02 CamaroSS

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?

2001NBMZ28

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.

wssix99

iTrader: (5)

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.

02 CamaroSS

On an obd 2 system, shouldn't a bad o2 cause the engine light to come on?

02 CamaroSS

it tool a hair over 6gal and the gauge read just over 1/2

02 CamaroSS

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.

wssix99

iTrader: (5)

This is normal. As you rev the engine, the upstream sensors will give a nice waveform. The downstream sensors will report less voltage and should be a straight line.

02 CamaroSS

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.

2001NBMZ28

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.

02 CamaroSS

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.

wssix99

iTrader: (5)

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.

02 CamaroSS

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.

LS6427

iTrader: (2)

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.

.

02 CamaroSS

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.

02 CamaroSS

If I had a leaky objectors or a bad air Intake temp sensor, wouldnt the mil light b on? A k&n filter wouldn't be affecting mpg, would it?

eseibel67

iTrader: (10)

Downstream sensors have NO effect on the computer's fueling decisions.

LS6427

iTrader: (2)

Failing 02 sensors do not always give a code or SES light until they totally fail. I have had five 02 sensors go bad in 12+ years.....I have never had a code or an SES light for any of them.

Did you use an OBDII scanner to look at them while the engine was rinning?

.

LS6427

iTrader: (2)

No way, not that much.

.

02 CamaroSS

Yes I did. The upstreams fluctuated from .02-.09. The downstream stayed between .6-.9.

02 CamaroSS

That is the complete opposite from what I have read.

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.