About three weeks ago it finally got warm enough to give my A/C its first run of 2017. It cooled down great but I noticed my blend doors are stuck on defrost (I know this is a common vacuum-related issue). I figured I would investigate it later, I don't really use my A/C that much.

Over the course of the proceeding three weeks my fuel mileage has improved by more than 10%. For the last five years I typically fill up at around 225 miles (that's when my low fuel light comes on during around town driving) but now it's not coming on until 260 miles. It's not an issue with the tank sender unit because it takes the same amount of gas (17.5 gallons) to fill.

I've heard of vacuum leaks being detrimental to fuel economy but I've never heard of it causing an improvement. My engine temps are normal, everything appears normal, other than the mileage. Any ideas? Could they be related or is it just a coincidence? I'm going to hunt down the leak that's preventing my blend doors from moving, fix it and see if it affects my fuel economy.

 

There's your problem right there.


NEWS FLASH: You have no idea what your mileage is. "Miles between fill-ups" is worse than meaningless. It fools you into thinking you know what's going on. You don't.

There is only ONE correct way to measure MPG. Record your mileage traveled, and record the actual gallons pumped each time. Then divide. Actual measured miles. Actual pumped gallons.

It's the correct way because it's the only way that gives accurate results.

Nothing else means anything. Until you are doing that, you are chasing ghosts.

 

Welcome back!!!

Fix the vacuum leak before drawing any conclusions.

I relocate the bottles to the engine bay.

 

Hey! Thanks for the news flash. I'm not sure where you got the idea I have no idea what my mileage is. Perhaps I wasn't pedantic enough in the interest of expediency and audience consideration. I check my mileage using both a WAAS-enabled Magellan Meridian Platinum GPS which is reset along with my trip odometer (~-5.2% undermileage due to larger tires) at every fill-up and a Garmin Nuvi 2797LMT. My average weekly mileage (averaging all adjusted sources) had previously been 14.187 mpg but has been 15.774 mpg over the course of the past three weeks, an improvement of 10.07%. I can extrapolate to the ten thousandth if that would help.

Thanks, Pete. I'll look after the leak and go from there. A cursory inspection was fruitless so I've probably got a journey ahead of me. In the mean time, I'll enjoy the improved mileage.

Good to be back

 

More like not accurate enough, or you have no clue what you are doing.

Miles traveled is only half the equation. If you think every fill up takes the same quantity of fuel, it's the second problem.

 

F1addict knows his stuff.
He just came back and I am glad he did.

I trust him and am willing to go down this road with him.

WELCOME BACK!!!

 

Jesus, dude, you're reminding me why I can't stand this place.

 

Mark, don't run him off. He's a great guy and doesn't deserve to be lumped in with some clueless newbs. I respect your opinion and assessment of many posts, but this one deserves great respect. I consider F1addict a friend and a huge contributor to this forum.

 

No obvious signs of a vacuum leak. I hooked a vacuum gauge to the line running to the fuel pressure regulator and it read 16.5 in/hg (.56 BAR). Not sure what it's supposed to be.

 

17 to 21 at sea level.

Did you spray for vacuum leaks?

 

I'm at 35 feet above sea level and fresh out of carb and brake cleaner. I'll pick some up tomorrow and start spraying.

 

Well, I procured myself a couple cans of carb cleaner and squirted the crap out of every vacuum line in the engine bay (including the vacuum ball). For fun, I squirted the brake booster (inside and out) and under the passenger side dash. Also, around the CAD.

Zero change in idle (my CAD is super clean, though). I'm still getting better than average mileage and nothin' but defrost on the blend doors.

 

Perhaps the actuator for defrost/dash is bad?

 

Maybe a silly question but I pay close attention to my fuel mileage as well . There have been several times over the past few years I've gotten better gas mileage from one gas station vs the next . Any chance you've been filling up somewhere different ? My S10 gets a consistent 21-22 mpg no matter how hard I drive . There have been a couple instances where I've gotten 25-26mpg just from going to a Caseys vs a Phillips 66 etc . There were no other changes and I drive 600 miles a week just for work .

 

It's possible. I need to replace the bulb in my HVAC panel so I'll check out the actuator while I'm in there.

Interesting. I hadn't really thought about it but this change in fuel economy occurred at about the time stations switched from winter to summer fuel.