Port Fuel Injection Vs. Direct Fuel Injection. Seriously, why do I care??? :P


Today’s discussion is about 2 different forms of fuel injection systems and why this actually does matter to YOU. Yes… YOU. The one that really doesn’t care about this. The one that doesn’t WANT to care about it and and in all likely hood will not read this post all the way through and end up in a shop sooner than later with a rather expensive bill. Do I have your attention now? 😀

So lets start with what the heck are we talking about here? Fuel injection is the system that gives the engine the fuel it needs to run. It’s responsible for giving the engine just the right amount of fuel at just the right time. Too much fuel, too little fuel, or fuel at the wrong time or delivered in the wrong way are all bad things for your engine and if any of these things happen, you will know it in the form of a rough running engine, low power, or an engine that just simply will not run at all. Now that we have that out of the way, lets talk about the 2 major kinds of fuel injection and just exactly why it even matters to someone who really just doesn’t want to think about it.

PFI (port fuel injection), which is the older system, and GDI (gasoline direct injection), which is the new technology, are the two kinds of fuel injection systems in use today on gas fueled engines. The major difference between the two is the placement of fuel injector and the fuel pressure required to operate the system.

In a PFI system, average required fuel pressure is anywhere from 35-60 PSI depending on the system. This system uses one fuel injector per cylinder. So if your engine is a V-6, you would have 6 fuel injectors. if it was an inline-4 cylinder, you would have 4 injectors, etc. The fuel injector is an electrical on/off valve if you will that when electrical power is applied, the internal valve opens and fuel is sprayed out. When the electricity is removed, the injector valve closes and fuel spray stops. This on/off happens very quickly. The amount of time the injector valve is on, or open, is measured in milliseconds. Sometimes single digit milliseconds. For reference, there are 1000 milliseconds in one second. That’s quick! The PFI fuel injector is located in the engine’s air intake manifold and is directed at the back side of the cylinder’s intake valve. We will discuss why the injector’s location is important a bit later.

A GDI fuel injection system is quite similar to a PFI system. Like the PFI system, the GDI system uses individual injectors for each cyllinder, they are electronically controlled and function much the same as the PFI injectors. The differences are that a GDI fuel injector needs upwards of 2000 PSI of pressure under heavy load operation. This requires a special mechanically driven fuel pump to produce that much pressure. The other significant difference is the injector location. Unlike the PFI system, where the injector points at the back side of the intake valve, the GDI injector is aimed directly into the combustion chamber, or cylinder as it’s more commonly known.

The PFI system on the left has the yellow injector above the valve. The GDI system on the right has the yellow injector below the valve, directly into the cylinder.

The injector’s location change is significant because fuel itself is a great cleaner. Because the injector in an older PFI system sprays fuel directly onto the intake valve, as seen above on the left, it is constantly cleaning carbon and build up off of the valve. The newer GDI injector is located after the valve as seen above on the right, directly in the cylinder therefore no fuel ever touches the valve and this allows large amounts of carbon build up to accumulate. When the carbon build up gets to the point that it interferes with the valve’s ability to seal on its seat, engine misfire and roughness begins to occur. If the build up gets too bad, the only way to remove it is to completely remove the intake manifold and use special media blasting equipment to blast the carbon build up off the valves and intake ports. In extreme cases, the cylinder head will need to be removed to remove the valves and replace them. This can be very expensive and is completely avoidable with regular induction cleaning services.

This photo shows on top, extreme carbon build up on the valves. On bottom, you can see what clean valves should look like.

We recommend on a GDI engine to have this service performed every 15,000 to 20,000 miles. Yes, that’s right. Carbon can accumulate to the point of causing runnability issues as soon as 40,000 miles. GDI systems offer greater fuel economy, more precise fuel delivery and more engine power. The trade off is some extra maintenance needs to be done to keep this system in top operating condition.

If you are unsure if your car or truck uses a GDI or PFI system, we are here to help! Visit www.auto-go.net or call us at 417-272-0091 and we will be happy to discuss this with you and determine your engine’s specific maintenance needs!

Now you know!