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Ihatecars
12-03-2013, 04:49 PM
I've recently noticed a major drop in my MPG, which made me a little concerned with the health of my little Roadster. After a decent amount of research and tinkering I've been lead to believe that my O2 sensor might be the cause of my heartache.
I'm not certain it's the sensor though. I've read that I might experience slight performance losses, I haven't had any sort of noticeable losses. I also did the diagnostic check by grounding out the TEN, but i didn't get any codes back. I also went through the old service record the previous owner kept and I haven't found anything that's said the O2 sensor was replaced. Which would explain it going bad, but I could have looked over something saying it had peen replaced.

Do you guys think it would be worth it to just purchased another O2 sensor and see if I get fixes my MPG problems?

And if I do end up getting another sensor, does anybody have an recommendations? Should I go for a universal sensor and solder it in, or go with one of the options that come with the female connection already attached? Any personal experience with either options?

Thanks in advanced MazdaRoadster! :bow:

The Driver
12-03-2013, 05:04 PM
When was the O2 replaced?

How many miles in your car?

When was the last tune up done?

Could it be that the catalythic converter is dying?

Ihatecars
12-03-2013, 05:40 PM
When was the O2 replaced?

How many miles in your car?

When was the last tune up done?

Could it be that the catalythic converter is dying?

Not sure. Couldn't find it in the service records I have.

Just under 50k.

100k Mazda service done in April of 08.

I haven't looked into the cat. I was well within the hydrocarbon threshold during my last inspection.

Thumper13
12-03-2013, 05:41 PM
Generally you would see a CEL if the O2 sensor is going bad.

As Driver mentioned tune up. Our little cars are very sensitive and HATE ignition wires over 36k miles,same with spark plugs.

You mentioned a MAJOR drop. How much less mpg are you getting than when you first calculated your mpg ?

Ihatecars
12-03-2013, 06:47 PM
I've read that the CEL doesn't always show up on the ODB1 vehicles because some sort of setting with the ECU. I have nothing to prove or discredit this apart from the thread on M.Net that stated this.

When I purchased the vehicle in March I did replace the spark plugs as well as the ignition wires with NGK products. I guess that'd be considered a tune up. Whoops. :oops:

I went from about 30 MPG on average, to about 20, 23, and 22 MPG on my last three fillups. (I use Fulley to track my fuel economy)
Admittedly, I have been doing a lot less freeway driving, but nearly 10 MPG seemed very alarming.

Thumper13
12-03-2013, 08:05 PM
I've read that the CEL doesn't always show up on the ODB1 vehicles because some sort of setting with the ECU. I have nothing to prove or discredit this apart from the thread on M.Net that stated this.

When I purchased the vehicle in March I did replace the spark plugs as well as the ignition wires with NGK products. I guess that'd be considered a tune up. Whoops. :oops:

I went from about 30 MPG on average, to about 20, 23, and 22 MPG on my last three fillups. (I use Fulley to track my fuel economy)
Admittedly, I have been doing a lot less freeway driving, but nearly 10 MPG seemed very alarming.

Oh Yeah, a drop of almost 1/3 is HUGE.................2 or 3 gals could be math...10 something ain't Kosher. No one thing stands out so what are you not telling us ? How is the car driving ? Are you using the same place to fuel up each time ? Maybe somebody has a gas pump meter that is effing the customers. Could be the gas has been sitting to long and maybe there is some water getting in, but not enough to make it run bad ?

With such a drop in fuel consumption you should be noticing a bunch of different stuff. At minimum you should be smelling a heavy fuel content in your exhaust,the gas has to be going somewhere.

With all my heavy duty mods ( 11:1 compression hot cams and a bunch of other stuff ) I'm still getting 23 to 27 mpg average,if you know me that's damned good. When I really do the happy go pedal dance maybe she'll drop to 18/19 mpg, at 7500 rpms race conditions ( living between 6000 and 7600 ) she'll drop to around 15 mpg

jnshk
12-03-2013, 08:23 PM
If your driving routine has changed, that is probably the more likely cause. However, other things like air filter, spark plugs, plug wires, and O2 sensor could easily contribute.

O2 sensors generally do not trigger a CEL until they are WAY off calibration, so you may have a sensor that's gone/going bad even if you have no CELs. Generally you will not notice a performance loss unless it's extremely off calibration. Aside from swapping it out for a known good sensor, I'm not sure of a good method for checking the O2 sensor on an OBD1 vehicle unless you've got one of those spendy scanners like a Genisys system.

If it's your cat, you will likely notice a rotten egg-like smell from the exhaust, and if it's really bad you will probably notice performance losses (especially when going WOT).

Ihatecars
12-03-2013, 11:15 PM
Oh Yeah, a drop of almost 1/3 is HUGE.................2 or 3 gals could be math...10 something ain't Kosher. No one thing stands out so what are you not telling us ? How is the car driving ? Are you using the same place to fuel up each time ? Maybe somebody has a gas pump meter that is effing the customers. Could be the gas has been sitting to long and maybe there is some water getting in, but not enough to make it run bad ?

With such a drop in fuel consumption you should be noticing a bunch of different stuff. At minimum you should be smelling a heavy fuel content in your exhaust,the gas has to be going somewhere.

With all my heavy duty mods ( 11:1 compression hot cams and a bunch of other stuff ) I'm still getting 23 to 27 mpg average,if you know me that's damned good. When I really do the happy go pedal dance maybe she'll drop to 18/19 mpg, at 7500 rpms race conditions ( living between 6000 and 7600 ) she'll drop to around 15 mpg

My driving routine has changed from about 20 miles of freeway driving and 10 miles of city, to about 15 miles of only city. I understand that it's a rather large change, but 10 mpg seems like far too much of an mpg drop.

As far as driving, everything seems to be the same. I haven't noticed any sort of performance changes, but there perhaps were gradual changes that I didn't notice because of the changes being so gradual.

I haven't been using my typical gas station actually. I changed jobs so I no longer work near the gas station I'd stop at.(I worked out in the country so gas was cheaper near my job)

I daily drive my Miata so gas sitting isn't a concern.

My exhaust doesn't smell any different. I park in a garage and haven't noticed a smell or gas seeping from the exhaust.


If your driving routine has changed, that is probably the more likely cause. However, other things like air filter, spark plugs, plug wires, and O2 sensor could easily contribute.

O2 sensors generally do not trigger a CEL until they are WAY off calibration, so you may have a sensor that's gone/going bad even if you have no CELs. Generally you will not notice a performance loss unless it's extremely off calibration. Aside from swapping it out for a known good sensor, I'm not sure of a good method for checking the O2 sensor on an OBD1 vehicle unless you've got one of those spendy scanners like a Genisys system.

If it's your cat, you will likely notice a rotten egg-like smell from the exhaust, and if it's really bad you will probably notice performance losses (especially when going WOT).

It might be a combo of my driving routine change(mentioned the change in the reply above this) and the O2 Sensor. My air filter, plug wires, and plugs have all been changed within the last six months.

A thread on M.Net said the exact same thing as you did regarding the cel not showing all the time with a malfunctioning O2 sensor.

I haven't noticed any unusual smells. And definitely nothing like rotten eggs. My only RX-7 didn't have any cats and it stunk badly. To the point that if I let it idle in my garage it would start to smell inside my house.

speedypenguin
12-04-2013, 01:16 AM
Doesn't hurt to change the O2 sensor. Chances are it's the original sensor, given your car's low mileage. IIRC, I paid <$30 for the pre-cat O2 sensor on my '94.

BoBo
12-04-2013, 01:54 AM
Doesn't hurt to change the O2 sensor. Chances are it's the original sensor, given your car's low mileage. IIRC, I paid <$30 for the pre-cat O2 sensor on my '94.

+1 02 sensor is cheap, most likely the OP still have the original one.

BoBo
12-04-2013, 01:58 AM
The OP could buy one without the connector and just splice it to your original connector to save money. I remember buying a new 02 sensor for my mr2 Turbo and the gas mileage vastly improved. I remember the 02 sensor being pricey at the time, so I just got the one you have to splice. But for the miata it should be dirt cheap.Bosche or denso brand is good. Rockauto might have a bosche for maybe $35. Your air filter might be dirty.

Ihatecars
12-04-2013, 02:58 AM
I think for now I'm going to go with a new universal *O2 Sensor*, and just see how it goes from there. In about a month I'll report my changes in MPG and see if there are any major differences.

BoBo
12-05-2013, 01:08 AM
universal spark plug? you mean some random spark plug? that might ruin the engine.Check if your tires are low on air or check your trunk if you left something heavy in there. Lift every corner of your car and spin the wheels to see if something is creating friction, Let us know that might be the culprit.

Ihatecars
12-05-2013, 02:44 AM
universal spark plug? you mean some random spark plug? that might ruin the engine.Check if your tires are low on air or check your trunk if you left something heavy in there. Lift every corner of your car and spin the wheels to see if something is creating friction, Let us know that might be the culprit.

I totally meant to type O2 Sensor. I'm not sure what I was thinking.:oops:
I know my tires are filled to a really low psi. I can't remember what though. It was August when I last aired them up. I also don't keep anything inside of my car apart from a few very light items.

I'll check my tires asap though. They might have leaked a little too much air. Especially with the rapidly changing climate we've had here in South Texas lately.

Thumper13
12-05-2013, 08:35 AM
One thing about an o 2 sensor going bad, if it's pulling more fuel because it's sending a lean condition to the ECU you should still smell the unburned fuel in the exhaust. Loosing that many mpg you have to be sending unburned fuel somewhere. The OEM engine isn't capable of using all that extra gas, even at 80 mph all day long..

Ihatecars
12-05-2013, 09:40 AM
I'll keep an eye out for any unusual smells.
I'm going to preform a slight experiment using a control of a single gas station, and alter a few variables. Such as my O2 Sensor, tire pressure, and driving habits. We'll see if there is any change or if I'm in bigger trouble than I'd previously anticipated.

jnshk
12-05-2013, 09:02 PM
FWIW, when my commute consisted of a route that was mostly in-town highway (40-50mph, but heavy traffic) with some in-town stop-and-go, I would generally get 27-28mpg. Now that my commute consists of major highway (65-75mph) and slightly more in-town stop-and-go, I am getting about 23-24mpg average.

psulja
12-05-2013, 09:33 PM
If you've done a tune up already and everything else looks like its performing fine it doesn't hurt to change the o2 sensor.

The car is performing properly, right?

If I was to do an o2 sensor I would buy this:
http://www.rockauto.com/catalog/moreinfo.php?pk=991627&cc=1190887

www.rockauto.com search for part number 2341024
$28.62 with the connector already in place rather than a universal sensor.

I have heard that it is not good to solder o2 sensor wires.


Dissimilar metals have a contact potential that varies with temperature. If the solder is between the two wires, any small temperature difference will create a voltage across the joint, hence giving a bad reading. This is called the galvanic effect and is the basis of how thermocouples are made.
Do not repair the wiring, the connector or the terminals. Replace the oxygen sensor if the pigtail wiring, the terminals or the connector is damaged. Proper oxygen sensor operation requires an external air reference. This external air reference is obtained by way of the oxygen sensor signal and heater wires. Any attempt to repair the wires, the connectors or the terminals results in the obstruction of the air reference and degrades the oxygen sensor performance

kung fu jesus
12-06-2013, 09:06 AM
A few thoughts and experiences with my '92 and this issue.

First I wanted to address your original question; the narrow band O2 sensors on an OBD1 car has a range that is within spec. The O2 can degrade over time and still be within range, not throw a code. For a replacement, I prefer direct OE replacements such as the NGK or such 'known' brands. I do not like the splice-in type because the cable is shielded and a break in the shielding can allow static from a number of sources to dilute the signal. That is just my opinion. I have and do use a spliced-in rear O2 on my '99 because I added a custom midpipe and '01 manifold.

For fuel economy to decrease that badly over that amount of time I would be suspicious of other systems causing the drop. In my experience, the first items I would check would be the brakes. I would check the pads for even wear, stuck calipers, dragging emergency brakes. I have had that cause a huge swing in fuel economy. Brake maintenance is pretty simple, inexpensive and preventative. Usually all I did was remove the pads, retract the caliper pistons to see if the were binding, inspect the rubber boots on the mounting hardware for tears, water in the grease or debris, clean the slides, re-lube with a high-temp silicone grease, re-install the pads, sometimes bleed the system (depending on the amount of track time the car saw). I usually did this annually, sometimes longer.

The tire pressures, as mentioned above, are also pretty common. These cars are sensitive to uneven pressures, but if the tire(s) are leaking slowly over time, you may not notice it as much.

Also, I don't know if TX does this, but in some of the northern states the fuel formulas are changed from summer to winter. IIRC, the winter formula doesn't achieve the same economy as the summer version, but it isn't that bad as you are noticing.

So, before throwing parts at the issue, check the simple, free stuff first, deduce the issue from that by eliminating the possible variables and move to the next one. If you you really still feel it is the O2, you can hook a simple volt meter to the O2 and drive around to see how it responds and reads. It's a simple 5v circuit and the ECU monitors the output and responds accordingly. A narrow band O2 dithers a LOT, but if the O2 is bad, the readings at the volt meter will be relatively flat or sluggish across the range of driving.

Good luck!

jnshk
12-07-2013, 10:31 AM
I would like to second the comments about "universal" O2 sensors. Ideally, it is better not to use them at all, but if you do then you want to crimp only. (Most of the time the wires are a stainless or othersuch metal which will not readily solder anyhow.) I believe that Bosch universal sensors include "posi-lock" connectors which allow you to securely join the wires by hand without any special tools, but as Steve mentioned you will still be breaking the shielding of the signal wires.

Ihatecars
12-19-2013, 12:13 AM
UPDATE!

Okay, so I decided to do a bit of an experiment using gas stations, and an O2 Sensor as the variables. I'll also try to make my driving routine as constant as possible for the sake of my experiment. After two fill ups at the same gas station, getting the same gas, driving nearly the same distances, under the same conditions I got a constant reading of 26 mpg using Fuelly as my MPG tracking calculator.

This leads me to believe that the gas station I'd previously used had either a bad pump(I don't believe I fueled up at the same pump constantly) or they're thefts.

Today I ordered a complete Denso O2 Sensor to use for the next portion of my experiment.

I'll report back once I've installed the new sensor and I've also fueled up a few times.

artcentermiata
12-19-2013, 12:21 AM
i dont think ive ever gotten much more than that before my rotrex...

i remember around 28mpg with normal driving and highway entrances (not eco driving, but not nutso driving)

now i dont get near that haha!

Ihatecars
12-19-2013, 12:33 AM
But now you get to listen to all that awesome whining! I'd loves a few mpg for that.

kung fu jesus
12-19-2013, 08:20 AM
I had the same pump issue happen to me on a recent roadtrip with another car. The fuel pump shut off and I hang it up thinking it was full. When I got in the gauge read full, but it started decreasing after a shorter time than before. I had also calculated my mileage on that fill up and it was much higher than the previous stops. I figure the cutoff in the pump kick on and probably still has 2 gallons or more to go.

Oddly, with this same car, I have had the cutoff NOT engage and gave the fender and immediate are near the car's filler a nice gasoline douching. :/

Ihatecars
12-19-2013, 07:08 PM
I'm excited to see what type of differences a new O2 sensor will make.
I'm not expecting anything too great, but I'm excited to be able to reach 30mpg again.

maharaj
12-19-2013, 07:17 PM
Just so you know, I hate all of you with your hybrid mileage miatas. Jerks. :P :D

Ihatecars
12-19-2013, 08:58 PM
I'm not too excited about my hybrid horsepower.