Fuel Trim Tech, 6G74, Simulated Manifold Leak, Lean Condition

[Salonika]

The link should work this time. I tried to post this earlier and the video wouldn't work.

Anyway this topic comes up from time to time and I thought it would be helpful for people to see how the engine control system responds to an air leak in real time.

In this video you will see that when unmetered air (air entering the engine AFTER the MAF) is allowed into the manifold to create a lean condition, the oxygen sensors detect it immediately and cause the computer to add fuel to richen the mixture and return it to the correct 14.7:1 ratio, known as fuel trimming.

Enjoy.

 

[evomaki]

Would the sort of air leak you mocked up cause a CEL and a trouble code, or does one need to connect a scan tool and check fuel trims?

 

[Salonika]

What I did in the video did not trigger a CEL. I don't think you would get a CEL unless the trims were excessive (I think they max out at + / - 25%) If everything is operating to spec (fuel pressure, injectors, MAF reading correctly, o2 sensors are good and all seals and gaskets are keeping air leaks at bay), your short term fuel trims at idle should be bouncing around 0. Air leaks though aren't the only driver of fuel trims. Fuel system issues can also change trims.

If the amount of air measured by the MAF combined with the fuel needed at idle is keeping the o2 sensors happy then no fuel trim adjustments are needed. Remember our trucks are blind basically when it comes to fuel, other than o2 sensors. We have no fuel pressure sensors. The ECU doesn't even know if the fuel pump is powered up or not.

As the engine ages, the mixture can be maintained and the engine will run even with minor problems. This is an area that you can find very little information on though, the trouble limit logic built into the ECU. Imagine low fuel pressure combined with air leaks, and you could get maxed out fuel trims. I don't think there is an easy way to make an air leak big enough to get past 10% to test it. Also no way to manipulate fuel pressure. I would love to try though.