Bad Gas Mileage

[stevenmamos]

I have noticed a drop is gas mileage recently...currently I have used half a tank and gone 99 miles. This tank reflects 100% city driving, however it doesn't do much better on the highway. Any suggestions on where I should look to improve this? My initial thought is maybe the O2 sensors, but I'm not sure. Thanks in advance.

[nos4blood70]

Are you sure you've actually burned half a tank of gas? Maybe your fuel gauge has gone whacky. It's a common problem.

Fuel gauge has a mind of its own
Problem: The fuel gauge doesn’t work correctly. You are sick of using the trip meter to judge your distance left.
Solution: You need to replace the fuel level sensor on the fuel pump assembly. Buying this part from GM will cost over $400. Thanks to Hans, we have a simple $40-50 fix. The fix involves buying a part from a GM dealer that is made for a recall on Tahoe’s. What happens that makes the sensors not read correctly, is the little fingers on the sensor wear out, causing false readings.
Bad sensor: http://i33.photobucket.com/albums/d61/b ... G_7569.jpg
Good sensor: http://i33.photobucket.com/albums/d61/b ... G_7572.jpg
Luckily GM made it easy to get the fuel pump out by making an access panel in the trunk. You simply remove the pump from the fuel tank, and swap the sensor arms, and reinstall. For a more detailed explaination with a video tutorial, see here: viewtopic.php?f=9&t=3463
Pics here: viewtopic.php?f=18&t=456
Part number needed: 15824379 ($43.30 as of 5/8/11)
Estimated Cost: $40-50 for the part from GM, $20-30 on ebay

However, if you rule this out, what is the maintenance like on the car?

[stevenmamos]

I've already had the whole gauge cluster replaced, and I've also calculated my gas mileage during each full up by the amount of gas I put in. It is a meticulously maintained vehicle and has new plugs and wires about 1,000 miles ago. I have also run fuel injector cleaner through the tank recently.

[01bonneSC]

What plugs and wires did you use?

[00Beast]

Like nos4blood said, did you fill up and get appx. 9 gallons? Or is the cluster showing 1/2? The sender in the tank is a common cause of failure, but we need to make sure we know what's going on.

• 1987-2005 Bonneville / Buick 3800 Maintenance List
  
  Grease: Ball Joints, Tie Rods Every 8-10k ( Green Grease or Amsoil Grease recommended)
  Air Filter Every 15k (Amsoil EaA, Purolator PureONE)
  PCV Valve Every 30k
  MAF Sensor Element Cleaning/TB Cleaning Every 50k viewtopic.php?f=48&t=1661
  Fuel Filter Every 15-25k (Purolator, AC Delco, WIX) http://www.pontiacbonnevilleclub.com/fo ... 45&t=23215
  Spark Plugs Every 30-50k (NGK TR55, AC Delco 41-101 or Autolite 605) (Never Bosch)
  Ignition Wires Every 30-50k (Napa Belden, AC Delco, Autolite Pro, NGK) (Never Taylor or Bosch)
  Oil & Filter Every 5-6k or Every 6 months (Valvoline SYNPower, Pennzoil Platinum, or Amsoil )(Mobil1, Purolator PureONE, Amsoil EaO, K&N or WIX) (Never Fram) viewtopic.php?f=45&t=214
  *Conventional Oil Every 3k/3 months(API SM/SN rating) (WIX (Napa), Delco or Purolator filter)
  Tires: Check Inflation Pressure & Visually Inspect Monthly Rotate Every 6-8,000 Miles
  Transmission Flush or Pan Drop Every 15-20k (Always make sure filter is changed when flushed) viewtopic.php?f=45&t=10193
  Cooling System Flush Every 25-50k or 3-5 Years (Peak or Prestone Mixes-With-Any Coolant Only) (Never Dexcool)
  Oxygen Sensor Every 80-100k (30k for non-heated) (AC Delco or Denso) (Absolutely never Bosch) viewtopic.php?f=48&t=794
  Accessory Belt Inspect every 15k, replace every 100k, or as needed
  Cabin Air Filter Inspect and Replace every 15k, or as needed in dusty conditions. (Note: 00+ Only) viewtopic.php?f=45&t=13626
  Evap Core Cleaning Whenever Needed (Note: 99 and older only) viewtopic.php?f=45&t=2432
  Brake Fluid Flush 3yr/50k
  Power Steering Fluid Replacement/Flush Every 100k or 5 years
  
  *Special Considerations*
  All Series II/III 3800's RPO's: L36, L26, L67 & L32, especially important on vehicles factory installed with orange Havoline Dexcool Antifreeze/Coolant: http://www.dex-cool.net/
  Lower Intake Manifold Gasket Replacement, every 80-100k (ignorance of this can allow coolant to enter your engine oil, this happening can cause irreversible lower engine damage, requiring replacement of subject engine)
  
  Series II, Naturally Aspirated, RPO:L36:
  Upper Intake Manifold Plenum Replacement, Every 80-100k (ignorance of this can allow coolant to enter your intake manifold, possible damage to occur can be hydrolock, irreversible damage to emissions devices, misfires, etc. Most times, engine replacement is required if ignored long enough.) viewtopic.php?f=53&t=1061
  
  Series I, II, III RPO Code L67 & L32:
  Change Supercharger Oil; Every 30-50k. (Intense Racing or GM Dealer) viewtopic.php?f=56&t=1690
  Supercharger Belt; Inspect Every 15k, Replace Every 100k or as needed.

7/29/2012 by 00Beast

[stevenmamos]

They are Duralast wires and AutoLite Platinum plugs. Already fixed the gauge issue and calculate my mileage with the distance I travel and the amount of gas I put in.

[01bonneSC]

Have you changed your LIM gaskets and upgraded to the new style UIM?

[stevenmamos]

I'm not sure the LIM has been replaced but I know for sure I have the updated UIM installed.

[01bonneSC]

You are an 04, not sure if they gave you the updated LIM gaskets or not from the factory.

Check between the cylinder head and the LIM. If the gaskets is black its the old version and should be changed. If its silver/aluminum looking, it is the new style and you should be ok.

Notice any coolant loss?

[00Beast]

No L36 got any upgraded gaskets in any year.

Not sure on the N/A, but on S/C, platinum plugs are bad. Nothing Duralast has proven to be any good that I've seen, however...

[LeSabre in Buffalo]

Were the plugs gapped correctly, or did they get installed without gapping? Those plugs should be gapped to 0.060", IIRC. If they're gapped under that, then gas mileage will go down.

To gap them, get a wire gapping tool, not one of those "coin" gappers. Bend the electrode, and don't force the wire into the gap.

[1oldman]

When the fuel mileage drops drastically on the ole green monster, I replace the o2 sensor. That has fixed the problem almost every time. - BC

[01bonneSC]

How many miles on the car?

[stevenmamos]

The plugs were gapped correctly. There are about 99,000 miles on the car.

[01bonneSC]

You could try taking the O2 sensor out and cleaning the soot off with some carb cleaner. See if that helps.

At that milage it wouldn't be throwing money away to put a new one in. Go with ACDelco as the brand.

[Arrowhead]

Have you checked/changed air filter lately? It's a simple thing that can/will waste a lot of gas if it's really dirty and restricts flow.

[stevenmamos]

I might try cleaning the sensors. Heard that air filter doesn't affect gas mileage, just performance...(http://www.fueleconomy.gov/feg/maintain.shtml). Also found out that there have been people in my neighborhood siphoning gas from vehicles at night...so just bought a locking gas cap hoping that this is what is happening. We shall see.

[00Beast]

No, a plugged air filter will kill performance AND mileage. Co-worker just went through this on his F150, lol. Filter was PACKED with crap.

Do you really believe what the Government says about your car?!

[01bonneSC]

Performance and MPG are directly related.

[LeSabre in Buffalo]

If you can't remember the last time the air filter was changed, it's a good idea to change it. Once every few years and 30-50k miles is generally enough for most cars. Filters get more efficient as they age and fill up, up to a point. After that point, they're too filled with junk to pass much air. So, just like the 3 little pigs, one doesn't want to change the air filter too often or too soon.