Reader Question: Dual Injection Systems?

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

Brian asks: I have been looking for a used car. But I want something a little older and am concerned about carbon buildup on these direct injection systems. I am having a really hard time finding which cars have multi-port injection. I don’t want something that is too old to not be able to daily drive. When I came across a 2008 Lexus is350 and found out that Toyota put in this d-4s dual injection system. They put it in this car as a way of cooling down the engine. Now of course this makes the whole thing more complex… but it is made by Toyota. I am just struggling here.

My reply: Indeed, it is becoming almost impossible to find a new without direct-injection, an expedient resorted to as a way to slightly increase mileage, with some performance benefit, too. The problem is that DI engines are more prone to carbon fouling of the intake valves, as you note. The solution – adopted by a number of manufacturers – is to add a dedicated PFI circuit to wash down the valves with gas, to keep the crud formation at bay. Of course, this is akin to breaking someone’s leg and then fixing it by giving them crutches. We’d be much better off without DI, which offers little substantive benefit to the car owner over PFI. But because it benefits the car manufacturers – in terms of helping them comply with government regs – we get saddled with the additional cost/complexity.

This is what comes of empowering government to design cars!

. . .

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  1. “… this is akin to breaking someone’s leg and then fixing it by giving them crutches…”

    Not necessarily, adding fuel before the valve offers a performance advantage due to inlet charge cooling/increased density. It’s some of the reason carbs were easily capable of making more power than early (pre-200x?) factory efi (on the same motor). The increased complexity of dual injection is still a potential downside though similar systems work great in racing applications (where money is not an object)…


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