O2 sensors and increased MPG?

Messages
3,653
Location
Clermont, Florida
I have been researching O2 sensors and most of what I read says they really only have a service life of about 80,000 miles. My truck has 4 O2 sensors and is at 108,000 miles. The MPG is suffering a little and I wonder if anyone here has changed their O2's out as a maintenance item (meaning there is no check engine light on) and if so did it help the MPG and performance at all? I have a 2001 Dodge Dakota with the 3.9 V6 and 5-speed manual transmission. I recently did a tune up with plugs, wires, cap and rotor and I use only name brand gas, Chevron or Shell. It gets a dose of Regane every 5,000 miles. My CEL is not on, I just wonder if new O2's will help. I'm interested in knowing how a new set of O2 sensors may help me and whether anyone else got any benefit from replacing theirs. I appreciate any info you guys can offer.
 
Messages
2,131
Location
Maryland
I would probably not change them until I get a CEL that's pointing at a malfunction with the O2 sensors. If anything, the pre cat, O2 sensors are the only ones that will have an affect on your gas mileage.
 
Messages
18,449
Location
East of IGO
The new sensors will make a difference if the old ones are not with in specs. Try a Auto R/X clean up first. You may be suprised with the results and it costs way less.
 
Messages
19,479
Location
Chicago Area
O2 sensors will usually read slowly as they age, but steady state driving will be OK. I'd get everything else right first. It is tough to check an O2 sensor properly. [it's easy if they are really bad] Clean the TB, fix all vacuum leaks, plugs wires, fuel system cleaner. Also brake drag and tires. Etc.. And only the FIRST [front] O2 sensors read the mixture - the second ones are after the catalytic converter[s] only to tell if there is a difference from the front ones.
 
Messages
4,998
Location
Milwaukee, WI
"And only the FIRST [front] O2 sensors read the mixture - the second ones are after the catalytic converter[s] only to tell if there is a difference from the front ones." This is correct, and very important if you're going to upgrade the upstream sensors to gain mileage back. I would agree, 100,000 miles is usually about it for an O2 sensor.
 

Jimmy9190

Thread starter
Messages
3,653
Location
Clermont, Florida
TB and IAC are clean as a whistle. That is always part of my tune-up procedure. Plugs are only 6 months old with about 15K on them and the plug wires are all OK. The engine pulls exactly 20 hG of vacuum at idle. Tires are new, alignment is perfect and there are no brake issues. My truck is in great shape and I take exceptionally good care of it. I have just noticed a reduction in MPG lately. I do plan on keeping it as long as possible. It's a good truck and the few problems I have had with it were all very minor. I do all my maintenance ahead of schedule and I wonder if my O2's need to be replaced due to their age and mileage.
 
Messages
19,528
Location
Lake Forest, CA
My '94 Lexus LS400 with 240k+ miles has 4 original O2 sensors. Gas mileage is suffering the last several months, about 2-3 mpg less than average and average wasn't that great either at about 17 MPG, but still the CEL is not on. I'm thinking about replacing the 2 pre-cat sensors, which are easier and a little cheaper than the 2 rear.
 
Messages
7,430
Location
beaver land EH?
subject title is somewhat misleading. It should be like this: Replacing O2 sensor(s) restore MPG? It does not improve your MPG unless you start your car with a lazy/defective unit(s) to begin with. Q.
 
Messages
1,562
Location
St. Paul, MN
Depends on how well made the original sensors were. Some last 300k, some last 100k. I highly recommend getting the vehicle specific sensors if you do replace em', the universal fit can be a PITA.
 
Messages
19,528
Location
Lake Forest, CA
 Originally Posted By: AcuraTech
Depends on how well made the original sensors were. Some last 300k, some last 100k. I highly recommend getting the vehicle specific sensors if you do replace em', the universal fit can be a PITA.
I agreed. Mine lasted more than 200k miles, the OE I believe is Denso. I'm looking OE exact fit from Denso, not universal fit even from Denso. The lowest Denso OE fit I can find is about $65-70/each for the pre-cat, which is about 70% less than dealer's.
 
Messages
19,479
Location
Chicago Area
I find that the intermediate stage of fuel going from winter to spring is the worst for mileage. A consistent pattern. It is probably not the actual blend, but I have a sneaking feeling that 'dregs' are being used up and sold at this time.
 

NJC

Messages
3,007
Location
Vancouver BC
As others have mentioned, they get lazy over time. And it's not something a multimeter can detect. I have a thread here somewhere with readings from a scope, and the cycle times (rich -> lean, vice-versa) are >500ms. They should be ~100ms.
 
Messages
1,304
Location
Kankakee, IL
My milage was down a few MPG from consistent 32 or so to 28-29 and then finally had it failing when it got wet. Once replaced milage went immediately back to 32+. So they do make a difference. If they get a bit off they get dirty and thus off some more. You may or may not get a total fix on yours. Depending on price not a bad deal to change them out at your milage.
 

hal

Messages
393
Location
NC
My 91 626 had a REALLY lazy o2 sensor, it's a 1-wire though and was not throwing a CEL. I was getting literally 23-24mpg before, and over 30mpg after changing it. Serious as I can be. It was probably on the verge of going completely bad.
 
Messages
25,046
Location
ON, Canada eh?
Since we are all comparing mileage. This last week I had 1 of 4 02 Sensors go bad in my Santa Fe (170K KM - 106K Miles). It set the CEL and I went and bought a replacement Denso one and put it in. Since then I have burn't 3 tanks of gas and I haven't noticed any different mileage or a different response from the engine. I'm still averaging just under 30mpg as I was before... So I guess the computer just used the functioning one on the other bank of cylinders to compensate along with the ones downstream?
 
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