Electric fuel pump for Mercruiser

[Donald]

I had a marine mechanic do some work on my 1987 Mercruiser 260. He concluded the mechanical fuel pump was not working and connected an electric fuel pump inline with the mechanical one and powered it by the starter. He also used up more than 5 gallons of gas doing various testing.

So I now have a different mobile marine mechanic that I feel better about. He wants to replace the mechanical fuel pump and rip out the electric one.

The engine would run for 20 seconds and then die. Which is consistent with it being wired to the starter.

I do not get why the prior mechanic wired it the way he did or how he could run the engine long enough to use up 5 gallons of fuel.

It's a 5.7L GM V8.

 

[ionize]

If the mech. fuel pump diaphram failed and let get gas into the crankcase/engine oil there went
several gallons of gas---check your oil level, if high, theres your fuel. No need for an electric pump on that motor-
put in a new mech. fuel pump and be done...chg oil as well--- 1st mech is an idiot

 

[spasm3]

I also have to ask if a mechanical replacement pump is available, why on earth put in an electric pump? I could understand if it was a high performance engine, needing more fuel.

I'd dump the oil to be safe.

 

[Trav]

If you are going to run an inline fuel pump you don't want to wire that way. Use a oil pressure switch with a fuel pump kill circuit, yu can Tee one in if you have a mechanical gauge.
One from a early GM carb w/electric pump should work fine in that engine and has an additional 5 psi fuel pump cutoff circuit. keyed Power in from relay (fused), power out and ground to a ground wire that's it, do not run it in series with the mechanical, depending on the pump he used you may need a fuel pressure regulator up stream near the carb.

Some Buick Riviera and Chevy Vega used electric pumps with a carb but don't go by Rock catalog they don't have the correct part listed they show the 69 Riv with a mechanical pump and 1 wire sensor which is not correct. This should work also.
This is a quick and easy job and the electric pump offers better stability and can also be jumped to drain the tank almost empty which is a bonus for a boat owner.

https://www.summitracing.com/parts/atx-os75

 

[Michael_P]

No to the electric. A mechanical fuel pump in your application is more reliable. All Mercruiser fuel pumps since the 1970's (or so) has a semi clear yellow tube on the top of the pump. This tube leads to the carburetor. It is in case the diaphragm leaks, the fuel is burned rather than dropping into the crankcase. If you pull that yellow tube off at the carburetor and it smells like fuel , you have a bad pump. Putting in an electric fuel pump is a hill jack way to fix the problem and will fail long before a Quicksilver OE pump will. Use a marine mechanical fuel pump. Not just some small block pump that happens to fit. Auto style pumps may work but has no provisions in the event of a diaphragm leak. Any marine mechanic that says otherwise is not a true marine mechanic.

 

[Trav]

Please explain how its more reliable than an electric. You say they anticipate a diaphragm leak by using the yellow hose to the carb for this very reason, how is that more reliable?

 

[Donald]

The problem I see with electric, is you need same type of relay you have in a car. On for 15 seconds when ignition is first turned on, off if no oil pressure. Relay has to be ignition protected (no sparks to ignite gas fumes) and able to get wet and work in salty environment without failure.

I have not found one.

 

[Donald]

The yellow tube from carb is to drain gas in the event of the carb float sticking open, not a failed diaphragm in the fuel pump.

 

[Trav]

Originally Posted By: Donald
The problem I see with electric, is you need same type of relay you have in a car. On for 15 seconds when ignition is first turned on, off if no oil pressure. Relay has to be ignition protected (no sparks to ignite gas fumes) and able to get wet and work in salty environment without failure.

I have not found one.


It doesn't need a delay with a carb, there is enough fuel in the bowl to start the engine, even cranking should build 5 psi and trigger the pump relay. I thought you could use a weatherproof relay under the console near the ignition switch or something. The old GM cars had no delay in the relay and never an issue as soon as the oil light went out the pump was running.

I am not a marine mechanic but in this case I cant see it making any difference, keeping the relay out of the engine compartment is the only difference I can see.

 

[juggernaut]

Originally Posted By: Donald
The problem I see with electric, is you need same type of relay you have in a car. On for 15 seconds when ignition is first turned on, off if no oil pressure. Relay has to be ignition protected (no sparks to ignite gas fumes) and able to get wet and work in salty environment without failure.

I have not found one.


Donald the 8.1 Mercruisers have a fuel pump relay (in fact they have 4 relays) on top of the motor. These would be your best bet along with the merc plug I would imagine if you wanted to go down the path of an electric pump.

 

[juggernaut]

FYI there is no delay built into the merc 8.1 relays - the relays are controlled by the PCM.

 

[Donald]

I was not saying the fuel pump needs a delay. The 10 seconds is for the fuel pump to pump for 10 seconds without oil pressure.

 

[Donald]

My goal is to get the mechanical fuel pump replaced and working.

Boats typically have an anti-siphon valve so the the pump need to overcome that valve with vacuum.

 

[juggernaut]

Originally Posted By: Donald
I was not saying the fuel pump needs a delay. The 10 seconds is for the fuel pump to pump for 10 seconds without oil pressure.



Probably bad choice of words on my part ....but yep essentially the same as the Merc 8.1 just that the fuel pump relays are controlled by the PCM rather than another input source. Ignition on and the PCM activates the relay to run the pumps for a preset 10 or so seconds to pressurise the fuel rail then the pumps stop running until motors are fired up. I don't believe the merc guardian strategy is employed on these motors during the start up.

Its a moot point though as I agree with you that I would replace the mechanical pump and go boating

.

 

[Donald]

Originally Posted By: juggernaut
Originally Posted By: Donald
I was not saying the fuel pump needs a delay. The 10 seconds is for the fuel pump to pump for 10 seconds without oil pressure.



Probably bad choice of words on my part ....but yep essentially the same as the Merc 8.1 just that the fuel pump relays are controlled by the PCM rather than another input source. Ignition on and the PCM activates the relay to run the pumps for a preset 10 or so seconds to pressurise the fuel rail then the pumps stop running until motors are fired up. I don't believe the merc guardian strategy is employed on these motors during the start up.

Its a moot point though as I agree with you that I would replace the mechanical pump and go boating

.



The Mercruiser 260 has no PCM to control any relays. So even if I got the same relay, it would not do what I need.

 

[Trav]

Originally Posted By: Donald
My goal is to get the mechanical fuel pump replaced and working.


Isn't that what mechanic #2 wants to do? Is there a problem?

 

[juggernaut]

Originally Posted By: Donald
Originally Posted By: juggernaut
Originally Posted By: Donald
I was not saying the fuel pump needs a delay. The 10 seconds is for the fuel pump to pump for 10 seconds without oil pressure.



Probably bad choice of words on my part ....but yep essentially the same as the Merc 8.1 just that the fuel pump relays are controlled by the PCM rather than another input source. Ignition on and the PCM activates the relay to run the pumps for a preset 10 or so seconds to pressurise the fuel rail then the pumps stop running until motors are fired up. I don't believe the merc guardian strategy is employed on these motors during the start up.

Its a moot point though as I agree with you that I would replace the mechanical pump and go boating

.



The Mercruiser 260 has no PCM to control any relays. So even if I got the same relay, it would not do what I need.


I realise that hence my reference to "another input source" to control the relay.

 

[Donald]

Originally Posted By: Trav
Originally Posted By: Donald
My goal is to get the mechanical fuel pump replaced and working.


Isn't that what mechanic #2 wants to do? Is there a problem?


Yes. But I am still unsure how mechanic #1 ran the engine long enough to go through 5 gallons of fuel with fuel pump wired to starter. There is no gas in the oil.

 

[DoubleWasp]

Carb/electric fuel pump systems on boats work off of an oil pressure switch or from the alternator "tach" output.

Long story made short is, there is an extra stub on the starter that only turns on when the starter is being cranked. This powers the fuel pump during cranking. Once the engine is started, the tach output on the alternator is energizing the fuel pump relay. If the engine shuts down, the fuel pump stops along with the tach output.

The oil pressure switch arrangement works a little differently. A delay switch engages the fuel pump for 10 seconds, and then turns off. Once the engine develops oil pressure, the fuel pump relay is energized by the voltage going through the oil pressure switch. If the engine does, the fuel pump stops along with oil pressure.

Some manufacturers prefer the oil pressure system, because it will also shut down the engine in the case of oil pressure loss.

Problem is that the terminals on those oil pressure switches like to corrode, leaving a boat stranded.

 

[BrocLuno]

Mechanic #1 ran the pump long enough to gas up his truck. Putting one inline and using the starter circuit is only for priming if you have low mounted tanks. It must be a low pressure pump with little resistance to fuel flow once the mechanical is working. It is not a proper replacement for a working mechanical pump.

Plenty of boats have burned to the water line because of improperly wired electrical pump that just keep feeding the fire as folks jump overboard ...

Use a good mechanical and be done with it (for now). Gas with ethanol will bring water into the gas and it will eventually destroy the new pump too. It's now a maintenance item ...