Rick's Free Auto Repair Advice

What is fuel trim?

Fuel trim and what it means

Engine computers use fuel trim to attempt to regulate the right amount of fuel to achieve maximum power, fuel mileage while also minimizing emissions. So they try to maintain a 14.7:1 ratio. If the engine were stationary, like on an engine stand, and you ran it at a fixed RPM it wouldn’t be hard to come up with a fuel flow to maintain consistent power and emissions. But that doesn’t happen in the real world. You’re always going up or down a hill, driving on smooth and rough road surfaces, in changing temperatures and changing speeds. So the engine computers are always changing the amount of fuel it meters into the cylinders. The factory programming takes all of these parameters into account.

Things change and that’s where fuel trim comes into play

But what happens as an engine wears? Valves don’t seal as well, a cylinder develops a weak spark or misfire, the camshafts wear slightly, a vacuum line develops a slight leak that introduces unmetered air or the piston rings and cylinder walls begin to wear. Now you need to add some kind of modifier to the factory programming. That’s where fuel trim comes in.

Long term fuel trim and short term fuel trim

The engine computer begins compensating by adding or subtracting a certain amount of fuel, from -25% to up to +25%. If the addition or subtraction helps reach the emissions goal, and the engine requires this modification for a long period of time, The engine computer will determine that it must make a semi-permanent adjustment to the factory programming. At that point it moves the correction in the long term fuel trim category and either adds or subtracts fuel on every start up and every trip.

What are troublesome fuel trim numbers?

Generally speaking, a fuel trim that exceeds 10% means you’ve got an engine or ignition problem. However, on late model direct injection engines, a fuel trim adjustment of as little as 7% can mean you’ve got a significant issue.

How to use fuel trim numbers

Let’s say you have a trouble code for a misfire in a single cylinder. If the misfire is a lean misfire, combustion that goes out too soon because there wasn’t enough fuel, the exhaust will have too much left over oxygen. The engine computer will try to compensate by increasing fuel trims. If the misfire was a rich misfire due to a leaking fuel injector, there will be too little oxygen left over and the engine computer will pull back on fuel.

In a vacuum leak situation, unmetered air gets into the engine causing a lean condition (not enough fuel for the amount of air in the cylinder). In this case, if you see a Misfire code, along with an increase in fuel trims, you should immediately suspect a vacuum leak.

©, 2019 Rick Muscoplat



Posted on by Rick Muscoplat


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