'04 Fuel economy down to 10 mpg from ~14 last summer

I'm in San Antonio, TX. My son got his 2004 about 8 mos ago. Started off ~14 mpg, now down to ~10. I assume 14 mpg is decent for a V6 PreRunner non 4x4 with 185,000 miles. Solid service record and shop gave it a tuneup when we first got it. Had misfire on cylinder 3 that was repaired along with new plugs/coils.

No engine lights or codes and he's learned not to be heavy on the gas. I've never experienced such drop in such short time. Winter blends? current "market"? I've heard mixed things about winter blends being less efficient. My own truck is F150 and economy has been stable so I think it's less about the fuel and more about the truck/driver.

I have been opting for No Ethanol lately just b/c near cost negligible.

I will note, however, that I noticed a mild "odd" smell when I start it up in the morning. I often have to move his truck out of driveway as we all go to work/school. It's only for a moment so not sure what to make of it as I don't know if it persists once warm.

Service records shows shock/strut service and also timing (chain?/belt?) replaced. Tire pressure ok. Air filter replace couple months ago.

What do y'all think or what should I try next?

 

I would think 14 mpg should be your minimum unless loaded up with a ton of gear. No good advice on where to begin your search but something ain't right.

 

I'd check for leaks

 

where to check? and how? visual inspection? instrument/tool?

 

Idk really, check where the injectors are. If the boots are deteriorated it could be leaking at the injectors. But I'm just guessing, throwing ideas. Alternately, clean the MAF. Don't know if a dirty MAF could make it want to burn that much gas. Can you confirm its really getting a steady 10 mpg? I ain't much help, just chewing fat.

 

I have an ancient F-250 with a 460 gas engine, it will get 10 mpg on the highway -- something isn't right with your pre-runner.

Most fuel economy issues have their roots in the drivers seat - "spirited" driving habits, long periods of idling, etc. Perhaps you and your son could swap vehicles for a week as a test.

Also how accurate are the economy calculations? To be accurate you need to fill the tank until it clicks off (once, don't keep clicking it as it damages the emissions system) and record the miles/gallons every time, keep a log and after a while you have good data to use.

 

Where do you get your gas at in SA? I used to live on the Northwest side and had a couple bad fill ups from the same gas stations that had glass smoking pipes in the store and rolling papers...lol. Try changing up a gas station maybe?

Winter blend boils down to conventional gasolines using propane/isobutate/simpler carbon molecule chains in the winter instead of octane (complex 8 carbons) because it burns more enviromentally friendly then octane at lower temps. Mileage isn't as good is the trade off. Once temps rise octane gets put back in the blendstock to get that octane number up to the number it needs to be (87/89 etc...). Not sure where blendstock is used though state-wise.
Learned that in San Antonio working for Valero .

 

Have been mostly Valero consumer to support San Antonio but also to have fewer variables - except for the recent Non-Ethanol trials. Maybe will have him try higher octane.

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Answers to questions posed: Mostly Citi driving. Not under load. Acknowledge driver is a huge variable. Agree on the swapping for 1-2 weeks to test. Steady 10 mpg is anecdotal from my son.


Thanks all. You've given me more to chew on and as weather changes we'll keep working at it. Trying to hold out on another Costly trip to the shop for guesswork.

 

I just got two really lousy MPG’s on fillips from Circle K (76 supposedly) and ARCO. Coulda just been a coincidence of winter blend or two lousy batches of gas. I’m going to give it 2 more fillups from Shell and Chevron before worrying it’s engine related.

 

Shoot my 12k lb diesel gets 12 mpg.
Locking fuel cap?

 

Lots of short trips around town with cold starts is brutal on gas. If he’s driving 5 miles to work and back it’s going to suck gas while it warms up.

 

O2 sensors should be replaced about every 100k miles. There are 2, upstream is the most important but good idea to replace them at the same time- upstream in the down pipe before the catalytic converter, downstream is in the straight pipe between the cat and muffler.

Plugs should be replaced with OEM (

$9 NGK BKR5EKB-11 NGK Spark Plug Stock # 3967

NGK BKR5EKB-11 or

$18 Denso K16TR11 Spark Plugs, Pack of 4

Denso K16TR11, substitutes not a good idea) every 30k miles.

Clean the mass air flow sensor and throttle body, otherwise it could be driving habits (has he been putting his foot in it?) and winter gas might contribute, but honestly, 14mpg isn’t great to begin with and 10 is bad.

 

Anecdotal could be off by a lot. Id confirm with real measurements on miles and gallons.

 

Freeway mpg is pretty Ok. City driving has been sucking the gas lately. I’ve got most of the above mentioned done except my O2 sensor (120k). Until recently tho, I was getting a lot better city mpg. No faulty O2 sensor symptoms other than recent lousy mpg. Again, I’ll give it another month or so before really getting at possible causes. It might be a seasonal gas issue, especially since my hiway mpg is still half decent.