The gas mileage is getting worse.

[Ryan Heo]

Hi.The gas mileage is 300~330 miles per full tank after I replaced new O2 sensors and knock sensor. (15~16 premium gal.) However these day it is getting wrose after replaced a PCV valve(OEM), spark plugs(NGK), fuel filter, and fuel pump relay. In addition I cleaned the throttle valve, IAC valve, and intake manifold by carburator cleaner. The last time was 255 mile per full tank. It is only feeded at Shell premiun. I don't have CEL right now.What's wrong? Thanks.

[bundy26]

Using A/C alot?

[Beancooker]

Okay, now is the time to really look at your driving style. Do you accelerate quickly, a lot? Do you drive the car hard? Do you do a lot of city driving? All of those factors have a big impact on fuel economy.

[Ryan Heo]

No~ No~ Man~

I'm sure Nothing is different. It was summer vacation. I tried to make good gas mileage. To clean the throttle body, IAC valve, and Intake manifold.Especially the intake manifold was spent almost 2 hours by a brush and two carburator cleans. I make almost new one.I don't know what's wrong?Help me~

[ZGadson]

If anyone trys to tell you that midgrade gasoline increases gas mileage they're full of $h!t. I just went through a full tank of midgrade thinking I'd get better mileage but instead of getting 34-38mpg like I have been, I was lucky to get up to 31mpg even when coasting. Using premium gasoline only burns cleaner and allows for more hp, and because of that, you go through it much faster than with regular unleaded. (Although running premium grade through the car once in a while is good for the injectors and it cleans the system.)

But, if you're using regular unleaded and still getting crappy mileage, you might try some new plug wires for those new plugs and maybe even some injector cleaner that goes in your gas tank the next time you fill up.

Also, I noticed that you put in a new PVC valve. Our cars learn driver's styles and change engine functions based on your driving, so I'm guessing that if you were to reset your engine computer it would start over learning your driving and you'd see some improvement. (Leaving your battery unhooked over night resets the computer.)

[Ryan Heo]

I will try to disconnect the battery tonight.Thanks guys.

[MaXimaSE]

yeah well don't feel bad Ryan..

I get 269.50 miles to a full tank running Sunoco 91 Octane.

I replaced the following

*Plugs (NGK Platinum Premium)*Fuel Filter (Autozone for the 98 Maxima)*PCV Valve (Autozone)*Air Filter (My Cosmo SRI comes Monday/Tuesday)*Cleaned the Throttle Body, and Deep Creeped my intake manifold..

I ran Prestone Total Fuel System cleaner.. on my first fill up with this car, but Ive gotten **** MPG since I had it.. 250-270 Miles to full tank.

Im going to try half a pint of seafoam in the gas tank, and then use another half when I do my oil change. You could also try the Chevron Techtron, I heard thats good.. as well as the BG44K.

I would have a CEL, If a O2 Sensor was blown.. same with the Knock Sensor, or do I need to spend $65 on a OBDII code thing from walmart..

I still need to clean the IACV/MAF and I don't really romp on it to much.. I been trying to chill out lately..

[Sentientbydesign]

I've been having bad mileage problems too. I never get great mileage, but recently I went from 340+ miles/tank (our tanks hold in the neighborhood of 18 gallons btw) down to 280 miles/tank.

I only run premium to avoid knocking since my engine is so high mileage.

The only thing that's been different is that my car has been running really hot because my a/c is giving me problems, but that's another story.

[Ryan Heo]

When I got the I30 first time, it has CEL for all the O2 sensors and knock sensor. At that time, it has 250~260 miles per full tank even octane 93 shell premium. After that, I replaced the all the sensors. I got upto 330 miles by the same gas. However I don't know what's wrong this time.

Anyway.The car had leak at the spart well. So it needed to replace the spark plug tube seal. To replae that things, it should be taken the intake manifold and valve cover off. I took them out everything and cleaned by throttle body cleaner and old tooth brush. I takes almost 3 hours but it has worth. After the parts cleaned, it is more smooth and quiet. So if you would like to clean the intake manifold, the sea foam isn't enough that.

Thanks.

[MaXimaSE]

well yeah, but If Im going to pull apart the manifolds.. Im going to replace the knock sensor, and install some new gaskets.. I don't know whats up with the MPG, but I dont wanna drive the Maxima if it only gets 269 miles to a tank.. thats ridicolous when Its $2.85 a gallon and about 15.5 gallons

the Haynes manual says that Excessive Fuel Consumption could be caused by the following

*Dirty or Clogged Air Filter Element*a problem with the Fuel Injection system*a problem with the Emissions Control system*Incorrect Timing

Im more leaning towards my Emissions Control (I got a 0309-0705 Evap Codes that haven't came back since I cleaned the Canister Control Valve) or my timing is way off...

[Ryan Heo]

Actually I'm doubting that the timing is way off because sometimes the start is too hard. It need to be long cranked. Also what is the canister control valve? Where is it? I want to try to clean that one.Thanks.

[ZGadson]

Its also called a purge valve. Its basically a solenoid that controls how much fuel vapor is released from the fuel lines. Driving more agressively will cause the valve to stay open longer and realease move vapor, in other words, a fuel regulator, which leads me to believe this could possibly be a reason your mileage is down...

Z

[Ryan Heo]

Where is the fuel regulator? I have a repair manual so if you let me know what page it is, I can find it.

[Sentientbydesign]

I know I sound like a broken record here, but I thought everybody should know this so they can get a better idea of their car's gas capacity.

The stated capacity for fuel is 18.5 gallons. I have to assume that includes what's in the fuel lines, filter...etc.

I have personally drained my tank to the point where it would barely start and I filled up 17.6 gallons into the tank.

Now that fact is immaterial if you're filling up, driving, and dividing the number of miles driven, by the displacement when filling up again, but to say that you're getting 2xx miles per tank and thinking that it only holds 15.5 gallons is off.

[Ryan Heo]

I think the fuel filter might be something wrong because after the filter was replaced, it is happen. I will try by OEM.Thanks.