Gas Mileage On Truck Just Plunged

99silverado1500 · September 4, 2007

On my truck the gas mileage just took a sh*t recently, no idea why. Power seems to be the same. I went on a 140 mile trip and used 12 gallons. This was a all highway trip. That equivalates to 11.6 mpg. Now I did go 95 mph for a joint total of maybe 5 or so of those miles. The rest of the time was 60-70mph constant highway. I know the 95mph runs didnt help it, but 11.6 highway come on. Gotta be something up with it. I gotta pull the plugs today to see what they look like but im hoping its something easy.

Dans Denali · September 4, 2007

O2 sensor.

04z71steve · September 4, 2007

yeah i think it might be a o2 sensor, in my gtp i was getting about 27mpg highway with a supercharger which was great... then it plunged to 14!!!!! took it in and it needed an o2 sensor... fixed her right up

99silverado1500 · September 4, 2007

Does anyone have the part number for a 99 5.3? This is one of the few sites I can access with this new firewall here at work.

denalitwin · September 5, 2007

Does anyone have the part number for a 99 5.3? This is one of the few sites I can access with this new firewall here at work.

before you rush off and start replacing parts, you maybe should consider all your options......unless your O2 sensor just went bad & set a hard code...

redsled2007 · September 5, 2007

Before you go off yelling "The sky is falling!" try another 140 mile trip again but this time keep it at 65 mph and see if it goes back to the same as before. When the computer shuts off at 95 mph (mine does, did yours?) gas just pores thru the engine. And you said you did this for 5 miles. Before I spent a lot of time and money on this "problem", I'd check and make sure I had a problem.

2004GMC1500CC · September 5, 2007

Before you go off yelling "The sky is falling!" try another 140 mile trip again but this time keep it at 65 mph and see if it goes back to the same as before. When the computer shuts off at 95 mph (mine does, did yours?) gas just pores thru the engine. And you said you did this for 5 miles. Before I spent a lot of time and money on this "problem", I'd check and make sure I had a problem.

Don't believe the computer "shuts off". I believe you hit the speed limiter, which unless you have some speed rated tires and some pretty decent brakes is asking for trouble. Nothing like a panic stop at 95 with a 5k lb vehicle. Not so much worried about you, more worried about me, the guy stopped in front of you.

pjmmoore · September 5, 2007

Hey all - since we're on the topic of O2 sensors I have a few questions about them...if an O2 sensor goes bad does it throw a code or does something else happen like a drop in mpg or something else. My '02 4.8 has about 65k and is now almost 6 years old - is it time for new O2 sensors? My truck is still running fine, but I'm wondering if changing these now is good or necessary preventative maintenance. Thanks all.

--paul

bryanhunt · September 5, 2007

Hey all - since we're on the topic of O2 sensors I have a few questions about them...if an O2 sensor goes bad does it throw a code or does something else happen like a drop in mpg or something else. My '02 4.8 has about 65k and is now almost 6 years old - is it time for new O2 sensors? My truck is still running fine, but I'm wondering if changing these now is good or necessary preventative maintenance. Thanks all.

--paul

There are two ways for an O2 sensor to go bad. One is that the heater takes longer to warm the O2 sensor up and the engine will have rough running immediately after starting up, and for the northerners on the board could have rough running during the hardest part of winter.

The other one is that the O2 sensor gets gummed up or burned, or any other numerous ways that prevent the O2 sensor from switching. This will set a fault code in the computer. You will lose fuel mileage because the computer will be stuck running open loop.

Some people claim that the switching point on O2 sensors can drift over time, but I don't know how much and if it even makes a difference that is noticable.

Draenor · September 5, 2007

Before you go off yelling "The sky is falling!" try another 140 mile trip again but this time keep it at 65 mph and see if it goes back to the same as before. When the computer shuts off at 95 mph (mine does, did yours?) gas just pores thru the engine. And you said you did this for 5 miles. Before I spent a lot of time and money on this "problem", I'd check and make sure I had a problem.

+1 Get a better baseline before changing everything.

Hey all - since we're on the topic of O2 sensors I have a few questions about them...if an O2 sensor goes bad does it throw a code or does something else happen like a drop in mpg or something else. My '02 4.8 has about 65k and is now almost 6 years old - is it time for new O2 sensors? My truck is still running fine, but I'm wondering if changing these now is good or necessary preventative maintenance. Thanks all.

--paul

Dont fix it if it aint broke.

cmoore74 · September 5, 2007

Ditto on new base-line before you start thowing parts at a problem that may not be real.

There are 4 O2 sensors on these trucks (pre and post cats for each side) and they are not cheap. It would be upwards of $200-300 to replace all 4 O2 sensors just in parts alone.

I have heard the O2 sensors may start to go around 100k or more without throwing a code, but that's a lot of $ if you're not sure it's a problem.

LewiSILVERADO · September 5, 2007

I was having one tank in the 13's, then a couple in the 17-19 range, then another 13. I found the perfect aftermarket accessory that seems to have cured this problem. I put a locking billet fuel filler door on, and the problem was solved! I guess that's the risk of parking outside. But in your case, I'd get the local parts store to run the codes to see if it's the O2 sensor.

jhm · September 5, 2007

On my truck the gas mileage just took a sh*t recently, no idea why. Power seems to be the same. I went on a 140 mile trip and used 12 gallons. This was a all highway trip. That equivalates to 11.6 mpg. Now I did go 95 mph for a joint total of maybe 5 or so of those miles. The rest of the time was 60-70mph constant highway. I know the 95mph runs didnt help it, but 11.6 highway come on. Gotta be something up with it. I gotta pull the plugs today to see what they look like but im hoping its something easy.

I would bet on the 95 mph causing the drop. Amazing how much fuel gets sucked up once you exceed the sweet spot, usually around 55 mph. You were probably getting about 6 mpg during the time driving 95 mph.

bryanhunt · September 5, 2007

On my truck the gas mileage just took a sh*t recently, no idea why. Power seems to be the same. I went on a 140 mile trip and used 12 gallons. This was a all highway trip. That equivalates to 11.6 mpg. Now I did go 95 mph for a joint total of maybe 5 or so of those miles. The rest of the time was 60-70mph constant highway. I know the 95mph runs didnt help it, but 11.6 highway come on. Gotta be something up with it. I gotta pull the plugs today to see what they look like but im hoping its something easy.

I would bet on the 95 mph causing the drop. Amazing how much fuel gets sucked up once you exceed the sweet spot, usually around 55 mph. You were probably getting about 6 mpg during the time driving 95 mph.

Aerodynamic drag force increases by speed squared. So the amount of drag force at 95mph is a bit over twice what it is at 65mph. So that means you're having to do twice the work. Halving your fuel mileage at that speed isn't a bad estimate.

On top of that you have to think about fuel consumption as a function of engine RPM which will futher lower the previous half estimate.

colosilverado · September 5, 2007

On my truck the gas mileage just took a sh*t recently, no idea why. Power seems to be the same. I went on a 140 mile trip and used 12 gallons. This was a all highway trip. That equivalates to 11.6 mpg. Now I did go 95 mph for a joint total of maybe 5 or so of those miles. The rest of the time was 60-70mph constant highway. I know the 95mph runs didnt help it, but 11.6 highway come on. Gotta be something up with it. I gotta pull the plugs today to see what they look like but im hoping its something easy.

I would bet on the 95 mph causing the drop. Amazing how much fuel gets sucked up once you exceed the sweet spot, usually around 55 mph. You were probably getting about 6 mpg during the time driving 95 mph.

Aerodynamic drag force increases by speed squared. So the amount of drag force at 95mph is a bit over twice what it is at 65mph. So that means you're having to do twice the work. Halving your fuel mileage at that speed isn't a bad estimate.

On top of that you have to think about fuel consumption as a function of engine RPM which will futher lower the previous half estimate.

Mine did that when the fuel pressure regulator took a dump...