Reader Question: The DI plus the PFI?

Here’s the latest reader question, along with my reply!

Michael asks: I understand that the way direct injection’s tendency to carbon-foul intake valves is being addressed is by adding a port-fuel circuit just to spritz the backs of the intake valves; i.e., not to introduce fuel into the cylinders for actual use creating power.Is this true?

My reply: Unfortunately, it is true.

Almost all new cars have direct-injected engines. What this means is that the injector (one for each cylinder) is screwed into the cylinder almost like a spark plug and shoots an ultra-fine mist of fuel at extremely high pressure directly into the cylinder. Port fuel injection systems mount the injector outside the cylinder, usually just behind the intake valve. The wash of fuel acts as a solvent as well as a fuel; it keeps carbon from crudding up the backside of the intake valves.

But, it’s less “efficient.” Not much – we’re talking gains in economy of perhaps 2-3 MPG overall by switching to DI – but that matters hugely to the car companies insofar as trying to “achieve compliance” with federal MPG mandates.

Which is why DI is becoming default standard in new vehicles.

As is an increased likelihood of carbon fouling, which creates its own set of problems – and expenses.

To remedy them, most manufacturers are adding the additional port-fuel-injection circuit to the system, for the sole purpose of backwashing the intake valves to reduce the carbon fouling problems.

But this doesn’t come free. You are now buying two fuel-delivery systems (DI and PFI) and assuming the inevitably higher chances of down-the-road repair costs that come with greater complexity and more parts.

You can thank Uncle for all of it.

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