gas mileage help PLEASE!!!

**Natch** · 11-02-2009, 07:00 PM

Need some help here.
I have a gorgeous 2002 Silverado, Z-71 LT with a 5.3 engine, 3.73 gears and only 53K miles.
Before I started making what I thought were gas mileage improvements, I was getting 15.8 MPG. Had a low neutral idle at 500 RPM and a slight miss every now and then, but only at idle. Otherthan that miss, she ran great. I have since gone to a flowmaster delta50 mufler into duals, a K&N air filter and I just put some iridium NGKs (.040 gap) and new high end Moroso wires and my mileage did not change. I use Lucas fuel system cleaner now and then and I still have the miss and lousy mileage. Easy acceleration and 60 on the hiway, etc...
What am I missing here?
Thanks in advance for any assistance provided.
.

**DIGS16** · 11-02-2009, 07:21 PM

do u have ur exhaust straight out the back and not slightly bent behind the rear tires? i heard if u have them straight out the back u get worse fuel milage...

**SmokeU** · 11-02-2009, 09:20 PM

a programmer should help I would think

but really if you want better fuel millage get a Malibu or something

**XS ive** · 11-03-2009, 08:40 AM

Originally Posted by DIGS16

do u have ur exhaust straight out the back and not slightly bent behind the rear tires? i heard if u have them straight out the back u get worse fuel milage...

Why is this so? Lack of back pressure due to a bend?

**Mean_Green_95** · 11-03-2009, 09:24 AM

Run some Seafoam or some GM TEC through a vacuum line. You might have some carbon buildup which would cause a loss of mileage.

**MTPockets** · 11-03-2009, 09:30 AM

Originally Posted by DIGS16

do u have ur exhaust straight out the back and not slightly bent behind the rear tires? i heard if u have them straight out the back u get worse fuel milage...

What! Do explain because I'm curious about this. I understand back pressure but I don't understand you statement.

**phoebeisis** · 11-03-2009, 02:38 PM

I know it seems counterintuitive, but usually the best gas mileage comes with the OEM components-intake, exhaust, spark plugs and wires.

Part throttle FE shouldn't improve with improvements that decrease intake resistance before the throttle plate , since you have to shut the throttle plate more to get the same amount of fuel and air in the CC. The total intake resistance will be the same. Changing to aftermarket parts can throw off any intake tuning that the OEM engineers have done. You only have a K%N filter-not a CAI, so it shouldn't hurt as long as it isn't over oiled.For reasons know only to GM Delco plugs usually work best.

The exhaust system mods probably hurt the exhaust tuning efforts of the OEM engineers.It can hurt part throttle FE.

Your mods didn't cause the miss-get it scanned to see which cylinder is missing.Could be anything electrical-coil etc.It could even be a poor contact where the FE is plugged in.

I would find the miss first-electrical or maybe FI .
Your mods shouldn't really do much either way in respect to part throttle FE, beside, it probably sounds good,and you already own them.

Find the miss- that is the cause of your FE problems. You had it before your mods, so it isn't your mods.What is your idle supposed to be? 500 seems a bit low? It could sure have something to do with the miss and poor FE.
Charlie

ps-You aren't using coolant, are you?

**DIGS16** · 11-03-2009, 05:46 PM

Originally Posted by XS ive

Why is this so? Lack of back pressure due to a bend?

From what i understood when i went to get my exhaust put on by a friend that owns a muffler shop is that if the pipes go straight out the back to the bed of ur truck, it has easier exit and no pressure going back to the engine, and thus the engine runs more free and spend more gas... obviously better performance but worse gas milage... thats how i understood it and thats y i went with just one side exit pipe lol cuz i cant deal with losing more mpg...

**silveradotrailblazer** · 11-03-2009, 06:17 PM

X2 what mean green95 said and try taking off your throttlebody and clean front and back of the throttlebody with a throttlebody cleaner and a SOFT brush, old toothbrush works good. Make sure you use a t/body cleaner and not a carb cleaner. Also clean the MAF sensor with MAF cleaner only. CRC makes a great cleaner for both.

**TJbear** · 11-03-2009, 06:29 PM

I also have a slightly bumpy idle. Cleaned the TB recently due to a sticky gas pedal (which fixed that) but didn't help with the bumpy idle. Also cleaned the MAF. Will be doing the plugs and wires at some point as they are original.