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Old 01-22-2015, 11:09 AM   #1
tmontague's Avatar
Drives: '08 2zr swapped Vios M/T
Join Date: Oct 2012
Location: Hamilton Ont.
Posts: 2,728
Fuel Trim Readings and EGR valve

Hey guys, I know that using fuel trim (short and long term) readings is a good way to test for vacuum leaks since both reading should read within about 5 points within one another. I used my Ultra gauge to test for vacuum wleaks a couple weeks ago on my fiancees Echo. The values were very close to one another during idle, and driving.

I wanted to check these values on my Yaris since I have a DIY oil catch can and wanted to make sure there are no leaks. The results are a little confusing:

on start up when EGR reads fully closed (0) both fuel trims are very close (virtually the exact same).
When on highway or under throttle EGR is either 100 or close to, both fuel trims are essentially the same (so far so good)

Here's the strange part:
-during coasting or idle when the car is warmed up the EGR read 0 (normal) but the furl trims are way off. Typically short term reads about 0-5 and long term reads 15-18.

I'm assuming because there are no exhaust gases coming back in to intake (EGR closed at idle due to no need for extra fuel) the long term fuel trim is set up to add fuel into engine to keep enough fuel in the mixture. If this is in fact correct then why is the Echo showing both values always closely corresponding but my Yaris is not?

This is not causing any issues with how the car runs, CEL's or fuel efficiency I'd just like to know if this is a normal thing to have happen or not. And if it is in fact normal, I guess the only way to check for vacuum leaks in the Yaris would be through either a cold start or while the EGR valve is fully open?

Coles note:
Fuel trims only correlate when EGR is open or while car is not up to operating temps. No symptoms shown in car. Long term fuel trim shows about 10-15 points higher when EGR valve is closed
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Old 01-22-2015, 01:28 PM   #2
Drives: 2007
Join Date: Feb 2014
Location: motown
Posts: 293
The old method is to spray WD40 while idling any leak will cause an idle change,
once the WD is sucked in. Primitive but works.
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