2004 merc 350 mag mpi high temps on new dry joint manifolds

[stang32]

i have an issue where this boat has readings on new manifolds and riser of over 180 degrees,
back story ,the boat was overheating terribly and the previous owner had another mechanic run tests where he pulled the thermostat and "allegedly" pulled the risers for inspection. which he said were fine. i stepped in after that point., running the boat found that there was no water coming out of the transom when running on muffs the boat went into overheat, raising the idle speed caused the hose to blow off of the elbow from the pump to the cooling system. when the hose blew off, the temp went back to normal. further inspection found the manifolds and risers terribly corroded with rust.. (original from 2004) now fast forward to now, i replaced the manifolds and risers, and all seamed well, until i was called and informed the boat temp did not raise above 100 degrees. tests with infrared heat gun found the entire system did not go above 100 degrees including the manifolds. i found that the thermostat was removed and not in the system( which the PO said was replaced) and installed the correct one. after that this issue has arrised. i hope that makes sense.

 

[achris]

Runs cold without a thermostat, overheats with one.... I'd be double checking that all the hoses are where they should be (other mechanics working on it means checking for anything that might have been 'butchered' in an attempt to make it work by someone who doesn't understand how it should work in the first place)....

Here's how it should be hosed up....

 

[stang32]

its a bravo set up abd i should have mentioned that the engine temp stays fine, around 165 even though the manifolds are hot

 

[dubs283]

its a bravo set up abd i should have mentioned that the engine temp stays fine, around 165 even though the manifolds are hot

No mention of inspection/replacement of sea water pump/impeller and associated cooling plumbing to/from the pump. Closed cooling or raw water?

 

[achris]

its a bravo set up abd i should have mentioned that the engine temp stays fine, around 165 even though the manifolds are hot

Check to make sure the right tubulator gaskets have been put under the elbows, and in the correct orientation. If they've used closed gaskets, you'll get exactly what you're seeing. And no need to pull them, just check the egdes....

 

[stang32]

the gaskets are correct based on the set up., no spacer, restrictor gasket.
the system is raw water cooled.
the PO had the seapump impeller replaced when the overheat started happening. once i got involved, the boat was out of the water and i was told the thermostat was removed for inspection and then replaced wth new. which i later found out to be false, so when i was running it on muffs i found that no water was exiting the boat. and the water was not flowing in, it was being pushed out around the muffs, the temp was getting high and i raise the idle and the hose coming from the water distribution housing blew off and the engine immediately started to cool down. it was obvious the water was not making it through the manifolds, when i pulled the elbows to see all the corrosion, rust.
anyways, i replaced the manifolds/risers and ran the boat on muffs and it cooled nicely and the water flowed out the exhaust. i did notice the temp was not climbing but i felt the reason was because it was on muffs
and the water coming from the house was very cold and pressured. it wasnt until the boat was in the water and we saw the temp still didnt go up that i found the thermostat was missing. .
do you think the impeller can be bad even though the engine temp is staying within range?

the hose routing looks to be correct.

i think based on all the other inacurate information i have been fed, it may be possible this impeller was not changed as the said it was.
i just don;t see why the engine will be within range if the impeller is bad and how does the engine stay @165 when the manifolds are climing to over 200 degrees.