7.4L combo sea water/fuel pump maintenance

[Dave M.]

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.

 

[Dave M.]

Now on to the fuel pump part. Remove two bolts, and the fuel pump is removed from the housing. You will need a new fuel pump gasket when you do this. The fuel pump itself is pretty standard, as you can see in this picture of it removed.

Sticking out the right side of the fuel pump in the upper left is the arm that moves to pump fuel. It rides on a lobed cam on the pump shaft. You can see this cam in the housing in the main unit in the photo.

Here is where the issues begin. First, there is the possibility that there will be a little fuel leak past the gasket in the fuel pump, and it will get into the housing. In a normal engine, this would just go into the multiple quarts of engine oil, and be dumped in the next oil change. And never be noticed.

So second issue is that this reservoir is very small compared to the quarts of oil in an engine block, so dilution of the oil with gasoline will occur much more quickly if there is any or movement of fuel through the gaskets at all.

Third issue is that I think the normal maintenance does not call for replacing this oil, it only calls for removing the plug and topping it off. I could be wrong about this part.

Fourth issue, if oil gets diluted, then you are more likely to get wear of the arm and the cam. So you will start to get metal contamination of the oil. Again, it is only a small amount of oil, and there is no pumping and filtering to get rid of it. So it will try and settle to the bottom,where the shaft bearings are! :smt013

Fifth issue, if the oil gets diluted significantly with gas, it is more likely to leak by the bearing seals and get out, taking the oil with it.

So when you take it apart, look at the fuel pump arm, and check for wear. Here is what mine looked like, which appears OK to me.

Look down at the pump cam, it should also be nice and smooth, and not show signs of wear.

Lastly, here is a photo looking down inside the pump housing, past the cam, and into the end of the housing where you can barely see the bearings. This is where you really don't want any worn metal particles to remain.

So what you really need to do, with this thing taken apart, is to flush out the old oil (and maybe gas), get out any contamination, and refill it with fresh oil.

If you take care of it, knowing the issues, it should take care of you. If you don't, well, you could just be caught with a non-working fuel pump and sea water pump.

If I remember right, on mine, the first indication of a problem was of a small amount of leaking from the front seal where the pulley is, and I could not decide if it was oil or gas. So now I think it was both. After a flushing and oil change, it now has no leak that I can detect.

 

[osd9]

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.

Good Old dish detergent works well....Just changed both impellers a few months ago....

 

[Dave M.]

Good Old dish detergent works well....Just changed both impellers a few months ago....

OK, thanks! I did not want to stop writing and look it up to see if there was any particular recommendation. I've been working on getting the photos cropped, sized, named, moved to the web, and wanted to just get this done. :thumbsup:

 

[J Levine]

Dave,

How about some pictures of a frustrated DIY owner beating on the aluminum housing with a brass hammer to remove a frozen bearing? I totally rebuilt both of mine last winter and hope they hold up for a while because it is a PITA, as is just changing the lube, and yes removing the unit is really the only way to do it. Just be happy you only have one to mess with, as two are more than twice the fun.

 

[Dave M.]

Dave,

How about some pictures of a frustrated DIY owner beating on the aluminum housing with a brass hammer to remove a frozen bearing?

OK, do you have a picture of this to share? I am envisioning somehow a 'penalty box' section, like in hockey, where if you don't do it, here is your punishment! :smt018

 

[mrsrobinson]

Sounds all to familiar:

http://dotnetrocks.blogspot.com/archives/2006_07_30_dotnetrocks_archive.html

We purchased a new sea water pump and a new fuel pump, about $475 in parts. I sold the old sea water pump on eBay for $100 and I sold the fuel pump on eBay for $25, so all in all a $350 repair.

 

[Dave M.]

Read your blog, it sounds like your is quite difficult to get to. Mine is easy, with my single engine in frontwards.

Also sounds like we have similar problems. That is not too bad, if you know about them in advance then you can take care of them. I hope these posts help someone catch them early on.

 

[sbw1]

My 1993 37 Express had the mechainical fuel pumps. We changed the oil in the sumps every year by removing the screws and sucking out the oil and refilling them. They were never taken off the motors. Access to everything on that boat was outstanding.

 

[mrsrobinson]

How did you suck out the oil?

 

[Dave M.]

And I am curious which screws you took out. Do you mean the plugs in the sump case? In the photo way above, where I have labeled a plug "Oil check access plug", it looks like there may be another plug below it, where the circular writing says 'See Owners Manual'. I sort of see the outline of a plug, but no slot or anything you could get a tool into to remove it.

It would be nice to do this job without taking it off the engine, as taking the fuel system plumbing apart yearly won't make it improve with age.

 

[mrsrobinson]

There is only one screw, a drain and fill up screw. I was thinking I could stick the oil pump hose (the pump I use to change the engine oil) down in there if it was small enough but it's not. I guess with an adapter I could reduce the size of the oil pump hose.

 

[Dave M.]

My 1993 37 Express had the mechainical fuel pumps. We changed the oil in the sumps every year by removing the screws and sucking out the oil and refilling them.

:grin: OK, so this is not my day to have my brain turned on. I could not figure out why screws was plural. Guess I thought you were going to run your 37 Express on a single engine.
:smt021

Lemme see if I can ftp some files...

 

[sbw1]

My mechainic did the job and I believe he used a large syringe with a small tube attached to it. We didn't do this for 2 years and he suggested that I do it. Good thing as it had a fair amount of fuel in the oil. I had him change it twice in 1995 and annually thereafter. Labor charge was for one hour which is the minimum charge at my marina. He did show me the location of the screws so I could do it myself. After the oil condition improved I just included it in the work order for the winter layup.

 

[Rocklobster]

Dave,

I have heard that some people convert this setup by replacing the fuel pump with a electric pump. This way they use a standard water pump on the pulley and an electric one mounted somewhere easy to get at.

Have you heard of anyone doing this? I'm trying to figure out the details and wondered if it's worth going through the trouble.

Thanks,
Joe

 

[PlayDate]

I like electric fuel pumps so when I bought my 37' Express, I replaced both mechanical pumps with Holley Marine Red Pumps with built in pressure regulators. This is a weekend project.

These are the parts you need:

2 Holley Red Marine HLY-712-801-1 $117 each
2 Holley Fuel Shutoff Switches $25 each
2 Block off plates $6 each
2 Electrical relays $3 each
2 Brass 1/8 npt tee fittings $2 each

Misc.

2' of marine rated fuel line, gasket sealer, hose clamps and a number of 3/8" fittings depending where you locate them (straight/right angle/splices).


Pay particular attention to the installation diagram for the safety switch, it will explain how the wiring is to be done for the switch and the pumps. The switch kills the circuit if oil pressure drops below 5 psi.

The only reasonable location on my EC was on the stringers beneath the fuel tank valves. This is where the hose descends from the valves and makes the turn toward the front of the engine. I installed them at the turn (using a 45 degree fitting and a straight fitting on the other side of the pump). Access was easy.

I'll post a picture to show the mountings if you are interested.

Step 1 Turn fuel valves to off
Step 2 Remove fuel pumps and install block off plates
Step 3 Install Fuel Pumps along the fuel hose after the Water Separator and before the Fuel filter.
Step 4 Remove the factory oil pressure switches and reinstall them using the tee fitting and the safety switch.
Step 5 Wire according to the Safety switch wiring diagram.
Step 6 Recheck all hose clamps
Step 7 Open fuel valves and start engines


The messy part is dealing with fuel. Just be careful and make sure you have plenty of ventilation (and a fire exstinguisher nearby). Changing impellers is much easier with the mechanical fuel pumps removed.

Don't get me wrong I like mechanical pumps too. I just don't like combination units that require you to mess with fuel when all you want to do is replace an impeller.

The Holleys have been in place for five years. Of course that was just the beginning then I got into redoing the top end of both engines to increase performance.......that's another story...

 

[keokie]

Yep, I've had my share of experience with those mechanical pumps. I always used Merc's gear lube in the resevoir to stand up to the gas. Also, Jabsco makes a much better impellor than Merc for those sea pumps. Jabsco's has one more vane and is more substantial.

That system does have one nice feature. If you lose the belt that drives the water pump, it will cut off fuel before the motor overheats.

 

[Rocklobster]

Play Date,

I'd be interested in the pictures of your setup. Sounds like a nice setup.

Many thanks,
Joe

 

[PlayDate]

Joe,

I'm travelling this week. I'll have a couple of pictures posted by the weekend.

-John

 

[PlayDate]

Here you go... The last picture gives you a view of where they are placed in relation to the fuel valves. I forgot to shoot a picture of the safety switch but it is very easy to install. The existing oil switch is located on the right rear of the engine block as you face the engine. Just remove the factory switch, insert a 1/8 npt tee fitting, install the factory switch on the right angle of the tee and the safety switch on the straight part of the tee. The wiring of the safety switch is important since it shuts down the fuel pump if oil pressure drops below 5 psi.

One more thing. You will notice a vent hose leaving the port pump. This hose attaches to the same spot on the carb as the mechanical pump did. You can't see the hose on the starboard pump because it comes out behind the pump.

-John