Petrol fuel pump when on gas??

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antimony
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Petrol fuel pump when on gas??

#1 Post by antimony » Wed Nov 28, 2018 10:56 am

Hi. Quick question, when switching to gas, what happens to the petrol pump? Does it turn off or continue cycling petrol from the tank? So do I need to replace the petrol filter as frequently as normal if running mainly on gas? Cheers

Brian_H
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Re: Petrol fuel pump when on gas??

#2 Post by Brian_H » Wed Nov 28, 2018 11:09 am

Can depend on the system in question and the vehicle. The general rule is they get left as they are and just pump fuel round the system, you just end up with more going back via the fuel return than normal.
If the car doesn't have a fuel return this may not be the case, though a filter usually indicates that it has a fuel return as far as I know.

In the case of an engine thats a carb type and using a single point system, you'd usually shut it off and let it run the system dry as you switch over.

Personally I'd aim to change the petrol filter at least every couple of years, I've seen the state of them when left longer, and given that all petrol now available has some biofuels content it can cause a buildup of rubbish in the filter. Its not a bad idea to run it on petrol occasionally either just incase you need to use the petrol system (if its a sequential type system that starts on petrol and switches to gas this is usually enough to stop any major problems there, just try to run though whats in the tank occasionally and top up with fresh). Though alot of people don't change the filter at all, regardless of being on gas or not.

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Re: Petrol fuel pump when on gas??

#3 Post by Gilbertd » Wed Nov 28, 2018 10:53 pm

As Brian has explained, on most systems the pump still runs and simply circulates petrol. However, one thing to be wary of. Even though you aren't using petrol, make sure you keep at least a couple of gallons in the tank. Petrol pumps don't like being run dry so the fuel pump can burn out leaving you stranded if you run out of gas.
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Re: Petrol fuel pump when on gas??

#4 Post by LPGC » Thu Nov 29, 2018 9:56 am

If we knew what vehicle it is and what LPG system is fitted we could probably say if the petrol pump will still run. If it's a sequential slave system it's over 99% likely that it will still run.

Different schools of thought on changing petrol filters...
1. One school of thought says there isn't as much chance of new dirt getting in the tank if you don't fill it up as often, so even with the pump running all the time the filter won't accumulate as much dirt as if you were filling up more often, once the petrol in the tank has been continually filtered it should be very pure and then there's nothing to make the filter dirtier.
2. Other schools of thought will be concerned about what happens to modern petrol as it ages... potential gums / laquers build up etc.
Probably different takes on that depending on whether the petrol pump does/doesn't run while the engine is running on gas. I'd be inclined to go with 1 regardless of whether the pump does or doesn't run while the engine is running on gas.
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Morat
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Re: Petrol fuel pump when on gas??

#5 Post by Morat » Thu Dec 06, 2018 10:18 am

I went through a bad patch of destroying fuel pumps on my Jeep a couple of years ago. I was very pleased with whoever had cut a whole in the boot floor so I didn't have to drop the tank every time!
The current one seems to be lasting, but I'd be happier if it wasn't running all the time.

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Re: Petrol fuel pump when on gas??

#6 Post by LPGC » Thu Dec 06, 2018 10:52 am

You could put it on a switch, turn it off when it's switched over to LPG and you have a lot of gas in the tank, turn it back on when you know you're getting low on LPG.
Could make it fully automatically switch but it's a bit more involved than might first seem... If it's wired to the LPG solenoids (through a relay) the pump will turn off as soon as gas solenoids open but gas solenoids will open a short while before it actually switches to LPG, especially with one at a time cylinder switch-over.. so you'd want a relay with a delay. An automatic system ought to monitor gas tank level and switch the pump back on when level is low, but to avoid the pump switching on/off with fuel slosh would also involve a timing circuit.
A minority of modern LPG ECU's can be setup to do the above (timing and switching on low gas level) using one of their switched 12v outputs to control a simple external relay.
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Re: Petrol fuel pump when on gas??

#7 Post by Morat » Fri Dec 07, 2018 6:18 pm

I don't suppose that includes the baby BRC that I have? :)

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Re: Petrol fuel pump when on gas??

#8 Post by LPGC » Sun Dec 09, 2018 6:28 pm

Afraid not mate.

Back in the day when a lot of 6 cylinder Jeeps were converted with mixer systems some installers disabled the petrol pump when running on petrol, often by cutting into the fuel pump wiring just forward of the petrol tank and fitting a relay energised by LPG solenoid wiring in that area. A big difference in mixer systems and sequential systems like yours is that mixer systems rely on the driver to switch fuels, sequential systems might switch back to petrol when the driver isn't expecting it, if the petrol pump isn't running when that happens the engine will just die.
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