My fuel milage woes, faulty o2 sensor?

JDMMA70

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jetjock;1588112 said:
I just don't agree with throwing parts at a problem.

O2 sensors are the most needlessly changed parts on cars. Pretty sad when one considers how easily tested they are, at least for use on OBDI vehicles. That said, without going into detail about what the TCCS does when open loop, a dead sensor should not cause fuel economy to suffer to that extent unless there are other problems. I say should because it's possible but unlikely.

You can also check the sensor off the car using a propane torch. Look in my old SME section for the procedure. Kind of moot now though because you went and bought another. Since troubleshooting this symptom should have started by checking the loop anyway you're back to square one. You simply arrived there the most expensive way. Try it, let us know, and we'll go from there.

Keep in mind *accurate* fuel economy can be difficult to ascertain. Everything I'm telling you is predicated on your claim of 16 mpg. You didn't say how you arrived at that or even what your typical mileage is. It matters because say, if your mileage is higher the problem could be the O2 sensor alone. The point is we're trusting the figure you supplied to be accurate. If it's not we're pissin' in the wind...

I got the figure from filling up the tank and recording the milage until the tank was empty. I emptied the tank in one day doing mostly highway driving. I did the o2 sensor test a few minutes ago. Test came out bad. According to the TSRM it says to replace the o2 Sensor. While i was doing the test maintaining engine speed at 2500 white smoke poured out the exhaust, smelt it, it was fuel, like it was running slightly rich, almost as if the timing was off. I checked the timing as per TSRM and jumping the two terminals. Its still at 10' BTDC.

gennro;1588262 said:
http://www.cygnusx1.net/Supra/Library/TSRM/MK3/manual.aspx?S=FI&P=20
http://www.cygnusx1.net/Supra/Library/TSRM/MK3/manual.aspx?S=EM&P=6

That should you get started with figuring out your issue. As you can tell there is a lot of things that could cause this issue. Leaking cold start injector, base timing being off, etc

CSI i replaced december of 08 along with a new gasket from Toyota everything was torqued to spec.
 

jetjock

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Jul 11, 2005
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Well, it's going to run rich (.9 lambda) with a bad O2 sensor but not rich enough to cause that mileage loss and certainly not enough to smoke assuming everything else is working. It appears you have other problems and as was pointed out a lot of things can cause them.

And although you didn't say in what manner the sensor was "bad" testing it per the TSRM is only valid when there aren't other problems. When there are the sensor can still be good but fail the TSRM test. Therefore to really test it you have to use other techniques or test it off the car. It's not called "closed loop" for nothing you know...
 

JDMMA70

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jetjock;1588401 said:
Well, it's going to run rich (.9 lambda) with a bad O2 sensor but not rich enough to cause that mileage loss and certainly not enough to smoke assuming everything else is working. It appears you have other problems and as was pointed out a lot of things can cause them.

And although you didn't say in what manner the sensor was "bad" testing it per the TSRM is only valid when there aren't other problems. When there are the sensor can still be good but fail the TSRM test. Therefore to really test it you have to use other techniques or test it off the car. It's not called "closed loop" for nothing you know...

Well the CEL flashed Code 12 after the smoke. After i cleared the code and did it again no more smoke, and the code didnt reappear it only did it when i performed the test which was odd to me. I did the test with the multimeter and probing the diagnostic box, since i couldnt remove the sensor without cutting it. Now after thinking about it do remember working on the car during my head swap in June of 08. I had Code 21 after starting it up with the new head. I was going to buy a new o2 sensor but my father looked and it and decided that it was the plug not making a solid connection since the clips were broken (But the plugs were together and not loose). He cut off the plug ends and saudered the wires together. The code went away after that. The only other things i can think of that i havent checked is the egr. However there is a code for it 71 i believe? Wouldnt it throw a code if the EGR was indeed malfunctioning which leads me to believe its something else, but i just dont know what.

Ive already checked the CSI and time switch.
Fuel injectors if they were leaking. They werent
Spark plugs are new and gapped correctly.

What i havent done but is on my list is a compression test.
and reshim the valves (they are on the loose side of the specs currently)
 

jetjock

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The sensor may indeed be bad. I'm just trying to teach you something you may not know.

Lets say the engine is in fuel control with the sensor cross counting like it's supposed to. Then lets suppose something happens to enrichen the mixture enough to drive the sensor hard over. To use gennro's example lets suppose the CSI springs a leak. You come along, do the TSRM test, and find the sensor hard over in the rich direction (800+ mv) and not cross counting.

Is the sensor dead? Or it simply doing it's job reporting a mixture so rich the system can't compensate? Chicken or egg? There's no way of knowing without employing other techniques or opening the loop. The same goes for a full lean condition...
 

JDMMA70

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jetjock;1588404 said:
The sensor may indeed be bad. I'm just trying to teach you something you may not know.

Lets say the engine is in fuel control with the sensor cross counting like it's supposed to. Then lets suppose something happens to enrichen the mixture enough to drive the sensor hard over. To use gennro's example lets suppose the CSI springs a leak. You come along, do the TSRM test, and find the sensor hard over in the rich direction (800+ mv) and not cross counting.

Is the sensor dead? Or it simply doing it's job reporting a mixture so rich the system can't compensate? Chicken or egg? There's no way of knowing without employing other techniques or opening the loop. The same goes for a full lean condition...
Ah alright that makes sense, i get what youre saying.

Ok then assuming the new O2 sensor changes nothing because theres no real way i can employ other methods of testing the current O2 sensor. What are some other things i should look into that i havent already thought of?
 

jetjock

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Sure there are ways of testing it. Try forcing it the opposite way you found it. For example if you found it full rich make the engine lean by pulling the brake booster line off. If it's lean shoot some carb cleaner into the booster line or feed some propane in there. Any HC based product will do.

Or try quickly snapping the throttle wide open then closed. If you can get the sensor to fully swing both ways (> 800 mv when rich and < 200 mv when lean) using any of these methods it's working. If you can't, take if off the car and use a lit propane torch on it. If it still won't swing it's done for.

Or just put the new one in and hope for the best...
 

JDMMA70

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Ok something else caught my eye today. The Water Temp guage wasnt in the middle like i would think it would normally be. (Now it will go to the middle if i sit in traffic long enough)

sm_photo_missing.jpg


Does that look right? This is after 30min of highway driving. If not would it be the thermostat, or the temp sensor and if so which one. Both the temp sensor are only 3 years old though. I was leaning more toward the thermostat if this is an issue. Other things to note the car has a new waterpump, and OEM fan clutch. Coolant was also flushed 2 months ago and is now running G-05.

However even if this is an issue i still dont think its enough to cause that poor of fuel economy.
 

JDMMA70

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Ok so i put the new o2 sensor on today and tested the resistance of the old one now that its been cut off. 3.7ohms is what i got the spec calls for 3.0-3.6, also my other observation is it sounds like somethings rattling around the inside of the old o2 sensor. Sounds like tiny rock rolling around the inside. Is that the ceramic? Yeah something is sticking out of one of the tiny holes on the sensor end.
 

jetjock

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Are you *still* messing around with this? :)

Sounds like you're measuring the heater and that will change with temperature. You can't measure the resistance of the Nernst Cell. In fact it can be a bad thing to do. Sounds like it's busted though...
 

JDMMA70

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Well yeah since i wanted to see if it was indeed bad, i wanted to do that oil heat test shown in the tsrm since i dont have a propane torch. I want to see how the mileage will be now, filling up tommorow and im going to document the miles driven.
 
Oct 11, 2005
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Is it cross-counting now?

One way to get bad mileage is to have a bad thermostat that keeps the car in warmup mode permanently.

With a good working thermostat the car should show a normal temp on the gauge within a mile of city driving from a (summer time) cold start.
 

JDMMA70

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Havent yet had a chance to check it yet, as its raining outside. Will as soon as the sun comes out

well i need to replace the thermostat o ring gasket anyways so this is an excuse to put in a Toyota t-stat and throw the duralast one away.
 

JDMMA70

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jetjock;1589723 said:
3p is right. You need to start checking this stuff it you want to get anywhere.

Btw who is that in your avatar? Lol, not a huge fan of anime but she's starting to make my diaper damp...

Lol! It's Makina from Shikabane Hime, and yeah i know the t-stat slipped my mind i totally forgot about it.
 

Nick M

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JDMMA70;1589740 said:
Lol! It's Makina from Shikabane Hime, and yeah i know the t-stat slipped my mind i totally forgot about it.

Your guage reading would probably not be where it is if the thermostat wasn't open. Yours is hardly outside the realm of normal. Also, the ECU and guage are different sensors. But, always check, you never know.
 

JDMMA70

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Picked up a new Stant Superstat 195'F t-stat going to install that this weekend.

Ok so i put roughly 5 gallons of gas in the car today and avg. 20-21mpg highway (avg 65mph). 18mpg with the a/c on. So its improved quite a bit. Im wondering if this is as good as its going to get.