Causes for sudden MPG drop?

[donovan75]

I've had my '99 4wd Sport ed. (automatic) for 6 months now and the average MPG dropped from 19-20 to 15-17 in the past few weeks.

What are the most likely causes for this? Here's a couple of things I tried but both had no effect:

- replaced 2 tires- one had a slow leak and was almost always under-inflated. MPG stays the same

- removed Yakima ski racks. Coincidentally the MPG drop started when I installed racks. I have since removed them and there's no effect. The average MPG is for combined city/highway (The 15-16mpg excludes the time I'm using 4wd driving around ski areas.

- tried to see if fwy miles had any difference. None. (Regular SF-LA road trips with cruise control on 80mph also produces a crappy 16mpg average).

The last major tune I had was the 15k mile tune last July. Have since put 10k miles on it. I drive about 2000 miles/month 80% freeway, 20% city

In comparison my 99 V6 4WD Tacoma (5 speed) gets a consistent 20mpg.

Thanks for any tips or ideas!

 

[RunnerUp]

you are almost ready for a new air filter, so instead of getting the cheapy paper one you have to replace every 15k get a true flow filter, it will increase hp and mileage.

what kind of gas do you run?

and where did you get the truck from? condition of it?

 

[trd_style]

i read several articles from a website that, the more HOT air you get into the air intake, the more you save gas. but you lose hp of course. because the more cold air you get, the more hp you will get. but yes, changing the air filter might be one of the problem. also, change oil maybe? some ideas you need to look through. :

 

[donovan75]

you are almost ready for a new air filter, so instead of getting the cheapy paper one you have to replace every 15k get a true flow filter, it will increase hp and mileage.

what kind of gas do you run?

and where did you get the truck from? condition of it?

Thanks for the filter suggestion. I'll look at the True Flow and K&N filters.

I run 87 gas. I bought the truck on Craigslist from an elderly couple (one-owner car, full records, excellent maintenance, 106k miles)

 

[RunnerUp]

oh! i didnt know it had 106k miles... in that case


dont get the k&n, the performance gains supposedly arent that great and the oil gets on your MAF which you then need to clean because it will throw a CEL

you might want to check the throttle body butterfly for black "gunk" while you are at it, perhaps its time for new spark plugs? if there is gunk in your throttle body then you might want to try seafoam treatment.

 

[RunnerUp]

oh, and run 89, it runs a little better and is the manufacturers recommendation

 

[lee]

you always get worse mileage in the WINTER

 

[makakio]

My guess would be the air filter or simply a more aggressive use of throttle (feeling a little more stressed or cooped up than normal?). If you haven't noticed less power, I don't think it's fuel starvation...

 

[alotawatts]

how about

Bad Oxygen sensor = bad mpg

 

[makakio]

A bad O2 sensor would throw a CEL though, right?

 

[ebelen1]

A bad O2 sensor would throw a CEL though, right?

Not always. O2 sensors get lazy in time and can cause bad gas mileage.

 

[donovan75]

Thanks for all your help and suggestions guys.

My airfilter was NASTY. It had been changed in July last year (12000 miles ago). As soon as I bought replacement generic FRAM filter from Kragen, my mileage was back up to around 19-20MPG

 

[niquito]

How would you know a O2 is bad with no CEL? Can you visually inspect it?

 

[alotawatts]

O2 sensors

Bad O2 sensors can throw CEL's, and cause poor milage and cause rough idle.
They can be tested with a meter but when in doubt buy a new one..........and get a direct fit for your truck not a universal type.

 

[donovan75]

Ahhhh F#&*! I'm back down to 15-16mpg after 2 weeks.

Any other things I should check out? Air filter's clean, tires are properly inflated, Yakima ski racks removed. I had consistent 19-20mpg from June-December, and the winter here is mild (50-60s)

This is killing my gas budget! My 99 Tacoma 4WD with the same V6 engine is on 220k miles and still gets 19-20mpg.

 

[NViejeP]

May seem like a stupid question but how do you check your MPG? Just by looking at your gas tank and estimating?

 

[Lexusbstrdchild]

fill your tank. keep track of the miles with your trip meter. when you fill up again, look and see how many gallons you put in. divide the miles you drove, by the ammount put in and you get mpg.

 

[AUSig]

The tank is 16 gallons, correct?

 

[makakio]

Are you driving it like you drive your Tacoma? If so, even though it has the same engine you're lugging around a sh*tload more weight, which could cause a 4-5mpg drop, easy.

Just an idea.

The only time I've seen a 4-5mpg drop on mine is when I use 4WD. At this point, I'm thinking the only possibilities are:

1) driver input

2) O2 sensor (because you haven't had it checked)

I can't think of anything else you haven't checked that would cause this kind of mpg difference without other very visible signs of malfunction (timing, spark, fuel flow, vibration, noise, smoke, etc). It may be time to see a tech...

 

[Xandarin]

Ok call my crazy, all me a "noob". However, have you greased the u-joints on all your driveshafts? If you have 4wd you have four of them. (Where the shaft connects to the tranny and also to the differentials. There are these small nipple things placed on the U-Joints in a few spots. Just get a greese gun and some standard grease and load them up.) I did this on the 4runner I just bought, and I noticed a HUGE MPG jump. I dunno if this is what helped my gas mileage or not, but this is the only thing I've done so far. I was getting like 14-17 and now I'm pushing 20-22 easy.