Replace Impeller in 7.4L Mercruisers

BostonPaul

New Member
May 13, 2015
24
Boston, MA
Boat Info
1994 Express 400
Engines
7.4L Mercruisers
Took the boat out for spin around the harbor today and the starboard engine began overheating. I shut it down and made it back to my slip on the port engine. Started it back up and I had no water coming out the exhaust. Did some trouble shooting and it looks like I need to replace the impeller. The water heater and the gas pump appear to be all part of the same unit. I don't want to end up taking half the boat apart to change an impeller. What is the correct way to replace the impeller on a 7.4L Mercruiser?
 
All due respect Paul but your fuel (gas) pump has nothing to do with the raw water pump. The best way to address this is to remove the raw water pump from the motor, put it on a bench and take the back off and replace the impeller. And while your at it do the other motor as well. Very important!.... Inspect the bad impeller and make sure it is intact. If there are any vanes missing you need to find them as they are in the motor somewhere. They usually end up in the heat exchanger.

If this sounds like something you don't want to tackle have your regular mechanic take care of it. They do these all the time. But if you want to give it a try here's a link to a video on You Tube showing how to do it. https://www.youtube.com/watch?v=UxLfF2abE_o
 
I just did this last week. Pretty straight forward:

1. remove serpentine belt
2. remove 2 rear hoses on pump housing
3. remove 4-5 bolts on pump housing to remove entire assembly from motor.
4. pull off cap and pull out old impeller.
5. twist in new impeller (may need some grease)
6. reasssemble.
 
With all due respect Norcal boater and GrimmSpeed you are both providing wrong information. The boat in question is a 1994 and the 7.4 of that year did not have a serpentine belt and the fuel pump actually bolted onto the water pump and was driven off of a cam on the shaft of the water pump.

I've never changed one of the vintage so can't really say what needs to be done but it appears that the fuel pump can be disconnected from the water pump fairly easy to get it out of the way.
 
I remove the 2 fuel fittings, the clear tubing from the diaphragm, the 2 water hoses and remove the fuel pump and water pump as 1 unit. I believe there is only 2 or maybe 3 bolts holding the pump on. I don't even have to loosen the belt on mine:) I have done it twice so far.

HTH

Jeff
 
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YOU ARE HERE: MERCRUISER > 7.4L BRAVO (GEN. VI) GM 454 V-8 1996-1997 > 0F800700 THRU 0K999999 () > SEA WATER PUMP
 
I remove the 2 fuel fittings, the clear tubing from the diaphragm, the 2 water hoses and remove the fuel pump and water pump as 1 unit. I believe there is only 2 or maybe 3 bolts holding the pump on. I don't even have to loosen the belt on mine:) I have done it twice so far.

HTH

Jeff

+1

And, while you have the whole pump with bracket and fuel pump attached on the deck (or bench) change the gear oil that runs the fuel pump cam. The fill/drain is the big flat head screw on the side. Refill it with just enough gear oil that it dribbles out when in normal operating position.
 
The fuel pump is definitely part of the assembly. Going to give it a shot tomorrow night so I will let you know how I make out. Thanks for all of the advice.
 
We have the same engines and frankly having the mechanical fuel pump as part of the assembly just makes it a much tougher job especially on the Starboard engine. If you haven't done the job before, start with the Port engine so that you can understand how things come apart and go back together. The reason why is that removing the Starboard one is by "feel" since the raw water hose clamps are hidden by the housing. Be careful with the housings because they are ABS plastic and are easily broken. So... be careful removing the hoses and do not pry against the housings.

Make sure your fuel valves are in the off position and then disconnect the fuel lines and remove the entire assembly. The rebuild is a very straightforward process. If the impeller is missing vanes....you must find them or they can and will block the cooling system. Usually, they are stuck in one of the pump's passages.

As I mentioned, I really don't like the fuel pump and water pump combination. I replace my impellers every two years so I have done it on my boat 8 times since I bought it and countless times on other boats. After the second or third time, I installed Holley Electric fuel pumps and ditched the mechanical pumps which made the job a lot easier to do.

John
 
I've always worked on newer models but plan on trying to replace mine this spring. The fuel pump/water pump combo doesn't sound like fun.
 
On some Mercruiser 7.4L engines (like my 1995), the water pump and fuel pump are one unit. On the pic I have included, the fuel pump is gold casting and the water pump is the black plastic housing.

I love this website, but you often get people making suggestions who have no idea what their talking about (i.e. the guy who stated "with all due respect"). I also had people try to correct me when I inquired about this (and I knew they were wrong).

The water pump gets spun but the accessory drive belt. That shaft also pushes a lever arm that pumps a diaphragm in the fuel pump. You can either just unbolt the whole assembly and work on your bench, or I was able to simply unbolt the black pump hosing bolts and pull that out to get to the impeller. Depends on access.

pump.jpg
 
I believe if you want to get rid of the sea water/fuel pump combo you can install a conventional belt driven sea water pump and a low pressure electric fuel pump. The fuel pump must be wired thru the key switch as well as a low oil pressure shut down sending unit. If I had that set up it would be gone in a New York minute.
 
And, while you have the whole pump with bracket and fuel pump attached on the deck (or bench) change the gear oil that runs the fuel pump cam. The fill/drain is the big flat head screw on the side. Refill it with just enough gear oil that it dribbles out when in normal operating position.

YEP - do this! This is some awesome engineering. Not only is impeller replacement a frustrating, time-consuming, knuckle-busting task because of lazy and bizarre design choices, but the whole damn thing is bolted to a design that's EVEN WORSE - the notorious mechanical fuel pump.

Give the gear oil a sniff test. If it's contaminated with fuel, the relatively-inexpensive upper diaphragm assembly is leaking fuel into the oil chamber of the very-expensive lower assembly. A $150 Sierra replacement is now part of my spare parts bin.
 
I think this was done due to back in 1995 gm stopped using mechanical fuel pumps. on gen v blocks I don't think there is a spot for a mechanical pump. everything went FI
 
I changed my impellers. Above there's a lot of comments about the fuel pump and water pump being together. Mine are separate and they were easy to change. I took off the circulate pump hose, removed 3 5/8 bolts that hold the pump in place, then removed the hoses from the water pump. Then replaced the impellers on deck. Took about an 1.5 hr to do everything that's removing the furniture to pull the decking and everything.
 
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