull and the T).  :)

Ignore the copper tube in the tranny filler; just pumped out fluid for a change.

Note that my strainer is on its side; it auto drains every time I pull the boat out, so there is never much in it.  But when I launch and start the engine, the pump has to fill up the hose from the hull to past the T (filling the strainer) since all of that drained when the boat was pulled out of the water.

One comment on how long it takes for water to come out of the exhaust when running it out of the water.  Due to quagga/zebra mussel inspections that we have to periodically get, the boat has to be 'dry'.  When I know I need an inspection, I stop on our rather steep ramp when pulling out and let water drain out of the exhaust--sometimes even gunning it as we come out to blow water our (there's a lot of water in it).  I have done that in preparation for an inspection, and then later after inspection later hooked up a hose to run it at the house for some reason, and it can take 1-2 minutes at idle sometimes before it all fills up.  So when I start it in the driveway, I always put my hand on the pump housing.  As long as it is cool, I know everything aft of the pump is just filling up.

(and no, it's impossible for these to be dry on the inside; if I was a mussel inspector, I could fail everyone if I wanted to....  ;)  That said, they know all of us from the ski club and most of us launch exclusively at our site, our boats and bilges are clean and dry, etc.  So rarely does anyone fail inspection (but I digress)..