would creating a small vacuum leak after your MAF sensor - Fuel Economy, Hypermiling, EcoModding News and Forum

IsaacCarlson · 09-08-2009, 03:20 PM

lead to a small increase in mileage by leaning the mix a little or would the o2 sensor/vac sensor pick up on that?

tasdrouille · 09-08-2009, 03:41 PM

The O2 sensor will pick it up and adjust the fuel trim accordingly.

IsaacCarlson · 09-08-2009, 04:26 PM

Tygen1 · 09-08-2009, 07:01 PM

Unless the leak is so big the ECM can not compensate

tasdrouille · 09-08-2009, 10:51 PM

In which case it'll just ignore the MAF, throw a code, and trust the 02.

dcb · 09-08-2009, 10:53 PM

it will code, and stumble. O2 cannot react quickly enough to make it smooth.

tasdrouille · 09-09-2009, 08:26 AM

I'll unplug my MAF on my Elantra to test. I'm pretty sure there is a default map the ECU uses as a baseline if it can't get a response from the MAF.

dcb · 09-09-2009, 08:41 AM

no maf signal at all is different than air bypassing the maf. You will probably get a "no maf" code instead of a "too lean" code.

I just replaced a leaky intake hose that was downstream of the maf. Had some major flat spots on acceleration (and a too lean code).

Tygen1 · 09-09-2009, 01:32 PM

Well, I recently had a vacuum hose disconected after the MAF, drove four days that way and the mpg inceased by about 1. There are other factors though, all new accessory pulleys and time belt pulleys were the reason the vacuum line became unpluged. Pluging it back in this week has returned me back to the normal mpg. I can't totally explain this, just an observation until I get more time to test it out.

moorecomp · 09-10-2009, 11:43 AM

This would not really be a vacuum leak, it is an intake air leak. There is no vacuum in the air intake runner, only vacuum is after the throttle plate. However, an air intake leak would allow unmetered air to bypass the MAF. This would result in lean condition, but a small leak may not throw a code, as long as the adjustments made by the ECM keeps the O2 sensor information in the proper range. The ECM does not only adjust fuel trim; but also the ignition timing and on our engines, the variable cam timing (exhaust cam). The O2 sensors operate in a range that is considered normal. Maybe you were running on the lean end of normal?

I believe you are running the 5.6K resistor in your IAT? The air intake leak would have the same affect, it would be fooling the ECM into thinking there is less mass of air flowing through the intake (since some would be going around the MAF not through it), so it would adjust the fuel trim, timing and cam. Maybe the leak was acting similar to your resistor?

PS: Let me know which line that was! I want to test it out.