This is an issue that came up on the old board, and I thought I would repost the information here that we learned.
On many 7.4 liter engines used in the mid-90's (I don't know the date range), Merc had a design where the sea water pump and the fuel pump were on the same belt driven shaft. The sea water pump part of this are typical. You should change the impeller frequently, normally annually. But there is not much mention of what to do with the fuel pump. Plus, what you need to do is more than what is written, I believe.
First, here is a picture of the whole unit removed from the boat. When you do impeller maintenance on this thing, you might as well go ahead and remove it, at least every other year IMHO.
In the image above, I have labelled where the plug is that allows you to check the level of the oil in the fuel pump housing. Just below that you can barely make out in circular writing "See owners manual". Not exactly a big red flag!
More on that later, but let me deal with the impeller first, because that is the easy part. In the case of the unit I have shown, the impeller housing is a single piece design, the newer design. Probably most of the older multipiece units have been replaced with this type by now.
So you can just remove the bolts that hold the impeller case to the rest of the assembly, and pull the case and impeller off the shaft. Here is what the impeller will look like still in the housing. You will normally replace the impeller, and maybe the housing too if it shows internal wear or has other damage.
Here is what the shaft looks like where you just removed the impeller housing, impeller, and wear plate. Nothing surprising here. There is a bearing and seal inside there around the shaft, and in this photo there is no evidence of a seal leak.
Between the impeller and the fuel pump base there is a removable wear plate. It looks like this, when worn. At the time, I think I did not have a replacement on hand, so just flipped it over when I put it back together to put the worn side away from the impeller.
When putting the impeller in the housing, I think you should put in some lubrication to keep it from running dry before water pickup starts. Not sure what you should use.
On many 7.4 liter engines used in the mid-90's (I don't know the date range), Merc had a design where the sea water pump and the fuel pump were on the same belt driven shaft. The sea water pump part of this are typical. You should change the impeller frequently, normally annually. But there is not much mention of what to do with the fuel pump. Plus, what you need to do is more than what is written, I believe.
First, here is a picture of the whole unit removed from the boat. When you do impeller maintenance on this thing, you might as well go ahead and remove it, at least every other year IMHO.
In the image above, I have labelled where the plug is that allows you to check the level of the oil in the fuel pump housing. Just below that you can barely make out in circular writing "See owners manual". Not exactly a big red flag!
More on that later, but let me deal with the impeller first, because that is the easy part. In the case of the unit I have shown, the impeller housing is a single piece design, the newer design. Probably most of the older multipiece units have been replaced with this type by now.
So you can just remove the bolts that hold the impeller case to the rest of the assembly, and pull the case and impeller off the shaft. Here is what the impeller will look like still in the housing. You will normally replace the impeller, and maybe the housing too if it shows internal wear or has other damage.
Here is what the shaft looks like where you just removed the impeller housing, impeller, and wear plate. Nothing surprising here. There is a bearing and seal inside there around the shaft, and in this photo there is no evidence of a seal leak.
Between the impeller and the fuel pump base there is a removable wear plate. It looks like this, when worn. At the time, I think I did not have a replacement on hand, so just flipped it over when I put it back together to put the worn side away from the impeller.
When putting the impeller in the housing, I think you should put in some lubrication to keep it from running dry before water pickup starts. Not sure what you should use.