Tech Tip - Fuel Supply

Here's another quick tech tip/info article, this time on how important it is to understand your Fuel Supply in your EFI vehicle.

The fuel in an injected vehicle is delivered under high pressure, for a few important reasons. Firstly it prevents vapourisation of the fuel, especially on hot days. Secondly it allows much better control of the delivery into your engine. When it is injected under pressure it allows the fuel to atomise which makes for a more efficient burn in the combustion chamber. The less atomisation, the more unburnt fuel and hence, bad emissions.

Okay lets get to the nitty gritty. First we need to identify the main components in the fuel system, starting from the source, the fuel tank.

First in line is the fuel pump - probably the most important component in the system. It's job is obviously to supply fuel, duh! EFI pumps are great at pumping but pretty bad at sucking, in fact you could even go so far as saying they suck at sucking! For this reason they are usually placed inside the tank at or very near the bottom. There are some that are external, and you will usually find that they have a pre-pump inside the tank to feed it. At this point it is important to mention that the pump supplies volume, and this importance will be outlined later.

Next in line is the filter. This is obviously important to filter out contaminants that might wreak havoc on other components in the system. You can NOT check an EFI filter by blowing through it! Try it on a brand new one and you will know what I mean :P

After the filter, the fuel goes to the fuel rail where the injectors are mounted. The injectors are switched on and off rapidly by the ECU (Engine Control Unit) so that the precisely correct amount of fuel is supplied to the engine.

After the rail is the pressure regulator. This is responsible for maintaining correct fuel pressure and bypassing unused fuel back to the tank. Understanding this is paramount to correct diagnosis of fuel systems and you will be surprised how many "service professionals" fail to understand what I am about to tell you!

Now, the best way to test a fuel pump is to check the fuel pressure, right? Wrong! When you check the fuel pressure, all you are doing is making sure the pressure regulator is working. Granted, if you have a dud pump you may measure little or no pressure but guess what, your pump will show signs of failure long before it drops fuel pressure! What you really need to measure is return flow. This is basically how much fuel "reserve" you have in the system. Your fuel pump needs to supply volume under pressure, so free flow or pressure alone will not tell you how good a pump is. For example, lets say that the fuel reserve on your car with a good pump is 2 litres per minute, and your engine under maximum power requires 1.5 litres per minute of fuel. In this case, on the rare occasion (for most people) that you have your foot flat to the floor you still have half a litre of fuel being returned back to the tank. Hop in the doc's DeLorean (I hope that Flux Capacitor is working...) and go forward a few years when your pump has done a lot of work and is close to worn out and it has dropped it's ability to supply a good flow of fuel. It is now only returning ONE litre of fuel back to the tank, and this is okay for normal driving but you are noticing a lack of power up hills and for acceleration. This is because once your engine requires more than one litre per minute of fuel, your poor pump can't supply it anymore and your engine starves of fuel. But wait, your pressure is still okay when you check it with the car idling! If you were to check it under full load you will find that it drops as soon as the enigne's fuel requirement overtakes the ability of your pump to supply it.

This is why checking fuel supply is way more important than checking pressure, and here's the great part - you can buy the tools to do this for just a few dollars! A good quality EFI pressure guage kit could cost many hundreds of dollars, and is still an important tool for the service professional but way out of reach of most home mechanics and DIYers. All you need is some basic tools to remove the return line and a jug or bottle to measure the amount of fuel. Run the pump for 30 seconds, measure the amount of fuel captured, double it and there is your flow rate in litres per minute.

Well so much for the "quick" tech tip, but I sincerely hope that you have learnt something.

Come back for more :)

The Doctor.

Any questions?....