Running on empty - what does it really mean?

[Martin]

The user manual tells you how much fuel is in the tank when the low fuel warning light comes on

Running completely out of fuel carries some risk of killing the high pressure fuel pump which will cost $800+ to replace

[sports racer]

How would it damage the fuel pump? If it's not pumping petrol it would be pumping air which wouldn't do any harm.

It wouldn't pick up any debris in the tank because there's a filter to stop that stuff going into the pump.

[Diesel_vert]

The presence of unburned fuel in the exhaust system, due to intermittent or inconsistent fuel supply, may cause damage to the catalytic converter.

For direct injection engines, a lack of fuel supply may cause damage to the high pressure fuel pump, because fuel acts as a coolant and a lubricant.

Do not continually crank the engine if the vehicle is out of fuel.

[wai]

The thing to remember is that the fuel gauge is really a very approximate indicator. The amount of fuel indicated will vary from vehicle to vehicle.

If you really want to know how much is there, you need to get the system calibrated. There really is not that much demand for a super accurate gauge.

Now if you want to know when you need something more than just an indicator, think of someone with an original Mini that had just a 20 litre fuel tank!

[Martin]

How would it damage the fuel pump? If it's not pumping petrol it would be pumping air which wouldn't do any harm.

Well explained by Diesel_vert

The presence of unburned fuel in the exhaust system, due to intermittent or inconsistent fuel supply, may cause damage to the catalytic converter.

For direct injection engines, a lack of fuel supply may cause damage to the high pressure fuel pump, because fuel acts as a coolant and a lubricant.

Do not continually crank the engine if the vehicle is out of fuel.

The following may help explain why the high pressure fuel pump needs fuel for lubrication and cooling

Normal fuel injection into the inlet manifold only uses 40~60 psi fuel pressure

The direct or stratified injection systems (into the combustion chamber) run fuel pressures up to 2200 psi on some vechicles (1,900 PSI on the FSI engines)
(The fuel rail is often thick walled to ensure it's strong enough to hold the pressure)
The internals of the high pressure fuel pump are created on a very expensive machinery and have incredibly tight and consistent internal tolerances (you could not make one on a lathe for example)
The high pressure fuel pump is not lubricated by oil
Creating ~2200 psi of pressure creates heat - fuel must be present for both lubrication and cooling

One of the tuning upgrades for the FSI engine was an upgraded high pressure fuel pump,
buying this outright costs $1,650 and even $1,200 if you exchange your stock pump
APR - High Performance Development for Audi, VW and Porsche Vehicles.
(There is no need to upgrade the TSI high pressure fuel pump)

[Rawcpoppa]

The presence of unburned fuel in the exhaust system, due to intermittent or inconsistent fuel supply, may cause damage to the catalytic converter.

For direct injection engines, a lack of fuel supply may cause damage to the high pressure fuel pump, because fuel acts as a coolant and a lubricant.

Do not continually crank the engine if the vehicle is out of fuel.

I'm trying to understand why there would be fuel in the exhaust system due to intermittent or inconsistent fuel supply. If the engine is running it should burn all the fuel that enters each piston as per usual no?

[Idle]

Quick reply:- No

[Rawcpoppa]

Quick reply:- No

Looking forward to more details.

[Idle]

Slightly less quick reply:

Well, for a start, petrol engines have an optimum air to gas (evaporated petrol) ratio at which they run best and give their best power to economy performance. My aging memory suggests 14.7 to 1, but anyway it's thereabouts.

Long periods below this will result in a hotter running motor — substantially below can cause valve damage and even burn holes in pistons.

Ditto above this will result in a colder engine and unburned fuel in the exhaust system and in the oil (when you see black smoke from the exhaust, petrol or diesel, this indicates unburned fuel exhausted from the motor, and will result in poor economy.)

This has been the case for as long as there have been internal combustion engines, and never more so than today, with compression ratios of petrol motors approaching those of diesels, manufacturing tolerances very fine and materials cut to the limit (and sometimes beyond...)

A quick google will bring you more detail on the subject — probably more than you can cope with.

[Diesel_vert]

As per above, an engine needs a stable and consistent fuel supply for combustion to occur normally.

An intermittent or inconsistent fuel supply may cause the engine to misfire, a condition where fuel is burned partially or not burned properly. This allows raw fuel to pass through the engine and into the exhaust system.

As the exhaust system in a petrol engine is normally very hot, raw fuel is ignited by the heat and causes a large temperature increase. This will cause damage to the catalytic converter, as well as other components in the exhaust system.

Almost every passenger vehicle fitted with a catalytic converter will have the same warning written in the owners manual.