Excessive Fuel Consumption
Thread Starter
Staging Lane
Joined: Jul 2010
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Excessive Fuel Consumption I would like to get some input on this Issue. I filled up today at.12pm and by 130 I had used a 1/4 tank. No racing or foot to the floor excursions. I went to to visit a friend and we sat in the car for about an hour. I was idling, thats all. I was thinking perhaps it evaporating out of the fuel cap? The car is an 02 w/z06 camshaft. perhaps a change of plugs? I did fix a couple vacuum leaks but that was days before I filled up today. the 02 doesn't have an egr valve that can be sticking. What suggestions do u guys have?
TECH Senior Member
Joined: Dec 2001
Posts: 5,399
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From: Jacksonville, FL
How much gas did you actually use, go back to the same pump and see how many gallons it'll take. After a fill up the gauge will read high past full for quite a while at highway speeds before starting to drop. It's not evaporating out the fuel cap, you'd be throwing P0440 for a large leak, or P0442 for a small leak.
Scan for codes and take a look at your O2 waveforms.
BTW - Gas doesn't evaporate that fast and if your cap was bad, you'd get an SES code for the EVAP system.
The scan will tell you a lot. I had a similar situation and I think it was a bad post-cat O2 sensor. It was flatlining on me and I think was causing the engine to run rich.
BTW - Gas doesn't evaporate that fast and if your cap was bad, you'd get an SES code for the EVAP system.
The scan will tell you a lot. I had a similar situation and I think it was a bad post-cat O2 sensor. It was flatlining on me and I think was causing the engine to run rich.
Thread Starter
Staging Lane
Joined: Jul 2010
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How much gas did you actually use, go back to the same pump and see how many gallons it'll take. After a fill up the gauge will read high past full for quite a while at highway speeds before starting to drop. It's not evaporating out the fuel cap, you'd be throwing P0440 for a large leak, or P0442 for a small leak.
it tool a hair over 6gal and the gauge read just over 1/2
Thread Starter
Staging Lane
Joined: Jul 2010
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Here's an update. I put a scan tool and watched some real time data. The upstream o2 sensors are working as designed. The downstream sensors however are only operating on the .700v-.800v range. Being generous, .700v is the lowest voltage I read. Looks like ill b replacing 2 downstream sensors 2morrow.
Here's an update. I put a scan tool and watched some real time data. The upstream o2 sensors are working as designed. The downstream sensors however are only operating on the .700v-.800v range. Being generous, .700v is the lowest voltage I read. Looks like ill b replacing 2 downstream sensors 2morrow.
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Thread Starter
Staging Lane
Joined: Jul 2010
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So you're sauing there is nothing wrong with them? I can see the variation in voltage being slow or minimal, but they still should be seeing lower voltage at points. These NEVER went below .700v and that's being generous.
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Verdad Gallardo TECH Senior Member
Joined: Dec 2001
Posts: 5,399
Likes: 3
From: Jacksonville, FL
Don't just replace the post cat sensors, they just monitor the cats. You need to get your actual average MPG by filling up, driving, filling up again and doing the math. Hard to tell your useage by the gas gauge - especially after sitting at idle for so long.
Thread Starter
Staging Lane
Joined: Jul 2010
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I het anywhere from 130-150 miles/tank around town using 1-4 shift. Just now with 80% highway/20%town driving on this tank, I am at 170 miles.
2001NBMZ28 is correct in that you really need to quantify the problem with accurate gallon measurements before you start taking action - unless you can scan for something obvious.
The behavior you describe is correct for the O2 sensors. If one was bad, it would show you 0 volts or you would see one very different than the other.
A stock car should see anywhere from 24-30 mpg on the highway.
The behavior you describe is correct for the O2 sensors. If one was bad, it would show you 0 volts or you would see one very different than the other.
A stock car should see anywhere from 24-30 mpg on the highway.
Thread Starter
Staging Lane
Joined: Jul 2010
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14gal x 24mpg=336mi/tank. I am getting half that. Even with a z06 cam, only getting half of the low end mileage(24), is wrong. More power equals more mileage, provided u keep your foot out of it. I use the 1/4 shift. Hell, even around town, say 12mpg=168mi/tank and I'm not even getting that and I no these cars get better than 12mpg around town. I have a 67 Camaro 1/4.10's and a tko600 and it gets 200mpg/tank around town. I no a fuel injected engine with a 3.42 rear should be getting better than a carbbed 4.10 setup.
Car has 105k on it. Pulled #1/#3 plugs and they looked good. I don't smell gas anywhere so an external leak seems unlikely. I'm thinking perhaps the fpr? I am going to run a fp test. I read the spec is 53-65 with engine of and should increase by 10psi with engine running.
Car has 105k on it. Pulled #1/#3 plugs and they looked good. I don't smell gas anywhere so an external leak seems unlikely. I'm thinking perhaps the fpr? I am going to run a fp test. I read the spec is 53-65 with engine of and should increase by 10psi with engine running.
Last edited by 02 CamaroSS; Aug 1, 2010 at 05:00 PM.
14gal x 24mpg=336mi/tank. I am getting half that. Even with a z06 cam, only getting half of the low end mileage(24), is wrong. More power equals more mileage, provided u keep your foot out of it. I use the 1/4 shift. Hell, even around town, say 12mpg=168mi/tank and I'm not even getting that and I no these cars get better than 12mpg around town. I have a 67 Camaro 1/4.10's and a tko600 and it gets 200mpg/tank around town. I no a fuel injected engine with a 3.42 rear should be getting better than a carbbed 4.10 setup.
Car has 105k on it. Pulled #1/#3 plugs and they looked good. I don't smell gas anywhere so an external leak seems unlikely. I'm thinking perhaps the fpr? I am going to run a fp test. I read the spec is 53-65 with engine of and should increase by 10psi with engine running.
Car has 105k on it. Pulled #1/#3 plugs and they looked good. I don't smell gas anywhere so an external leak seems unlikely. I'm thinking perhaps the fpr? I am going to run a fp test. I read the spec is 53-65 with engine of and should increase by 10psi with engine running.
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Thread Starter
Staging Lane
Joined: Jul 2010
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I get about 200-220 miles per tank with my 427ci. and I always stomp it getting on the highway. I have 3.73 gears too. You have a problem. A single leaky fuel injector can totally kill your gas mileage. But you would most likely have slight to medium (to terrible) misfires. Failing 02 sensor(s) also start to run rich, and it will utterly destroy your mileage too.
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I couldn't get 200 downhill with the wind behind me. As for the injectors, is a leaking objectors going to give u different voltage reference when tested than a properly working 1? I can't wrap my head around the downstream o2's being cat ONLY monitors and only staying at the either end of the voltage range. That would mean they would tell the pcm the cat isnt doing its job and the pcm would act accordingly. on the other hand, if the.sensors were.bad, the engine light would turn on.
Did you use an OBDII scanner to look at them while the engine was rinning?
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Thread Starter
Staging Lane
Joined: Jul 2010
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Yes I did. The upstreams fluctuated from .02-.09. The downstream stayed between .6-.9.
Thread Starter
Staging Lane
Joined: Jul 2010
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A downstream oxygen sensor in or behind the catalytic converter works exactly the same as an upstream O2 sensor in the exhaust manifold. The sensor produces a voltage that changes when the amount of unburned oxygen in the exhaust changes. If the O2 sensor is a traditional zirconia type sensor, the voltage output drops to about 0.2 volts when the fuel mixture is lean (more oxygen in the exhaust). When the fuel mixture is rich (less oxygen in the exhaust), the sensor's output jumps up to a high of about 0.9 volts. The high or low voltage signal tells the PCM the fuel mixture is rich or lean.