Non GL question O2 sensors

[ekvh]

I have a 98 Corolla that should be getting 30mpg but is getting 22. I feel I might have an exhaust leak beyond the first sensor which is dumping excess into the cat. The rear O2 raises the FPR to compensate. If I were to disconnect the O2 sensor in the back, the computer would cry foul. What if I hooked the second O2 sensor to the first? The computer would see perfect readings from the second, wouldn't it? I am hoping to get it fixed when it warms up, but for now, would this work? It's too cold to get to it now, although Sunday we might see 32.

Second question from anyone who knows, can I read the O2 sensors outputs by simply tapping in with a multimeter? If so, I guess I should do this first to see oif they are working. The car shows no codes.

 

[AApple]

If you mean first sensor and second as before cat and after cat, you cannot connect the two together. Well...you COULD, but the ECM would see the problem, and throw a code. The ECM is looking for a specific difference in the two sensors, before and after the cat. This is how it determines if the cat is working within its capacity.
You dint say what engine, but I'm assuming it's a 4 cylinder? A V6 O2 set-up would be different, as you would have a sensor for each bank of cylinders before the cat, then one after the cat.
An exhaust leak before the cat could certainly introduce excess, u-metered air into the cat, and cause the 2nd O2 sensor to "see" a lean exhaust, and then the ECM would enrichen the fuel to compensate.

 

[dan filipi]

I would listen to Aapple, he knows his stuff (or he's got me well fooled Lol)

 

[ekvh]

I know he knows his stuff because the voice in my head thought this was a cool idea. So apparently even the ecm says it can't be perfect.

Will the multimeter check them to see if they work?

 

[mcgovern61]

Hmmmm...not sure? I know that when I "Leaned" on my exhaust after the Cat got in my way, it fell off from rust and that was the second (02?) exhaust I had this happen with! (Is it fair to say that I sensed this 02 times?) :smilie_happy: :wave:

 

[slabghost]

I'd take it to a parts store. Many will check the ecm for codes for free. There might not be anything wrong at all. I know I've burned a lot of gas this winter just warming the car a little before going out the road. Cold temps also reduce tire pressure and thicken oil in the motor transmission and gear boxs.

 

[ekvh]

I expect a little drop, but 22mpg highway for a Rolla is poor. My Reatta did better than that. No codes showing right now. I'm leaning towards a leak, might even be exhaust manifold gasket. It's not loud, but it probably doesn't take much to put them off.

 

[slabghost]

How many extra miles did you put on the odometer spinning tires? Might indeed be a leak or just winter and maybe a dirty air filter. Or maybe old spark plugs. A lot can come in to play.

 

[ekvh]

It did have one wet plug a while ago. I'm glad you mentioned that. I will check it, but it was wet, as in full of water wet, around the plug. The only thing I can figure is someone decided to wash it, and the boot at the top was loose and allowed the water in. It has the deep plug in the center of the head with a long tube through the valve cover. This car sat for a year and a half. The plug was rusty on the outside, but the electrode looked normal. I will see if it is partially full of water. I don't think so because it doesn't have any trouble accelerating once warmed up, but it's worth a look. I am not going to throw parts at it ......yet.

I don't know of a good way of checking for an exhaust leak other than good ears, and mine are so shot, sometimes I can't even hear what I'm typing.

 

[slabghost]

Another thing that may be part of the issue is leaking or partially plugged injectors (fuel) .I'm not familiar with the rolla set up but some had a seperate start injector that was an (enrichening ) source for cold starts. Any injector issues will almost certainly impact mileage.

 

[ekvh]

I should do the other items first sometimes when I take off it bogs for a few seconds and then kicks in hard. I was thinking fuel filter/ pump but it's in the tank. I hope that's not it. I don't think there's an exterior filter to change. I have run several cycles of injector cleaner through it. Finished typing and did a search and there is an external filter right on the firewall. I think I'll start there.

 

[Dusterdude]

Change both sensors,they're cheap enough


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[ekvh]

Although fairly cheap, I am not changing them until I find out if it's from bad sensors or an exhaust leak, or other. They are due to be changed on regular maintenance anyways, but I would like to investigate the causes first. I'm a beginner--- an old beginner--- and I like to learn.

 

[Ansimp]

Dusterdude » Thu Mar 06, 2014 7:22 am[/url]":1oxsnhd1]
Change both sensors,they're cheap enough


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+1 as they do read poorly even if they don't fail totally with age..

 

[AApple]

It's hard to "test" O2 sensors without a scope. You can check resistance to see if they are shorted, or open, and you can check the resistance of the heaters for open/shorts, but checking the actual operation is tough with just a meter. About the best you can do is see if they are changing under operating conditions. When the air-fuel ratio becomes LEAN, the oxygen concentration in the exhaust increases and the oxygen sensor informs the ECM of the LEAN condition, which would be below .45 volts on a meter.
When the air-fuel ratio is RICHER than the stoichiometric air-fuel ratio the oxygen concentration in the exhaust gas in reduced and the oxygen sensor informs the ECM of the RICH condition, or above 0.45 V. With a volt meter, you may or may not be able to see them changing, as they should be changing hi(over.45v)/low(under .45v) rapidly. A scope is the best way to see if they are actually working, and if they are working properly. Properly working, the over/under voltage would be pretty equal, over time.
Confused yet?

 

[ekvh]

Not at all, my Reatta has a computer which displays the value of its one O2 sensor which is continually up and down from 0.07 to 0.85 and everywhere in between. I just priced the sensors, ouch around $150 for both. I can find them online for much less.

 

[joedrum]

sheesh what monsterous rip off

 

[Ansimp]

At that price it would pay to check other things first. Any chance the cat is blocked?

 

[AApple]

ekvh » Wed Mar 05, 2014 5:38 pm[/url]":23t0l236]
Not at all, my Reatta has a computer which displays the value of its one O2 sensor which is continually up and down from 0.07 to 0.85 and everywhere in between..

But...but...we're talking aboot the Toy.
Unless you have a scanner that gives you live data, how would you access the ECM O2 readings on the Toy? :headscratch:

 

[ekvh]

AApple, My "Not at all" comment was that I did get the explanation of how the sensors communicate with the ECM. Drove through another blizzard last night. More snow tonight. Is it ever going to warm up????