Unmetered Air leak = Better MPG???
 

So, this thread is in reference another thread I made in "Success Stories" where it seemed that changing my accessory pullies out suddenly gave me better mpg. However I found an unpluged line on my intake tube that was letting unmetered air in... Well, since I've plugged it back in my mpg has droped. My typical mpg reading at 50mph was mid to high fifties, now it's mid to high fourties. Maybe the ECM messed with the long term fuel trims when the leak was happening? So after getting a couple consistant commutes to see what the current mpg was, I found I was getting high thirties and maybe touching on fourty mpg by the time I got to work. This morning I unpluged the hose to see what would happen and I imediatly noticed a big change. In fact I was at 50.3 mpg when I got to work this morning and the mpg at 50mph was high fifties, low sixties.
To be clear, this is a hose which plugs into the intake tube between the MAF and Throttle Body. MPG before unplugging the hose was high forties on the way to work, after being unplugged it jumped up to high forties, low fifties, after pluging it back in it dropped to high thirties low forties. Now after unplugging is again it's high forties low fifties.
Does anybody have a clue why this is happening?
I don't imagine my SG is being fooled. I figured the ECM would just see what's happening at the O2 sensor and compensate if things were lean? Speaking of lean, I get a little ping when acclerating with this hose unplugged, so I am guessing it's a bit lean.

I would assume it's just a lean condition from extra air if the car isn't compensating for the leak with extra fuel. Does the leak not affect MAP readings?

Do you happen to know if your vehicle has a MAP sensor? I can't be sure without the diagram, but I'd think the MAF, the O2, and MAP sensors all work together to determine the amount of fuel injected. Often, if one sensor is faulty, then the PCM will throw out the faulty reading and rely on the other two. In your case, the O2 reading should have been coming in high, leading to additional fuel injected. However, the PCM may have assumed your O2 was faulty and relied upon the other two. If that was the case, you may well have been running lean.

No MAP on this car. The SG doesn't show any noticable change in the O2 sensor reading. It fluktuates between 13.5 and 15.5 ie. HI/LO. Wish I had a wideband...
I popped the hood and traced the route of the line, it's conected to some sort of valve that is also conected to the PBB and below the TB, so there is vaccum somewhere in this arangement, but that doesn't really mater... Why does unmetered air flow cause better mpg?

I have the same car with a manual trans, and as a test I disconnected this line to see what was going on. I had 50 miles on my last tank when I first disconnected this and ran it that way for the whole tank (400 miles with it disconnected). When I first disconnected it and started the car, there was a noticable stumble and it did not want to idle (not unexpected). I drove it anyway and after a few miles, it idled fine. At times I did see some high mpg numbers on the MPGuino for instant mpg's, and it felt as if the throttle seemed to be more sensitive. I was cautiously optomistic. Overall, the tank mileage was not any better than I normally get (42.67 versus 43.18 for the previos fill). Check the fuel log in my sig to see my history. I still have the line disconnected, to keep an eye and see if any long term benefits will show. So far, it appears this tank is going to be about the same as the last. As an aside, the weather is a little cooler and I expect the mpg's to go down with it. Hard to tell as of now what affect it is having. I have not had any pinging with this disconnected, but your motor had some performance mods I do not have, so that may have something to do with that.

If I had time I'd do some ABA testing...you got time Moorecomp?

BTW, I got nearly 44mpg on the way home tonight, which killed my last couple commutes with the line conected...34-38mpg. I'm begining to think I need to the fuel trims are messed up. I just need to get the SG programed to read them, then I can compare after reseting the PCM and running it both ways...

Is there any aftermarket fuel management readily available for your car? Sounds like it might be worth tweaking around with.

I am still tweaking my MPGuino setting for injector ms/gal, so I can only go tank to tank for accurate results. I have the vss pulses per mile dead on. Last tank was over 400 miles and I was within 1 mile between the odometer and the guino. My commute is out and back the same route on 2 lane roads. Approx. 90 miles of 55mph hwy and 15 miles of 35 mph city total. I will finish this tank with the hose off, and then put it back on for the next tank.

With our ZETEC engines having no egr, and using the VCT for emissions purposes, I wonder if the results you are seeing are a combination of the sensors and the cam timing.

I was thinking of something like this for tweaking the O2 sensors for trying to lean things out:

DIY Dual O2 Sensor U-Solder Kit (EFIE)

Having fun yet?

I wish I could come up with an answer to this other than to say that you're running lean. But I'm at a loss on this one, and couldn't properly diagnose anything without "being there".

Sorry, dude.