The Trainer #67 - When Fuel Trims Won't Trim

In the May edition of “The Trainer” we explored how the ECM maintains fuel control. We also talked a bit about the factors that can cause fuel control to get out of whack.

This month, we focus on what to do when it does get out of whack. System Lean DTCs are among the top 10 Diagnostic Trouble Codes technicians face in the shop and can be a challenge to correct. System Rich codes, while not quite as common, can often be even more difficult to isolate. It’s not hard to understand why the Engine Control Module sets these codes. That, of itself, is pretty simple. If Total Fuel Trim numbers exceed a programmed threshold, plus or minus, the ECM will turn on the Check Engine light. It’s the reason the trim drops or skyrockets that we have to figure out.

Most of us start by looking at the amount of air getting in and the amount of fuel being added, don’t we? After all, if it’s too “lean”, it must be too much air or too little fuel. The opposite is true for a “rich” condition, or so you would think. What if the code is set for only one bank? What then?

The ECM has to know how much air (by weight) is coming in and then calculates the amount of fuel (by weight) to add. It doesn’t do this intuitively – it has to be told, either by inputs from a variety of sensors or by its internal programming. Sensors that are misreporting may make the ECM “think” that the mixture is rich or lean, when it really isn’t. Even the software may be incorrect, at least under certain operating conditions, and a reflash may be the fix.

We’ll cover all of those variables and talk about efficient ways to check for, and isolate, the REAL cause of these DTCs. Be sure to watch!

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