PCars Petrol Fuel Pump Constantly Buzzing/Humming

[gerlewis]

Well, Im now running on LPG!

But, when I drive the car Im noticing a buzzing noise which I have worked out to be the Fuel Pump, as when I remove the primary fuel pump fuse, the buzzing stops.

Would this likely be due to the LPG conversion? or just coincidence that my FPR or fuel return or pump has a problem?
it could be the case that ive never heard it before because I was never really listening.

It sounds like the priming noise, but constantly buzzing rather than just when you start the car.

There is no real 'problem' as such, just a noisy pump, can I live with it or will it die? Also could I just remove the fuse and run on the other pump? (cayennes have 2 pumps, one on either side for off roading in case of fuel starvation).

My main question I guess is could this have been caused by the conversion, or not?
Any advice welcome! thanks in advance.

Geraint

[Gilbertd]

I doubt it's been caused by the conversion although the conversion may be a contributing factor. How much petrol do you have in it? Running on gas with an almost empty petrol tank is the best way known to man to burn out a petrol pump as the petrol cools and lubricates it. Make sure you always have at least a quarter tank of petrol. Been there, done it......

[gerlewis]

I have half a tank.

I dont understand how LPG conversions work, but could the car somehow be confused and be trying to pump petrol when it doesnt need to?
Rather than labouring, I would have thought the pump would have been idle as LPG has its own pump I would have thought.
Thats why I wondered if the conversion ecu could be causing it, sending it incorrect signals, or not telling it to 'rest'...?

Also, if I only need petrol for 30 seconds a day, could I just use the secondary tank, and remove the fuse from the primary (assuming it will start without the primary).

[Gilbertd]

There is no LPG pump and the petrol pump will run all the time. When running on petrol the pump delivers far more fuel than the engine is likely to need, a pressure regulator does as it's name suggests and excess fuel is returned to the tank. Think about it, you'll need far more fuel at full welly than at idle but the pump sends a constant flow, the amount being returned varies. When running on LPG the pump is left running so it doesn't seize up through lack of use or cause the petrol ECU to get offended as it thinks the pump has failed (as some of the cleverer ones do monitor it), all that changes is the volume of petrol that is returned to the tank.

If you've had an LPG tank fitted in the spare wheel well, it may be that some sound deadening has been removed or disturbed and the noise from the pump is now more noticeable?

[gerlewis]

So if the LPG conversion hasnt effected the cars fuel pump, then maybe its something ive had but havent noticed before.

Will it be OK to live with? is there any 'problem' as such? also the car has done 95k - is it 'normal' to expect this noise after that many miles?

Thanks Gilbert.

[Gilbertd]

Petrol pumps are normally pretty quiet if they are mounted correctly so not allowing sound to be transmitted through the bodywork and you have fuel in the tank. I can just about hear mine when I switch the ignition on (over the noise of the other things, air suspension pump, ABS pump, heater blower, etc) if I try hard but not once the engine is running. Although my engine isn't going to be as quiet and smooth as yours. Like I say, it could have been disturbed so the noise is now more noticeable. If you can get to it, try giving it a poke while running and see if it changes. I suppose you might have something touching it that wasn't before allowing the noise to be transmitted.

[gerlewis]

Thanks.

One question as an aside. If the LPG tank does not have a pump, how does the LPG get to the engine?

[demonicwillow]

Basically when lpg is compressed into a storage vessel it becomes highly pressurised, your tank will only fill 80% creating a vapour trap at the top of the tank this is where the pressure is so as you use the lpg the liquid is forced to the bottom of the tank, the lpg is forced under pressure in its liquid state up a tube which goes from the bottom of the tank to the outlet at the top (much like a dipstick tube) and travels via whatever pipework you have installed to your vaporiser where it is vaporised and then delivered to the engine, when you fill your tank with lpg it has to be pumped as transferring from the garage storage tanks to your tank the lpg pressure will try to equalise in each tank so your tank wont fill up, when your tank is around 80% full a valve in your tank closes the fill inlet leaving the pressurised vapour trap at the top of your tank, a good example of how it works is one of those cheap see through cigarette lighters which is basically the same principle

[Gilbertd]

If the LPG tank does not have a pump, how does the LPG get to the engine?

Or putting it simply, it's at roughly 10 bar pressure in the tank otherwise it wouldn't stay as a liquid. If it had the room to expand, it would become a gas.

[gerlewis]

Thanks both - amazing amounts of free knowledge on this forum.

i like

[gerlewis]

small update:

It only appears to smell after a cold start, just after change over form petrol (perhaps at the next junction). Also Ive noticed a slight spluttering if change over occurs while I jab the throttle while idling at traffic lights.

30 seconds later the smell goes and I dont smell it for the rest of my journey.

Could it be running rich and Im smelling unburnt fuel from exhuast?

This information is subject to change - as I do more journeys!

[gerlewis]

any votes for unburnt lpg escaping from the exhaust?

I will take this back to the installer I think.