Mercruiser 120(2.5L in-line 4)

Here is the problem. Everytime I'm at 3/4 throttle or above for a few minutes the temp goes way up. Then I slow and idle it goes back down. When it's on the muffs in the driveway with no load and cold water doesn't heat up, but I don't rev it high in the driveway.

So new impeller. Lots of water flow in. I put a new circulating pump on, not the problem, one that came off was just as nice.

So I assumed it was the manifold and/or riser clogged. Just to see I took the hose off the front of the exhaust manifold that dumps in from the thermostat housing and water squirted/bubbled back up the inlet tube(I didn't think that exhaust could get in the water passage).

Put a new manifold on, was so happy, carb back on, put a newer riser on(holds acetone, passages are clear). Took the hose off again that dumps into the exhaust manifold and water is spitting up a couple inches every couple seconds, like to the cycle of the engine.

What is doing this? Is my exhaust pipe that dumps after the riser boot plugged?

Oh it has done this both in the water and then does it now hooked up to ear-muffs

put a newer riser on(holds acetone, passages are clear).

Maybe the inlets are clear but what about the exits
Like the other forum, check the flow on a funnel or remove the elbow for inspection
To push water and exaust back into the manifold you have a restriction. If not in the manifold or elbow then its in the exaust pipe or outdrive. Pull the drive and using a hose in the inlet hole run the engine and retest it .

Bt Doctur said:

put a newer riser on(holds acetone, passages are clear).

Maybe the inlets are clear but what about the exits
Like the other forum, check the flow on a funnel or remove the elbow for inspection
To push water and exaust back into the manifold you have a restriction. If not in the manifold or elbow then its in the exaust pipe or outdrive. Pull the drive and using a hose in the inlet hole run the engine and retest it .

Click to expand...

The exits are clear on the riser as well and another brand new gasket. I will run it without the boot attached(after the riser) see if it sprays lots of water and exhaust and then i will know that is somewhere in the exhaust leaving the transom or the drive.

https://www.youtube.com/watch?v=shmjznMxmpw

possibly raw water pump cavitating or sucking air

that video is from a V8 with a HX ,totally different than a IL4 . He needs to pull the drive and start there.
Test with a hose in the inlet port . Hell he might even find an old shutter or two just behind the shift rod

.."...pump cavitating or sucking air "

Had an air leak on the way to the raw water pump and my motor did the same thing. Doesn't take much aeration of the inlet water to cause massive overheating.

Jeff