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| C5Dave | bad gas mileage - maybe the O2 sensors | ||
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Guru Newb
 
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my 1999 C5 is getting 11MPG and tried tuning it up.
might the O2 sensors need replacement? |
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Posted on: 2016/5/27 21:45
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| bogus | Re: bad gas mileage - maybe the O2 sensors | ||
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Grand Imperial Pooh-Bah
 
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Gang... a little backstory...
Dave here is a friend. I helped with the tune up a few weeks back. However, the O2 sensors are of an unknown age and the car has over 110k on it. So new O2s seems like a logical replacement. I will say this, if only for disclosure, but when he drives, its a short hop here, a short hop there. The car never gets into closed loop, so its constantly running on tables. He drove it to Palm Springs and suddenly, the MPG is now well into the 20s... The car is a 98 coupe, automatic, stock. Thoughts? |
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Posted on: 2016/5/27 22:25
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| j3studio | Re: bad gas mileage - maybe the O2 sensors | ||
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Elite Guru
 
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O2 sensors are a common cause of a sudden drop in stock C5 mileage. The word on the street is that it is the front two that matter. I just took the 2003 'vert out and there's nothing I can do legally that will drive the mileage below 15 mpg in open loop. |
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Posted on: 2016/6/1 21:13
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| BillH | Re: bad gas mileage - maybe the O2 sensors | ||
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The Stig Moderator
 
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Bad O2's should throw a code.
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Posted on: 2016/6/2 3:28
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| bogus | Re: bad gas mileage - maybe the O2 sensors | ||
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Grand Imperial Pooh-Bah
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I would agree... but they could also be slow, especially the upstream ones...
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Posted on: 2016/6/2 19:28
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| BrianCunningham | Re: bad gas mileage - maybe the O2 sensors | ||
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Senior Guru
 
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Clogged cats
stuck injector (should be running rich) |
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Posted on: 2016/6/2 20:36
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| bogus | Re: bad gas mileage - maybe the O2 sensors | ||
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Grand Imperial Pooh-Bah
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No performance drop.... no overheating... nothing to indicate clogged cats.
Could be a bad injector(s), but the pipes are reasonably clean. |
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Posted on: 2016/6/2 21:05
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| Matatk | Re: bad gas mileage - maybe the O2 sensors | ||
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Webmaster
 
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Replacing 02 sensors at 110,000 is basic maintenance, not throwing parts at it. I'd do a general tune up. How do the plugs and wires look? Fuel filter been replaced recently?
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Posted on: 2016/6/3 2:28
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| NelsonV | Re: bad gas mileage - maybe the O2 sensors | ||
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Senior Guru
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Dave-
Have you done a manual calculation of actual miles driven versus actual fuel consumed? I don't believe the average MPG, as indicated by the DIC, is a "true average MPG" of actual fuel consumption, but rather an average of many samples by the computer of instantaneous fuel consumption. |
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Posted on: 2016/6/5 6:06
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| bogus | Re: bad gas mileage - maybe the O2 sensors | ||
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Grand Imperial Pooh-Bah
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Quote:
I did new NGK plugs and MSD Wires just 6 weeks ago. I can't speak for the fuel filter,but it won't cause bad mileage... |
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Posted on: 2016/6/5 17:48
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| Matatk | Re: bad gas mileage - maybe the O2 sensors | ||
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Webmaster
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Quote:
We are all entitled to our opinions, but I disagree. A dirty/clogged fuel filter can affect the pressure, fuel atomization, etc and if it causes a lean condition will affect the ecm control of the fuel trims. A dirty air filter can also affect the mileage. I still recommend a tune up of basic parts at 110,000 miles. |
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Posted on: 2016/6/6 12:58
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| bogus | Re: bad gas mileage - maybe the O2 sensors | ||
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Grand Imperial Pooh-Bah
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I don't disagree... I just don't know the age of the fuel filter.
Also, with our more temperate climate, fuel filters don't have nearly with work to do... they do last longer. As for the air filter, I gave it a good shake out and it appeared to be serviceable. |
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Posted on: 2016/6/6 17:03
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