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How does no MAF systems work?

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5 years 7 months ago #23914 by blazerguy1983
How does no MAF systems work? was created by blazerguy1983
I have a 2004 Jeep Liberty with the 3.7L that doesn’t utilize a Mass Air Flow Sensor. How does this system calculate fuel trims? From what I understand it uses data from the MAP sensor, but I don’t understand how that works.

My fuel trims are a little high (lean) and range from +8 to +13 in the long and short terms. New down stream O2 sensors NTK. Can not find vacuum leaks. No codes or drivability issues. Any thoughts? Thanks!

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5 years 7 months ago - 5 years 7 months ago #23934 by Chad
Replied by Chad on topic How does no MAF systems work?
Fuel trims are based on upstream O2 sensor data. O2 sensors go rich, fuel trims go negative. O2 sensors go lean, Fuel trims go positive.



An engine without a MAF sensor is called a "Speed Density" engine. Air flow calculated based on Engine RPM, Manifold Absolute Pressure (Load), and Air Temperature.
"Knowledge is a weapon. Arm yourself, well, before going to do battle."
"Understanding a question is half an answer."
I have learned more by being wrong, than I have by being right. :-)
Last edit: 5 years 7 months ago by Chad.

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5 years 7 months ago #23936 by Tutti57
Replied by Tutti57 on topic Re:How does no MAF systems work?
What method are you using to look for vacuum leaks and what happens to the fuel trims with some rpms vs idle?

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5 years 7 months ago #23955 by Andy.MacFadyen
Replied by Andy.MacFadyen on topic How does no MAF systems work?
Speed density is really just a different way of measuring the mass airflow. If the engine computer knows the manifold absolute pressure and inlet air temperature it can use a look-up table to find the required injector duration and ignition advance required for a given RPM . There are also corrections for throttle position, fuel trim and of course fuel trim.
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