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Renix fuel trim.

outlander

NAXJA Forum User
Location
Columbus,Ohio
My short term and long term trims are up a bit and I suspect a vacuum leak so im trying to troubleshoot.
While researching fuel trims I came across an industry professional that made this statement:
On engines that use a manifold absolute pressure (MAP) sensor to determine airflow, a vacuum leak does not affect fuel trim because the extra air (pressure) in the manifold is still measured by the MAP sensor.
Sounds plausible....true or false?
 
Possible, depending on engine speed, but I'm calling B.S. Vacuum leaks are generally less noticeable at higher speeds when the engine is sucking in more air, but more noticeable at idle when the throttle blade is shut. If you suspect a vacuum leak, I recommend investing in a good vacuum gauge. I don't know about the Renix intake manifolds, but my H.O. has a few capped off ports where you can connect a gauge pretty easily. IIIRC, a good vacuum reading is 19-21 in/Hg at idle, but you can subtract 1 in/Hg off of that for each 2,000 feet above sea level. Also, make sure the engune has plenty of time to warm up (basically, operating temperature) before you take the reading. If the vacuum reading is low or you still suspect a leak, you can try spraying WD-40 around suspected or probable vacuum leak locations while listening for a change in idle speed. (Higher speed=you've found the leak/leaks.)
 
the iac is a stepper motor. it sets itself at shutdown it will just run for a set time to close or open. then it will step out to a predefined opening.

what are your fuel trims at?

and its true a vacuum leak with a MAP sensor will not effect mixtures just cause a higher idle.
 
What controls the renix iac circuit (besides the ecm)
I mean how does the iac know where to position the pintle...?

There is an ASD relay that keeps the power on the ECU for a few seconds after shutting off the key to allow the ECU to step the IAC to where it needs to be to start. My guess is that it runs it closed and then backs it off a certain number of steps.
 
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