Bad Gas Mileage [Merged 9-6]

[99GST]

I am still getting some pretty bad gas mileage.

I am only getting about 19.7 mpg with mostly city driving shifting at 4k.
I have already replaced my front O2 sensor and am looking to replace the fuel filter.

I have a 99gst with about 55k miles on it. Should my fuel filter already be bad?

What else should I check or change? I have NKG plugs and Accel wires are on the way.

Thanks,
Nathan

 

[Defiant]

Okay, we're wandering away from the topic. Start new threads, or go to those already discussing the fuel light, gauge, capacity, filling ability et al.

 

[blacklabel5281]

Hey guys. I'm having a huge problem. I'm only getting 150 miles with a full tank of gas. I have no clue why. These are the mods I have: HKS Bov Vented to atm (Yes I know that will cause a drop in gas mileage), Apexi Power Intake, Apexi N1 Exhaust and Downpipe, MSD 8.5 Ignition wires, NGK Spark Plugs (Pre Gaped - Not sure by how much), Dejon full upper intercooler pipping, Act 2600 and lightened flywheel, Tein Super Street coilovers and pillow ball upper mounts with a 2 inch drop so far, and some Autometer gauges.

 

[DROOPY209]

How do you filters look?

Fuel filter, pcv valve, air filter...

Do you have any bad idleing issues?

What PSI are your tires inflated to?

What gap did you set your NGK BPR6ES spark plugs to?

I need a little more info

 

[eclipsemustang]

are you sure your out of gas maybe its your gauge or fuel sensor

 

[andymoraitis]

After you've checked everything above, replace the front O2 sensor. Avoid the Bosch sensors that you have to splice and pick up an OEM unit from:

http://www.sparkplugs.com

Let us know what fixed it,

Andy

 

[dsmclipse97]

yea a bad front 02 sensor will destroy gas mileage.... found that out the hard way.

 

[blacklabel5281]

Both of my o2 sensors should be brand new. I got them replaced like 4 months ago. My Intake filter looks a little beat up. How do i check my fuel filter? Pcv Valve should be new. Thats for the oil right? As for idle sometimes if I rev at a stop or I hammer down in first and roll to a stop with the clutch in my car dies. Tires were just rotated and checked and are perfect. I havent tested the fuel gauge or what not but the low gas light doesnt come on. Lastly the plugs I have no clue what the gauge is on them I will try to check that tomorrow. What should they be set at anyone know? Thanks guys

 

[leet]

So, you know venting your BOV will reduce fuel efficiency, yet you're here asking us WHY your car is fuel inefficient?



EDIT: I've seen a few vented DSM's that have the stalling problem when coming to a stop in the manner you described.

 

[brute]

Why is the splice-in Bosch o2 sensor bad? That's what I'm using right now.

 

[PinoyBalla5987]

Are there any things I should look out for with my car because my mileage is absolutely horrible. The only modification I have is a CAI as far as engine mods. I'm getting about 15-18 mpg 18 and most of my miles are from the freeway. Please help

 

[donmagicjuan]

It could be a number of things, but I would say the most common cause of poor gas mileage is a failing O2 sensor. They fail lean, which causes the ECU to continue to add fuel to no avail, with herendous fuel economy being the end result. There's a quick check for it in the Haynes manual that basically involves checking the voltage across its terminals with the car running.

Good luck!

 

[dr1665]

Check your spark plugs and wires too. Be sure your plugs are gapped right and don't buy into the hype on aftermarket plug wires if you're not seriously modded. The $30 Bosch wires at Autozone will do you right and they have free lifetime replacement FTW.

As mentioned, O2 sensor failure is a common culprit as well, but I'm thinking you might have soot on your rear bumper (run your finger across it - is the dust black?) and a CEL.

 

[donmagicjuan]

Thanks for mentioning some of the other common causes, Brian. You're absolutely right that a faulty O2 sensor is usually accompanied by a CEL, but often it can take quite a while to meet the necessary conditions to throw the code. That's my two cents for now; I'll leave the rest up to the N/T experts.

 

[DanThMan1983]

A good intake cleaning with BG products is a really good thing you can do to help your fuel mileage.

 

[blacklabel5281]

Lol venting my bov isn't going to cause a 150mpg drop. And yes I think I stall out sometimes because of my bov when I roll to a stop.

 

[andymoraitis]

Why is the splice-in Bosch o2 sensor bad? That's what I'm using right now.

It's not that it's bad, but if you ever plan on running race gas, the Denso is much more resistant to lead contamination. I also like plug and play capability and the Denso is a direct OEM replacement.

 

[blacklabel5281]

What do you think of the DSMlink? Can I used it to help vent my bov properly? I would also like to bring up my redline and increase my stock boost a tad.

 

[1990talon]

Your getting off topic when did you notice this happening? how long? did it slowly drop or all the sudden did you change any thing recently?

 

[blacklabel5281]

It seemed to happen all of a sudden. And no I didnt change anything. Recently I drove up to columbus from cinci where I live. Without going over 3k and getting my boost up above 1/2 way and doin only the speed limit on the freeway I got 300 mpg. Thats with the bov not going off fully or anything and all freeway.

 

[Qwertty]

Check your alignment, 02 sensor output, plumb your bov back into the intake, And see how your A/F ratio is(kinda the same as checking 02 output). And if all that fails, Just drive slow!

 

[garrett21]

A boost leak can cause poor gas mileage, just search and you will find plenty of information on it.
A DSM link will not help with the stall out problem. Only setting up a blow through setup w/ a gm mass air flow and a mass air flow translator can help.

Also before you do any more posting I would suggest that you read the How To section and use the search button. You can learn a lot if you just read and learn from other people.

 

[1990talon]

man you got 300 mpg (miles per gallon) . Teach me how I can do that. I am assuming you meant to the tank. Is that city driving? That can reduce milage alot. aslo I took a long trip and got an exhaust leak at the manifoild to turbo gasket milage droped quite a bit.

 

[jibberishballr]

man you got 300 mpg (miles per gallon) . Teach me how I can do that. I am assuming you meant to the tank. Is that city driving? That can reduce milage alot. aslo I took a long trip and got an exhaust leak at the manifoild to turbo gasket milage droped quite a bit.

300 haha

Our cars run naturally rich from the factory so you could just need to tune it. Also as everyone is saying the front 02 sensor is usually the suspect. I'd do a boost leak test before anything just to make sure thats the problem. Anytime you boost you are going to get less mpg. Thats just my thoughts on it. Good luck!

 

[PinoyBalla5987]

I do have a check engine light on which I tried getting tested with other scanners. However they were unable to recognize the codes and said that I should try to bring it to the dealer and get it diagnosed there. As far as the soot in the rear bumper I don't think I have that and the spark plugs were changed not too long ago. Thanks for the input and I will most likely get it diagnosed first to see what is wrong and post it.

 

[donmagicjuan]

If you haven't tried an auto parts store, I would take it there to be read first, as they don't charge anything to just pull the code. I will avoid going to a dealer at just about any cost, though.