Electric and mechanical fuel pump?

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COjeeper
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Joined: Tue Apr 27, 2021 9:29 am

Electric and mechanical fuel pump?

Post by COjeeper » Wed Jul 21, 2021 12:12 pm

My ‘87 wagoneer has an inline electric fuel pump pushing fuel to the mechanical pump. Is this normal or did someone put the electric pump in and just didn’t want to remove the mechanical pump?

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tgreese
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Location: Medford MA USA

Re: Electric and mechanical fuel pump?

Post by tgreese » Wed Jul 21, 2021 12:39 pm

Some electric pumps can operate inline like this. Depends on the fuel pump.

There are reed valves in the mechanical pump that prevent backwards flow from the pump stroke. I expect they would allow flow through if there were pressure.

These carburetors often suffer from "dry bowl syndrome" when they sit for a few days. Varies a lot between vehicles. Sometimes an electric like this is added to prime the bowl for quicker starting. Not real safe to let the pump run constantly with the key on. There are safety devices including electronic and oil-pressure-dependent switches. Your ideal electric setup has these.
Tim Reese
Maine beekeeper's truck: '77 J10 LWB, 258/T15/D20/3.54 bone stock, low options (delete radio), PS/PDB, hubcaps.
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Topic author
COjeeper
Posts: 21
Joined: Tue Apr 27, 2021 9:29 am

Re: Electric and mechanical fuel pump?

Post by COjeeper » Wed Jul 21, 2021 12:48 pm

So would the best option be to bypass the mechanical pump and use a block off plate?

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tgreese
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Re: Electric and mechanical fuel pump?

Post by tgreese » Wed Jul 21, 2021 1:31 pm

Dunno. Does it work now? What kind of electric pump is it? Does it have the capacity to fuel the engine on its own? You could disconnect it and see if the Jeep runs, ie can the mechanical pump draw through the electric pump? If so, you could put a momentary switch on the dash and prime the mechanical pump when needed, or add the electric pump if or when you have vapor lock.

Certainly the mechanical pump is capable of fueling the engine. Mechanical pumps are reliable and durable generally. I'd remove the electric pump and see what happens.
Tim Reese
Maine beekeeper's truck: '77 J10 LWB, 258/T15/D20/3.54 bone stock, low options (delete radio), PS/PDB, hubcaps.
Browless and proud: '82 J20 360/T18/NP208/3.73, Destination A/Ts, 7600 GVWR
Copper Polly: '75 CJ-6, 304/T15, PS, BFG KM2s, soft top
GTI without the badges: '95 VW Golf Sport 2000cc 2D
Dual Everything: '15 FCA Jeep Cherokee KL Trailhawk, ECO Green
Blockchain the vote.


will e
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Re: Electric and mechanical fuel pump?

Post by will e » Wed Jul 21, 2021 6:56 pm

My thoughts, for what they are worth.

I have a carb and run an electric fuel pump bypassing the mechanical. There are a couple of advantages of an electrical pump. Like Tim mentioned, you can prime the carb bowls. You can set up a 'anti theft' switch to make it more difficult to steal. I carry a spare electric pump and when the working one dies, I swap it out. Takes about 15 minutes. A mechanical pump is a pain in the butt to swap out road or trail side.

Some will argue an electric pump pushing from the rear of the wagon will help reduce vapor lock.

I don't recommend to folks to run both. If the mechanical pump diaphragm ruptures fuel can get into the crank case. If this happens and it is not being fed by a secondary electric pump the amount of gas that gets into the crank case won't be too bad. An electric pump will continue to push gas into the crank case while the engine is running until the carb finally dries out. This will ruin the oil very quickly and might cause engine damage. It's a lot of 'ifs' but I just don't see a benefit of running both kinds for the damage that could happen.

The plus side to a mechanical pump is, well, it's mechanical. No wires to break, no fuses to blow. If your charging system isn't working the draw on the battery from just the ignition is pretty nominal so you will have more drivable miles if you are not running an electric pump.
81 Waggy 'WILL E' Retired
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letank
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Re: Electric and mechanical fuel pump?

Post by letank » Thu Jul 22, 2021 12:41 pm

Go electric, excellent way to prime the carb after 2 weeks to 6 months of non use. I have electric fuel pump in all the oldies... I used a carter P4070, a bit big and annoying to change $80ish . Currently testing (not driving off-road too much between the heat wave and the fires) the cheapo $50 edelbrock installed in the engine bay... bought a spare for easy swap.

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Michel
74 wag (349 Kmiles... parked, next step is a rust free body)
85 Gwag (227 Kmiles... the running test lab)
See Engine bits , Fuel and brake lines, Body work, Trany and TC

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