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

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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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Gordonjcp
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Re: Petrol fuel pump when on gas??

#9 Post by Gordonjcp » Tue Mar 05, 2019 11:03 am

I was wondering about this too - I've got a P38 running on gas and after sorting out some dodgy underbonnet wiring I left the injector emulators out and pulled the fuel pump relay since the pump had failed. Now I've replaced it, I'm wondering about using a relay to switch the fuel pump relay feed instead of breaking into all eight injector wires to fit the emulators.

Any downside on a single-point system? I did entertain the idea of fitting multipoint but to be honest that's not likely to happen any time soon.
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Re: Petrol fuel pump when on gas??

#10 Post by Brian_H » Tue Mar 05, 2019 11:21 am

Gordonjcp wrote:
Tue Mar 05, 2019 11:03 am
I was wondering about this too - I've got a P38 running on gas and after sorting out some dodgy underbonnet wiring I left the injector emulators out and pulled the fuel pump relay since the pump had failed. Now I've replaced it, I'm wondering about using a relay to switch the fuel pump relay feed instead of breaking into all eight injector wires to fit the emulators.

Any downside on a single-point system? I did entertain the idea of fitting multipoint but to be honest that's not likely to happen any time soon.
The main downside would be the loss of being able to switch back smoothly if you managed to run out. Yours being a GEMS p38 I think has a fuel return fitted, I know the Thor fitted to the Disco 2 doesn't have one, I'd suspect the same is the case with the Thor in a p38.

Provided enough fuel is kept in the tank it shouldn't cause the fuel pump to burn out (it is designed to run all the time the car is running after all, but would cut out when the engine stops due to lack of petrol and not end up running dry). I'd suggest at least 1/4 of a tank to be the level you want as the higher volume will keep the pump cooler

If you were to put a relay somewhere, I'd make sure its easily accessible just in case it ended up going wrong - Don't want to find you can't use petrol somewhere when you need to do so.

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Re: Petrol fuel pump when on gas??

#11 Post by LPGC » Tue Mar 05, 2019 11:46 am

Most single point controllers feature an output that can be used to control such petrol pump relay (yellow wire or wires on AEB stuff). Most can be set to delay switching off the petrol pump during LPG priming time for a programmable amount of time. Similarly most injector emulators such as Pitagoras can be set up to delay switching off of petrol injectors for a moment after gas solenoids are turned on.. There would be little point in petrol injectors continuing to pulse during priming time if the fuel pump immediately turned off as soon as gas solenoids opened but emulators don't feature such separate output to control a petrol pump.

You don't really need the priming time but it can help make for a smoother switch from petrol to LPG and can help prevent backfires during switching. If you have a slightly leaky petrol injector but don't run on petrol much switching off the fuel pump side-steps the problem so each cylinder sees the same mixture running on LPG.
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Gilbertd
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Re: Petrol fuel pump when on gas??

#12 Post by Gilbertd » Tue Mar 05, 2019 1:43 pm

Not sure if GEMS is clever enough (the old 14CUX on a Classic isn't) to detect what it will see as faulty injectors if you do away with the emulators and just cut the power to them. Easy way to find out would be to unplug one petrol injector while it is running on petrol and see if it flags a P code. Your Leo will have a pair of dangling yellow wires that are closed when on petrol and open when running on gas so could be used to cut the power to the injectors. Personally I'd leave the pump as it is, there's a return so by keeping it exercised and running all the time isn't going to do any harm unless you run the tank dry. I keep enough in mine that the little yellow light isn't on because it's annoying.....
'96 Saab 900XS, AEB Leo, sold
'93 Range Rover 4.2 LSE, Lovato LovEco, sold
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Re: Petrol fuel pump when on gas??

#13 Post by Gordonjcp » Tue Mar 05, 2019 10:04 pm

I've wired up the injector emulators, and yes the GEMS ECU does detect open-circuit injectors. It doesn't care much about them, it just detects them.

I might go for a fuel pump relay mod anyway since the new pump sounds like a hoover!

At least wiring in the breakout cables for the emulators was good practice for doing Miah's multipoint conversion...
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Re: Petrol fuel pump when on gas??

#14 Post by Gilbertd » Tue Mar 05, 2019 11:01 pm

My emulators are on plug in looms. Makes the area above the injectors a bit crowded but a piece of piss to connect and disconnect.
'96 Saab 900XS, AEB Leo, sold
'93 Range Rover 4.2 LSE, Lovato LovEco, sold
'98 Ex-Police Range Rover 4.0, Singlepoint AEB Leo, my daily motor
'97 Range Rover 4.0SE, eGas multipoint, a project.....

Proud to be a member of the YCHJCYA2PDTHFH club.

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Re: Petrol fuel pump when on gas??

#15 Post by LPGC » Wed Mar 06, 2019 2:16 am

I couldn't remember if Gems detects disconnected injectors but did know that there are no consequences if they do (doesn't switch to open loop or prevent ltft's shifting etc). Seen plenty Gems without an emulator, just a relay (or two, one for each bank) to disconnect the positive feeds to each bank's injectors... But I wouldn't be happy not fitting an emulator on a customer car, fixed too many big audio amps that had output trannies fried when not connected to a load.

Just got in from calibrating a car I just converted, another coming for conversion tomorrow (er today).
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Re: Petrol fuel pump when on gas??

#16 Post by Gordonjcp » Wed Mar 06, 2019 1:13 pm

Gilbertd wrote:
Tue Mar 05, 2019 11:01 pm
My emulators are on plug in looms. Makes the area above the injectors a bit crowded but a piece of piss to connect and disconnect.
Mine *were* on plug-in looms but the wiring around the plugs was all crusty and falling apart.
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Re: Petrol fuel pump when on gas??

#17 Post by LPGC » Wed Mar 06, 2019 2:06 pm

Incorrect polarity plug on looms were often fitted on P38 mixer system installs because P38 petrol injectors are wired with opposite polarity to the usual Bosch standard, this still works to turn off petrol injectors because with opposite polarity the emulator disconnects injector positive feed instead of the pulsing negative. But with opposite polarity the emulator doesn't actually provide any emulated injector load - the emulation is between the pulsing injector negatives and the emulator's switched live feed (usually blue wire to gas solenoids) so with reversed polarity looms the emulated load is from injector positive feed to positive switched feed which does nothing. There are even some older sequential system installs with some channels connected to injector positive instead of negative, again not always of much consequence with some of the older ECU's that were not truly sequential except for the lack of emulation... unless the ECU is replaced with a more recent one that is truly sequential.

Back in the days when mixer systems and plug on looms were more commonly fitted some suppliers used to advice checking polarity of every petrol injector plug on P38s as supposedly some P38s had some of their injectors wired with opposite polarity to others ( in same car same set of 8 injectors).. I don't remember seeing a P38 with an injector wired opposite polarity but I have seen plug on looms for emulators and for LPG injection ECU's with the quirk.
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Gordonjcp
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Re: Petrol fuel pump when on gas??

#18 Post by Gordonjcp » Wed Mar 06, 2019 3:35 pm

Looking at the Pitagoras I have, it seems that the resistors are just placed in series with the coils and shorted by the relays!
'97 Range Rover P38, GEMS, OMVL Millenium controller, R90E

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