Why would anybody care about fuel trims? What does it matter how the ECU feels?

All the fuel trim is showing you is an output parameter decided by the ECU.

I’m interested in finding the objective cause of my poor performance. Do I have a vacuum leak or not, that’s what I want to know.

I’m trying to study the real world. The objective world. Some clipped hose or gasket somewhere in the labyrinths of the engine. Air spiriting itself past a gasket. Not how the ECU is feeling this morning.

But it hit me like an automatic transmission tapped into reverse on the interstate: we don’t experience cars or roads or milkshakes. We don’t even experience the feelings of our own hands slapping our own foreheads.

All we can hope to experience is our own consciousness. That consciousness is wrapping itself around the outside world, but it’s the consciousness that constitutes our world.


That’s Husserl, who was Czech like Skoda. He stood on the shoulders of Hegel, Stuttgartian contemporary of Karl Benz. Husserl’s student was Nazi-loving Martin Heidegger, and Heidegger’s student in turn was Sartre, darling of fourteen-year-olds everywhere. But Husserl, unlike his prodigal clove-cigarette-smoking intellectual offspring, had a PhD in mathematics. Respect.

I don’t bask in the volatile organic aromatics wafting from the engine bay. I bask only in their smell. That smell is consciousness: the squishy placenta between us and the world.


So look. About that vacuum leak. Real-time data is coming off the OBD2. All of it is filtered through the ECU’s consciousness. Or would be if the ECU had a consciousness. It doesn’t, though, because it’s purely algorithmic. Its only consciousness is the collection of its actions.

The ECU never gets to consider who it is, or even what it is. It simply is. Its consciousness consists of its actions.

Sartre said humans are unique because we choose to be born. He later denied ever saying that. But his larger point stands: humans’ actions have many degrees of freedom. Even the most un-free human you can imagine, perhaps a slave, has many things he can do. Many dimensions among which he can act. An un-free person has a consciousness by which he can define himself, and a large collection of actions by which he can further make who he is, create himself in the existential sense.


An ECU? Not really. An ECU can give fuel or not. Time the firing of the spark plugs. Maybe speed up a downshift. That’s about it. Its degrees of freedom are few.

But whether consciousness is explicit, as in humans, or an implicit product of actions, as in machines, it is always consciousness of a world. Husserl famously called this aboutness: consciousness always is about something, while a physical body part just is. Like transitive and intransitive verbs.


That’s why it makes no sense to talk about consciousness in a vacuum.

What would your fuel trim readings be if your ECU were not even installed in a car?

How about your 0-60 times if you had no wheels?

So here we are. Monitoring the fuel trims. Watching the ECU act. And trying to discern the reality of the world the ECU is seeing through its actions.


Suppose you had to draw a terrain map while flying in a low-flying airplane. You weren’t allowed to look out the window. But you were able to look at the pilot’s actions in flying the plane. Watching when she turns or climbs to avoid obstacles, you could make some attempt at a terrain map, couldn’t you? Without ever looking out the window directly. Because in phenomenology, you can’t see out that window directly — you can only see the ECU or the pilot or some other form of consciousness.

Sartre had a similar thing about watching a person stumble around a dark room bumping into walls, except Sartre said that’s basically how we live our whole lives, at least until we learn where the walls are. That’s how the ECU lives its life — and with autonomous tech, it can even learn where the walls are.


Fuel trims are revolutionary as a diagnostic method. They’re amazingly deep. It’s like the urban legend about Eskimos having fifty-eight words for snow. If your ECU can only act by adding or cutting fuel in response to a whole range of outside-world phenomena, isn’t it going to develop a whole complex range of responses? Like the mythical snow-describing Eskimos.

If your fuel trims are chronically high or low, say ten points above or below even, something’s not right. The ECU shouldn’t be reacting like that all the time. It’s like the low-flying pilot who’s always trying to pull at the control wheel to bring the nose up. Something’s going on.

Years ago, when I wasn’t sure whether I trusted fuel trim theory, I diagnosed worn-out fuel injectors on my Land Rover that way: chronically high fuel trims meant there wasn’t enough fuel getting shot into the combustion chambers.


And now for the vacuum leak.

Think of your ECU’s lebenswelt. Its lifeworld. When the car is just idling, the vacuum leak is there. Like a stubbed toe. The extra air’s presence is huge. It’s, well, extra. Oh shit oh shit there’s too much air kick in more fuel. The fuel trim will be high.

But when you press the throttle down, you’ve just introduced a whole load of gas and air into the engine. In this situation, that stubbed toe isn’t so prominent anymore. The vacuum leak’s air — if it’s not a huge wide-open leak, at least — is pretty insignificant compared to the air coming in from your open throttle. So the fuel trim falls off. Because your ECU’s consciousness is no longer dominated by the sight, or the pain, of that extra air from the vacuum leak.


Do the old propane-spray trick and you’ll either gloriously self-immolate or be able to isolate where the vacuum leak is. When you spray near the leak, the ECU’s consciousness will go nuts. STFT will shoot up. All we can observe is how the ECU is acting. How it experiences its consciousness. All we’re seeing is that the ECU is adding fuel. But the power of phenomenology, or of fuel trim theory, is by knowing the ECU’s actions and consciousness we are studying not just the ECU, but, because of aboutness, we are studying the vacuum leak itself.

Read up on fuel trim theory. It’s a powerful diagnostic paradigm. You can get into fuel trims with a simple Bluetooth OBD2 dongle and Torque.

Short-term fuel trim is immediate adjustment and gets reset at engine restart. Long-term is what it sounds like, long-term. Use them in unison to identify problems: generally reading the relatively static LTFT number to find that there is a chronic problem of some sort, viewing immediate change in STFT to track down exactly what the problem is.


Remember we’re only experiencing our consciousness, not the world outside directly. “Well, that’s just like, your opinion, man.”

But avoiding the sticky parts of intersubjectivity is easier for cars than for people: presumably you care only about what your car’s ECU is experiencing, not what every other car’s ECU would think of the matter. And, keeping part numbers and revision dates constant, ECUs are identical to one another. Human brains mostly aren’t.