You say, you put in the water and it takes forever, does it flow? and then run fine?

The last time I dropped it in the water, I had to rev the hell out of it several times and then wait an uncomfortable amount of time (HOT!) before it finally started to flow. I then ran the boat for a few hours and it continued to flow. When I pulled the boat out of the water, I let everything drain out and then backed back into the water and tried it again with the same results. I did pull the water pump and the impeller looked perfectly fine to me. I did notice that when I took the pump apart where the end of the impeller rubs, there are some very fine circular grooves where it revolves, but nothing that looks like a problem. Maybe there should be no grooves at all? Anyway, I hooked everything back up and was able to get water flowing with the garden hose on the out drive. I need to submerge it in standing water to see what it does next. There's still something about it that doesn't seem quite right. Thanks for your replies.

I have followed this thread for a little while and last week I would have posted "I have no problem". Well, checking the boat out before going out this weekend. I found three of the four exhast tubes warp and one with a split in it. I have only ran the boat twice on a hose to flush it after last weekend in the ocean. So I guess I'll check my impellers to rule them out.

Those "grooves" that are wore into the brass end-plate are why I got new pump assemblies, under warranty. Again, hot is relative, it sounds like it runs fine when your in the water and running the boat.

I do the same as BillR. Once I have an oil pressure reading, I just blip the throttle up to about 1500 and she starts pumping.

Where will cooling water debris most likely end up in a 496? Oil cooler?, Heat Exchanger?
If the heat exchanger can it be cleaned out be removing the end cap?

I know on my old 454 mag the oil cooler capture most things.

The oil cooler is the first place debris will go. The inside of the oil cooler is like a honeycomb. It will catch larger debris. This is where I found a piece of impellar fin that was causing me a problem. :mad:

If debris gets to the heat exchanger the end caps can be take off. The heat exchanger is also honeycomb shaped inside. I had mine off looking for a cooling problem. (before I found the 2 impellar fins blocking the oil cooler).

Interesting. There seem to be many of us having this problem after changing the impeller. My pump housing was find but I still have an issue drawing good pressure when on the hose. It never had the problem before...

Has anyone tried contacting Merc direct to discuss?

You guys are scaring me
 

I'm hopeful I'll get my first ride in this weekend.
I have never had a problem, yet. Original impeller
over 200 hours and never started it out of water.
Maybe that garden hose wont flow enough.
I'll post my results. JC

Re: 496 Mag HO water circ. problem
 

Has anyone found a cure for this problem? I am having the same or similar problem, but one motor seems to be worse than the other and it also seems to run a good 10 degrees hotter or so. I have tried back flushing the system and have new impellers and so on. No change.

Thanks for any info.