I have a Mercruiser 6.2 with less than 35 hours on it. Recently had an issue with the impeller and thermostat triggering the alarm system. In doing a complete check over after changing these parts I did a complete run down with an infared thermometer and found I have a temperature difference on both the manifolds and riser from side to side. I also have about 14 PSI at 3000 RPM. The starboard side reads aprox. 25 degees hotter than the port side. On further checking both sides seem to be getting the same volume flow of water from the thermostat housing but distinctly different on the output to the exhaust. I believe this may have been this way since new. Outside of some kind of a blockage in either the riser or the manifold does anyone have any suggestions?

You stated that you had a "impeller" problem, did you loose the impeller and have to replace it? If so you might have some small pieces of the imeller clogging something. If you have thru transom exhaust can you look and see if the water flow in the exhaust looks even?

Gasket not matching up on the riser?

The impeller was not missing any pieces, it was just fatigued and the water flow is very uneven out of the exhaust.

The risers are what they call dry fitted. there are no water chambers to align between the manifold and the riser. Only the exhaust flows between the two.

If the water flow is uneven then logic would indicate an obstruction is somewhere. Can you reomove the exhaust and backflush? Or even backflush the whole system? Also do know by chance what your water pressure is?

Forgot to ask if the manifold that is 25 degress hotter is the one that seems to have the lower water flow/discharge out the exhaust?

Water pressure at idle aprox 2 PSI and about 14 PSI at 3000 RPM and the higher temp side is showing far less water flow thru the exhaust. I've actually started wondering if Mercury has the wrong riser gasket in the poor flow side. They have 2 different gaskets that will fit this. One allows water flow thru the front and rear and the other blocks the flow on one end.

boatnuts,

I thought in post #5 you said that the risers are "dry fitted" and that no water flows thru them only exhaust. As for as you water pressure seems a little low to me.

Unlike the older style manifolds & risers that had water chambers up all 4 sides of the riser, the dry fit have a cast in tube like water chamber on the exterior of the front and rear.