Ok to switch off fuelpump?

[Eminent]

As some of you may know my car runs on LPG. This means that it only use petrol when starting the engine.
So my question: is it ok to switch off the fuelpump as soon as it runs on LPG? The pump makes an annoying sound and i suspect that there is still a bit fuel passing from the fuel injectionpump to the injection valves.

[Benz Dr.]

Wouldn't the system be rigged to shut off gasoline once the engine fires? Seems like you would want to shut the pump off after the enginer is running.

[Eminent]

Thanks for your answer Dr.
The engine starts and runs at idle on gasoline. As soon as i hit the throttle a little vacuum plunger (which is mounted at the front of the FIP, see attachement) pushes the rack inside the FIP backwards to cut off the fueltransport to the injection-lines. At the same time the LPG system takes over. But i suspect there is still a bit of lack of fuel trough the injection-lines. So by switching off the fuelpump this problem would be solved (and the annoying sound also). But will this be harmful for the FIP? I can imagine that the fuel is resonsible for some cooling/lubrication also.
Thanks.
Rini.

[Eminent]

Nobody 

[ejboyd5]

I'm afraid very few of us have any experience with a combination of LPG and a mechanical fuel injection pump, however your argument about the lubricating properties of gasoline in the pump sounds logical.  I'm curious as to the delivery system you employ for the LPG; is it plumbed into the intake manifold or otherwise?  How about a picture or two of the entire set-up?

[450sl]

Best to shut off your fuelpump whilst running on Lpg like lmost every lpg system on IPbased engines does.

[Naj ✝︎]

 

My thinking is that the injector pump would get too hot if you switch of the electric pump and there is no fuel flowing thru it to cool it.
But then, what do I know? (not familiar with LPG)
naj

[Eminent]

Well, Naj. Thats my biggest concern. And as long i am nut sure it isn't harmful i leave as it is.
But let me get you a bit familair with LPG.
In thrunk of the car there is anorher gastank filled with LPG.
From there the LPG is transported to the evaporator (see pic.1). The evaporator uses the cooling system to evaporate the liquid LPG.
Once that is done the gas is transported to a nozzle which is mounted between the intake manofold and hose to the airfilter (second pic).
The injectors are not used for LPG as it goes straight through the manifold.
The FIP is cut off by a vacuum plunger which pushes the rack inside the FIP backwards (pic. 3).
See, your learnmoment from today 

[Naj ✝︎]

Thank You, lesson learnt.

The fuel pump on my 280 was very noisy when i first got it.
When i went to take it off, i found it was not properly installed (early long pump) and fell off from its round rubber mounting and was resting on the coffee can.
With new bearings and correctly installed, the nasty noise went away.

naj

[Shvegel]

With the rack in the minimum or zero fuel position the pistons in the pump are still traveling Up and down as much as at full fuel. not having any flow through the injection pump or worse yet having the fuel drain from the injection pump while it is running just sounds like a recipe for some very expensive repairs to me.
 

[Benz Dr.]

Those pistons are constantly lubricated from engine oil pressure while it's running. So, you can't burn up the pump by running it without fuel - I think. Fuel flowing through the pump helps to keep it a bit cooler but the main idea is more about keeping fresh fuel flowing through it so that vapour lock is prevented. In this case you will have no fuel running through the pump and since you are not burning any gas, it may not matter. I would call a pump expert and ask them what you should do.

[Eminent]

Well, the main reason i would cut off the fuelpump is rough idling when the engine is at temperature (because the thoughts that there may be a lack of fuel when the vacumm plunger pushes the rack in the FIP backwards).
But looking at my own pictures in this topic i think i found the problem.
The LPG nozzle is in front of the throttle butterfly. It's almost closed at idling so nearly any LPG can pass trough.
The hose mounted at the right of the nozzle goes to the WRD where normally the air filter is fitted.
As you know there is no vacuum when the engine is hot. So there is no LPG transported from the nozzle to the WRD and thus to the intake manifold either.
Nearly no LPG through the butterfly and no LPG through the WRD means, in my opinion, to little fuel to get a smooth idle.
Hope i am in the right direction now.
Everybody thanks for your thoughts and interesses.
Rini.

[ejboyd5]

What regulates the quantity of LPG delivered to the engine as the rpm increases and decreases?

[Eminent]

The vacuum regulated by the throttle butterfly. Same as this regulate the quantity of air what goes inside.

[Shvegel]

Dan,
The entire upper plunger from the annular groove in the barrel runs in fuel. With a machined tolerance between the plunger and barrel of around 2 millionths of an inch it is so tight that there is no physical seal between the fuel and the oil section of the pump.  I was in a shop in Miami that rebuilt my pumps for the generators on the ship I work on and the mechanic showed me how if you hold the plunger in your hand to warm it a little more than the barrel  the plunger will no longer fit into the barrel.  If the outlet check valve on the electric pump or the outlet check valve on the injection pump leaks you run the chance of running the upper pump element dry.

[Eminent]

That sounds as an advice to keep the fuelpump running...

[Benz Dr.]

Dan,
The entire upper plunger from the annular groove in the barrel runs in fuel. With a machined tolerance between the plunger and barrel of around 2 millionths of an inch it is so tight that there is no physical seal between the fuel and the oil section of the pump.  I was in a shop in Miami that rebuilt my pumps for the generators on the ship I work on and the mechanic showed me how if you hold the plunger in your hand to warm it a little more than the barrel  the plunger will no longer fit into the barrel.  If the outlet check valve on the electric pump or the outlet check valve on the injection pump leaks you run the chance of running the upper pump element dry.

This is all for diesel pumps. Actually, you shouldn't touch the injection elements with your bare hands as the acids from your fingers can ruin the finish.

113 injection pumps have a small groove the runs around the injection elements. The oil line running to the pump supplies oil to this groove which acts kind of like a piston ring to hold fuel above the oil grooves. Gasoline would quickly wash away any sealing properties if the oil line were to fail or plug up as this system needs a constant supply of fresh oil on 280SL's. 230SL's are similar but the oil doesn't circulate through the pump and it only goes as far as the oil locks.

As long as your engine is running, oil will be sent to the injection pump and it doesn't matter if your fuel pump is running or not because the elements will be lubricated. Your question is about running fuel through the pump or not and I don't know that answer.

I have a question though. Why are you running this system on your car?     

[Eminent]

1 Ltr gasoline 98 cost about 1.86$. A liter LPG cost about 0.75$
On a tank from 70 liter is that amost 80 dollars cheaper.

[Rodolfo]

When you drive a pagoda let's say 5000 km a year, it makes no sense putting it on LPG.
But North of our border they make sometimes serious efforts for saving money ( ;-)  )

Beware also for backfires in the air inlet.

[Eminent]

Well, the PO did the effort so why shouldn't i leave it as is. The money i save i can use for other projects on my W108.
And even with an earlier moment of ignition (which is necessary by driving on LPG as it has on octan rate from about 108 a 110) i never had backfires.