Hi,
I have a problem with my 2000 502 MPI. I had a broken impeller (one blade was already missing) so i put in a new set, impeller, wear plate and housing. First test drive i saw more than 35psi water pressure (35 is max on my gaffrig) and the big water hose from t stat housing to waterpump has been so big that the serpentine belt cut it. I looked everywhere to find the missing impeller blade, but couldn't find it. The water pressure is taken where you can drain the block near the starter. So the blade couldnt be before the oilcooler. And when it went trough the oilcooler it should be so little pieces that they are already out. Courius is, when the boat is in no gear (0mls/h) and 5000 1/min there is 25psi water pressure, when it drives 50 mls or more there is more than 35psi. Could there be a problem with the new raw water pump? Any other ideas?
Thanks from Germany / sorry for my bad english
Andre

The missing piece is blocking the water flow somewhere. Have you checked for blockages in the hoses going to the exhaust. The missing pieces usually get caught in the oil cooler since it is the first thing downstream of the raw water pump. Because The water hose after the oil cooler is expanding then it has to be on the exit side of the engine water circuit.

x2

I checked the t stat housing, the hoses to the exhaust and the exhaust. Nothing in and a very free flow. I think the pieces couldnt be as big to block anything when they went trough the oilcooler.

Disconnect the hose that runs from the Raw pump to the oil cooler, at the Raw pump end... remove the water hose inlet at the T-Stat housing (on the right side when looking at the engine. Flow water from a garden hose through it and this will backflush the oil cooler.... the pieces could very well be stuck inside the cooler.

Did you put the tstat spacer back in orientated correctly + with the tstat on top ?

Did you know with the tstat closed, it closes off just one way the water goes to the top of the tstat housing where the exhaust gets its water ?

Yes, there are two levels to the tstat housing.

The blockage has to be in the engine or the exhaust - If it was before the engine than the hose connected to the engine would not have swelled. Every few years I pull the ends off of my oil cooler and push a brass rounded end brazing rod through each tube. You would be surprised at how much junk you will get out and how many tubes are blocked.

Lets ask is this a saltwater boat, if so how bad or not is internal corrosion , & scale built up in the t stat housing and necks and in the exhaust manifolds & risers water jackets ? How old is the water circ pump ? Is all of this stock OEM set up and equipment ?

Not knowing - you did not mentioned that this was a closed cooled app ? so is it or not ?

Is the impeller and housing a stock OEM set up and a stock OEM replacement that you installed ? Is your water pick ups stock in the outdrive or do have any other means for water intakes or water inlets ?

Just asking because 8 out of 10 times up here nothing is stock so very hard to figure anything out including a fix. I have learned the hard way and now will ask all indepth questions about what the set up is and what is OEM or not designed and installed.

Its a saltwater boat, low water pickup in the drive, always flushed and kept on a lift, older water circulation pump, 10 years old stainless marine exhaust, replacement raw water pump i think, no closed cooling.
The high pressure happens only when i drive more than 50mls. And the fault is from one day to the other. I only changed the raw water pump because there was no water flow.

...deleted my previous response....

On second thought, your restriction is likely after these coolers - I just re-read about your water pressure pickup. Sound like thermostat housing and beyond is where you need to look (restriction in thermo housing or exhaust).

I had this one time, but this caused low water pressure.