TERRIBLE GAS MILEAGE!! Really upset..

**flybird** · 02-13-2012, 12:59 PM

okay here it is.. I have a 99 3.8 Firebird. I AM GETTING 13MPG Houston TX City..

The engine is brand new, not just rebuilt with old parts. Im talking about from bottom to top everything is GM new including long block. Engine is bored over .30 and heads are shaved.

Everything I have replaced bc of this bad MPG problem are Fuel Filter, Fuel Regulator, Spark Plugs, O2 Sensors, Knock Sensor, Knock Sensor Wiring. I had several codes and when i replaced all of these items, they are gone now. I repeat I have NO CODES. I am very aggrivated bc this is not making any sense to me. I drive to school and work and to home. I range 2000 rpms and 2500 rpms so i can improve on my MPG.

MODS: just a K&N Cold Air Intake and Performance chip.

I am very lost... I have no idea what to do. The reason I have a v6 is to save money for school.. This is really annoying and making me want to get rid of this...

Any help??

**pajeff02** · 02-13-2012, 01:36 PM

What sort of mileage are you expecting? Auto or stick?

***SMWS6TA*** · 02-13-2012, 01:40 PM

First clean the MAF. K&N filters no matter how good you let them dry have oil in them and as the air passes through the filter it picks up oil with then attaches to the MAF sensor. Use some MAF cleaner, you can pick it up from any parts store. Spray and let dry do not use a towel.

Either ditch the K&N and use a paper filter or get use to cleaning the MAF every few weeks

2nd - performance chip - this could be the main problem. They do go bad, are you able to disconnect and run with stock tune?

Contact this member - Smittro

he is our resident V6 guy.

**Too Fast** · 02-13-2012, 02:17 PM

+1 on the MAF being contaminated.

**chris1974** · 02-13-2012, 02:19 PM

+1 on the chip. The whole motor is new and running on the old tune?

**flybird** · 02-13-2012, 02:22 PM

its an auto

I cleaned the MAF sensor roughly 2 weeks ago. But i do hear what your saying. Clean it every so often makes sense. I can disable chip now. I had no idea that they can go bad??? I bought a expensive one too.. Not one of those cheap ass $40 ones. But yeah i will pull it off.

I was starting to think that i had a fuel leak(checked did not find one) or some1 around this area were siphering gas. (Im sure i spelled siphering wrong) lol

**35th-ANV-SS** · 02-13-2012, 02:44 PM

Other than terrible gas mileage, how is the car running overall?

**flybird** · 02-13-2012, 03:04 PM

Good! No problems, no missfiring, sometimes after a warm start it has abit of a stumble on start like it hasnt got fuel to start but nothing major.

**024mula** · 02-13-2012, 03:18 PM

Back to the Basic...........Is the emergency brake cable stuck?

Or check the fuel line after the fuel pump by running the car and about 1500 RPM for about 15 mins and check if there is fuel under the car.

**35th-ANV-SS** · 02-13-2012, 03:19 PM

My next question was do you smell fuel at all.

**flybird** · 02-13-2012, 03:43 PM

Checked for leaks none. Ill do what you said run it 1500 rpms. And no brake is not stuck.

**flybird** · 02-13-2012, 03:44 PM

No none at all. Checked for smells often. Nvr.

**pajeff02** · 02-13-2012, 04:22 PM

Other things to check that make a difference -- correct tire pressure, wheel alignment in spec, any brakes dragging (in addition to the parking brake), plugs and wires in good shape, etc. What is your rear gear ratio?

**flybird** · 02-13-2012, 05:10 PM

Tires are brand new, 600miles z rated. Pressure, balance, alignment is all correct. Calipers stick just a tad but not much to make a difference. Rear end is stock. Limited slip 3.23.. Plugs are new and wiring, 300miles.

**Too Fast** · 02-14-2012, 02:21 AM

What about the engine temp. If your thermostat is not there, or is opening way too soon, then it will usually throw a code, but not right away. If it is always running below ~165, then the pcm has the car running way rich. I've seen this happen before....check engine temp with a scan tool preferably, for more accurate readings.

**MrKid** · 02-14-2012, 04:24 AM

How many miles on the rebuild? Gas milage improves after break in.
Are you running cats? Are they original? They can clog a bit without throwing a light.

**0rion** · 02-14-2012, 04:43 AM

any exhaust leaks pre O2's? Is there an excessive amount of soot inside the exhaust tips? What "chip" do you have? Is it an iat tricker or does it flash the pcm?

**Smittro** · 02-14-2012, 04:50 AM

Originally Posted by flybird

Good! No problems, no missfiring, sometimes after a warm start it has abit of a stumble on start like it hasnt got fuel to start but nothing major.

Originally Posted by flybird

okay here it is.. I have a 99 3.8 Firebird. I AM GETTING 13MPG Houston TX City..

The engine is brand new, not just rebuilt with old parts. Im talking about from bottom to top everything is GM new including long block. Engine is bored over .30 and heads are shaved.
Everything I have replaced bc of this bad MPG problem are Fuel Filter, Fuel Regulator, Spark Plugs, O2 Sensors, Knock Sensor, Knock Sensor Wiring. I had several codes and when i replaced all of these items, they are gone now. I repeat I have NO CODES. I am very aggrivated bc this is not making any sense to me. I drive to school and work and to home. I range 2000 rpms and 2500 rpms so i can improve on my MPG.

MODS: just a K&N Cold Air Intake and Performance chip.

I am very lost... I have no idea what to do. The reason I have a v6 is to save money for school.. This is really annoying and making me want to get rid of this...

Any help??

Originally Posted by flybird

Tires are brand new, 600miles z rated. Pressure, balance, alignment is all correct. Calipers stick just a tad but not much to make a difference. Rear end is stock. Limited slip 3.23.. Plugs are new and wiring, 300miles.

1) .30 over and shaved heads but no tune,

2) performance chip for a non modified 3800,

3)hung calipar,

4) K&N junk filter system..

All can contribute to poor MPG's..

13-14mpg city is about right for these considering they only get ~26 mpg's highway but no tune after internal mods is likely going to be your issue here..

**2002ssslp** · 02-14-2012, 05:16 AM

Originally Posted by flybird

okay here it is.. I have a 99 3.8 Firebird. I AM GETTING 13MPG Houston TX City..

The engine is brand new, not just rebuilt with old parts. Im talking about from bottom to top everything is GM new including long block. Engine is bored over .30 and heads are shaved.

Everything I have replaced bc of this bad MPG problem are Fuel Filter, Fuel Regulator, Spark Plugs, O2 Sensors, Knock Sensor, Knock Sensor Wiring. I had several codes and when i replaced all of these items, they are gone now. I repeat I have NO CODES. I am very aggrivated bc this is not making any sense to me. I drive to school and work and to home. I range 2000 rpms and 2500 rpms so i can improve on my MPG.

MODS: just a K&N Cold Air Intake and Performance chip.

I am very lost... I have no idea what to do. The reason I have a v6 is to save money for school.. This is really annoying and making me want to get rid of this...

Any help??

You have some issues here. Take it to a good Dyno shop and have everything tested. My 02 SS with 350rwhp gets 17 city and 27 hwy. I think it may be your computer.

**Smittro** · 02-14-2012, 05:24 AM

Anytime you change displacement and mill heads you're going to need a REAL tune..

ESPECIALLY, with a drive by wire TB.

A "performance chip" which is'nt really too usefull on OBD2 cars anyways is NOT, or even near to, a real tune..