Much lower MPGs- where's the culprit?

B

BigBrother

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My 06 4R, with ~40K miles on it, recently had a major dip in its average MPGs. I would usually get about 18.5 city, 20 highway. It dipped to about 16/17 pretty suddenly, about 400 miles ago, and I just took it in today for scheduled maintenance. They told me, unrelated, that I needed a spark plug change. I passed on it to price shop, but once I got it out of there, I was getting about 14 in the city.

This is a very reputable place (huge Toyota dealership with great customer service), so I don't suspect they missed anything major in their 40K maintenance, but I need to solve this problem stat.

Could it actually be the spark plugs? I haven't the faintest clue what could cause such a marked drop so quickly.

All other variables (gas, driving habits, etc.) have remained constant.

Thanks.
 
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Interesting development. After turning the 4R on several times and seeing the MPGs lower and lower each time, I suspected something..

I left it in idle and saw the avg MPGs drop by a tenth every 10 seconds or so- it constantly does this- while idling! Once I start actually driving it raises back up, but if I'm in idle it drops- tick tick tick, lower and lower.

So thankfully, it's probably not anything mechanical- but electronic. Anyone ever seen anything like this? Thoughts before taking it to the dealer??
 
There are a lot of variables that will affect mileage.

The first that comes to mind is winter fuel vs. summer fuel. Many municipalities swap over and the winter fuel will cause a MPG fall-off.

Tires, tuneup, intake/exhaust restrictions can come into play.

Driving style, traffic patterns, cold weather warm ups and snow can have a significant impact.

A change in wheel/tire size will affect the on-board readings and calculated values if you haven't recalibrated the tire revs/mile.

Are you relying on the on-board computer or are you hand calculating?
 
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Atlanta_man said:
It's working good. Sitting still, burning gas and making no miles, should run the AVERAGE mpg down.
Click to expand...

Somehow I doubt this. After the average has been calculating for several days, I sincerely doubt sitting in park should cause the following over a 1:30 span:

11.9
11.8
11.7
11.6

And yes, it was down at 11!

I'll have the shop check it out.

-BB
 
Idling will DEFINATELY cause the average MPG reading on the readout to drop. This is an average. You're using fuel and going 0, nada, zilch miles. That's why you appear to get such better gas mileage when you fill up at a turnpike gas station and roll right onto the highway as opposed to filling up and sitting at a few traffic lights before getting on the on-ramp.

But if that is your average after tooling around town, I'd be concerned, too (unless you're dealing with a LOT of stop and go lately). Any chance you're losing fuel somewhere?
 
I don't think losing gas will show on the mpg meter. It calculates fuel usage by counting the pulses to the fuel injectors and integrating the pulse width applied by the f.i. computer.
 
BigBrother said:
Somehow I doubt this. After the average has been calculating for several days, I sincerely doubt sitting in park should cause the following over a 1:30 span:

11.9
11.8
11.7
11.6

And yes, it was down at 11!

I'll have the shop check it out.

-BB
Click to expand...

Did they replace the plugs? Did the reset the ECM with the new plugs?

I'm not sure you have a problem though... Let
 
Where are you located at? My first guess is winter fuel mixture which will run less MPG for you. Summer I'm up around 18-19 on a V8, winter I'm usually down to around 15-16.
 
What you are describing is completely NORMAL!!!

The MPG readout will go down if you sit and idle. This is more apparent at the beginning of a tank of fuel. If you have only driven 30 miles on the current tank and idle for a extended time the MPG readout will decrease faster (less miles to average with) than if you have driven 300 miles on a tank.(more miles to average with).

Also remember it is a running average not a mean average like you calculate with gallons used and miles driven.

As for the Spark plugs. The V-6 requires the plugs be changed @ 30k miles to maintain your emmisions warranty per the maintenence guide. I have changed the plugs twice in my 04 and never seen a dramatic increase in MPG. I used NGK & Denso plugs 1 time each. It is due for a change right now but I will go to an iridium plug this time since I do not have any emmisions warranty left. Expires @ 80K miles.

If your mileage dropped suddenly after leaving the dealer it is possible they let it sit and idle for a long time. So your mileage readout for that entire tank will be askew. You may be still getting your normal mileage hwy & city but due to the extended idle time early in the tank it will take a while of hwy driving to bring the average back up for that tank. But it will never get as high as if you had just got on the hwy from a gas stop and drove a hwy speeds for the entire tank.

It is also NOT an instantaneous mileage readout. The only way you can sort of make it that way is to reset the MPG readout while driving at a constant speed. Then you will see instantaneous MPG for a short time.

Like others pointed out in the winter your MPG will be lower due to a couple things. 1, Lower temps extend the time needed for all the various fluids to get to optimum operating temps thus lowering MPG. Rearends, trannys, Transfercase, and the engine all are more efficient when the oils are warmed up to normal operating temps. 2, Winter blends of fuel will cause you to get lower mileage. 3, Colder, denser air takes more power to drive through than warm dry air. hense lower MPG. 4. Before the engine warms up to a set temp the computer runs it on a preset set of parameters (not exactly designed for optimum MPG) more or less a safe set of parameters or a closed loop set. The computer then changes to an open loop and runs off what all the various sensors are telling it once the engine reaches the preset temp.
 
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By the way, in case anyone cares :), I had the spark plugs replaced as they recommended, and WAM! My MPGs shot up. I was actually getting 25.5 avg on the highways! I'm still waiting to burn through a whole tank of gas to do the math myself, but looks like all is back to normal. So moral of the story- if you're seeing a decrease, change those plugs! ;)
 
EH007 said:
Actually, the 4Runner will burn a bit less than a half gallon of gas per hour while idling.

greasefingers had an excellent post a while back to explain. You can find his post here:

http://www.toyota-4runner.org/showthread.php?s=&threadid=24717&perpage=15&pagenumber=2
Click to expand...

That's awesome. I can't believe how low the power output is at idle!:awais: I would have guessed higher than that. I would love to see what the idle fuel flow was in D vs N. I am assuming greasefingers numbers are in N.


:cheers:
 
Yes, it is surprising, isn't it? I also have a scan tool that records data (OTC) and I confirmed greasefingers findings. I have a V6 4Runner, and my numbers were slightly better than his V8, which of course should be expected.

Overall, both are close to 1/2 gallon per hour. And to think people shut off their engines to save gas when idling. Sure, it saves gas, but not nearly as much as most people think.
 
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