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Q1: is a fuel demand valve necessary for a recently made Moeller 6-gallon on-deck fuel tank?

Assume that a fuel demand valve is not necessary.

Q2: what is the reason that a fuel demand valve is no longer needed with new-design Moeller 6-gallon on-deck fuel tanks?

Q3: why did each fuel tank in the original design have its own fuel demand valve?

The new Moeller 6-gallon on-deck fuel tank includes a primer bulb.

Q4: is a primer bulb needed [for my particular boat with a 2014 Mercury 90-HP EFI engine]?

What prompts these questions is a sales representative at [an unidentified] WEST MARINE store states that a fuel demand valve is now obsolete on Moeller fuel tanks because the fuel tank has been redesigned. But I haven't seen any evidence of this in the marketing or enclosed directions. The sales representative pointed to an "old style" hose kit that included the fuel demand valve. He then said that [this particular kit was] the single remaining package [in their stock], and they don't plan to stock [this version of the "hose kit"] anymore. The sales agent was adamant that he was right, but he could not offer an explanation other than "you just don't need it anymore."

BACKSTORY: The original Moeller 6-gallon on-deck fuel tanks were beginning to crack on my 2014 170 MONTAUK with 2014 Mercury 90-HP EFI engine. I replaced those tanks with new [Moeller 6-gallon on-deck fuel tanks]. These new tanks came with a hose assembly with a primer bulb and quick-connects, but without a fuel demand valve. See the new tanks in Figure 1 below.



The old fuel tanks had a fuel demand valve in the supply hose line. See Figure 2 below:



I would prefer that each fuel tank have its own quick-connect connector mounted onto the tank itself to eliminate the tanks having their own short fuel hoses which flop around the boat during transport.

Note: the topic of fuel demand valves has been debated in threads which date back to 2013 or older.

Your mention of a fuel demand valve was a new term for me. I had to look up that device.

My understanding is that the purpose of a fuel demand valve is to prevent the fuel supply hose to the engine from becoming pressurized by build up of pressure in the fuel tank, which I presume would occur due to heating of the fuel tank when exposed to strong sunlight. The reason for preventing the pressure in the fuel tank from being transferred to the fuel supply hose is to prevent too much fuel pressure to occur in the outboard engine's fuel system. With a fuel demand valve, fuel can flow to the engine only at the rate that the engine's fuel lift pump creates sufficient negative pressure to pull fuel from the tank, that is the tank pressure cannot force fuel to travel to the engine on its own.

ADK Whaler wrote:Q2: what is the reason that a fuel demand valve is no longer needed with new-design Moeller 6-gallon on-deck fuel tanks?

I can only presume that the elimination of the fuel demand valve for use with a new Moeller 6-gallon on-deck fuel tank was related to some new method or technique incorporated into the fuel tank where the pressure in the fuel tank is held to a very low level, perhaps by some method of venting the fuel tank to the atmosphere to release the pressure.

I recommend you contact Moeller directly and inquire with them regarding what element of their 6-gallon on-deck fuel tank has changed so that there is no longer a need for a fuel demand valve.

ADK Whaler wrote:Q4: is a primer bulb needed [for my particular boat with a 2014 Mercury 90-HP EFI engine]?

Traditionally an outboard engine used a fuel lift pump that only operated once the engine was already running. These old-style engines had a fuel lift-pump that used an air-motor driven by pressure and vacuum pulses taken from the engine crankcase area. This meant that no fuel was being lifting until some time after the engine was started and sufficient action of the air-motor was able to operate the lift pump. To provide fuel to the engine before trying to start the engine, the fuel hose connected to a traditional outboard engine always included a primer bulb. The primer bulb was routinely used prior to engine start to lift fuel to the engine. The appreciation of firmness or resistance to further pumping in the primer bulb by the engine operator was a sign that the engine fuel system had been primed, that is, that fuel had been lifted from the fuel tank to the engine, and the engine was ready to be started.

Also, in traditional outboard engines with carburetors, the fuel system was vented to the atmosphere in the carburetors, which allowed fuel to evaporate in the carburetor bowls. For this reason it was necessary to prime the fuel system with the fuel line primer bulb to be certain that all carburetor bowls were filled with fuel before trying to start the engine.

Modern Outboard Engines and Primer Bulbs
In modern outboard engines the use of carburetors has declines, with the result that the fuel system is typically not continually vented to the atmosphere. Thus fuel in the engine fuel system is not prone to evaporate when the engine is not running. Also, fuel injector systems generally have a fuel distribution manifold and a fuel-cooling system in which a volume of fuel is maintained at all times, and an electric fuel pump maintains that distribution at a fixed pressure during engine operation and at engine starting. In some modern engines the fuel lift pump may also be electrical, but in other modern engines the same air-motor type lift pump is still used. The air-motor pump will rapidly lift fuel to the engine once the engine starts by using the fuel retained in the fuel high-pressure system, manifold, and cooler to get started.

One way to tell if your modern outboard engine has an electrically operated fuel pump is to remove the engine cowling before engine start, turn the ignition key switch to the RUN position, and listen for the sound of the electric fuel pump operating before trying to start the engine.

In modern outboard engines a primer bulb is not required, but a primer bulb can still be used. The primer bulb can be useful, both for priming the fuel system if there has been a long interval of no engine operation, and as a diagnostic tool for assessing problems in the fuel system.

For more about fuel line primers see

A Primer on Primers
https://continuouswave.com/whaler/reference/primer.html

[The fuel system seen in Figure 2 above] is terrible setup.

The gray hose is not alcohol-resistant and is prone to degrade and contaminate the carburetors and fuel injectors. Replace any fuel hose that does not have alcohol-resistant printed on the outside.

With a traditional manual vented cap or the new EPA compliant cap, a fuel demand valve is not necessary.

You only need a single fuel line with a primer bulb near the tank connector.

Jim and Phil, thank you for the quick reply. I will reach out to Moeller, as suggested.

From Attwood, a manufacturer of this type of product:

The integrated fuel demand valve, used in conjunction with integrated fuel systems, is for below deck use, eliminates pressurized fuel from reaching fuel distribution lines and your engine and ensures that fuel is provided to the engine on demand rather than as a result of pressure in the tank.

Hi, as a follow up to the open question about the fuel demand valve. I spoke to two Mercury-engine mechanics and the mechanic at our local Boston Whaler dealer. All three of them strongly recommend that I include the fuel demand valve. In their opinion, the pressure from the EPA tanks makes the demand valves necessary.

For carburetor engines, this prevents flooding, and for EFi engines it reduces premature wear on the pump. There were some technical aspects to the EFI fuel system that were mentioned to make their point. I didn't fully understand the nuances of the EFI pump system, but the basic concept made sense to me. Therefore, I've decided to keep the original set up with a demand valve for each tank.

The new Moeller vent caps seem to work as designed. The tanks expand modestly, but nothing like my old cracked tanks. In hindsight the old caps on the replaced tanks weren't venting properly, if at all. I'm glad I replaced them, but ballooning tanks still set off every alarm in my lizard brain.

Thanks again for the advice.