Water not exiting exhaust, at idle speeds

you can follow the good flow til it gets bad. i had some heating issues so i spliced in a short section of clear tubing so i could see the flow.
do you have a flow diagram? its nice to see it on paper as a reference. get a temp gun and follow the hoses til it gets hot
 
Have you checked the belt tension on the water pump(s)?

Have you physically dove under the boat and checked the seacock for an obstruction?

Have you pulled off the inlet hose on the sea cock? Is there anything in that hose, seacock, etc?

Is this on both engines or just one?
 
belt tension checked
dive under boat no
inlet hose no
sea strainer checked and clear
only one engine
told my mechanic to look at it tommorrow
 
That boat wasn't by any chance designed with an alternate exhaust route was it (stationary outlet vs, cruise speed)?

Does that side sound different than the stbd? Does the engine exhibit back-pressure issues ( if not , the problem wouldn't be in muffler , flapper or exhaust hose, but would likely have to still be in the RW only passages.)
If you disconnect the RW hose at the riser inlet and put the hose in a 5 gal bucket, can you fill the bucket in 15 seconds at 4000 rpm (engine cold, gear in N)? What flow at idle?

Do FWC Mercs have the spring-loaded RW flow control balls? (I'd imagine no, ...but maybe?)

Do you have RW system lubed/cooled dripless shaft seals as an alternate route for water at low flow??

On the suction side of the RW pump are all hose clamps tight and strainer seal intact to prevent air intrusion until moving boat scoops water to allow proper RW pump priming?
 
I wouldn't be surprised if you find seaweed or something stuck in the inlet hose.
 
Just gotUpdate from mechanic
he had all hoses unhooked to check for blockages. Hoses off both oil coolers, poked thru coolers, ok. Took water pump apart again. Poked thru heat exchanger, ok ,
Just bypassed heat exchanger w hose w no change. Yet flow to exchanger seems same pressure as other motor.
 
I think you should delete your pix now so it doesn't make viewing so difficult.

Did you connect water hose directly to the riser and check flow out the exhaust at different flow rates?
 
I did not think thermostat would affect raw water part of cooling system? would it?
and with a bad tstat would engine temp go down with higher revs?
 
What if the thermostat was stuck?

Edit: WTF am I thinking? No, of course a thermostat will not effect the raw water side. There's too many cooling issues that I'm getting them confused now!
 
latest update from mechanic, he is frustrated too he is going to bring back a pressure testor tomorrow for the heat exchanger. as there seems to be a little coolant missing after top off this morning
 
pump. Don't assume a device is good just because it is new or changed. Make sure it has the right impeller. Could be water leakage around the impeller if it is not the right one.
 
i think he needs a compression test. Maybe a head gasket went.

Why i say that missing coolent.
 
*update*
I took boat out this weekend, prior to going out, I tightened up all the exhaust hoses and filled up the heat exchanger, burped the air from thermostat housing, and viola it was working 100% normal pegged at 160 firm no deviation all day even at idle, water coming out exhaust, not sure what happened, i think what ever was blocking got jostled by mechanic removing and and poking everything :)
 
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