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bad mpg & black soot on ground

11K views 41 replies 11 participants last post by  3X4X4 
#1 ·
I just bought a 1997 2.5 wrangler. it's getting horrible mileage. filling up after a 100 miles. I did a full tune up and changed the coolant temp sensor and map sensor today and still no good. it's throwing black soot and condensation out the tailpipe on the ground. i looked under the jeep to see the cat is gone and no downstream O2 sensor. I'm not wanting to just keep putting parts on it just to get no result. it's not smoking going down the road. when I rev it no soot or condensation until I left off the gas it will then come out. autozone was giving O2 sensor codes. I need help please
 

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#16 ·
The downstream 02 sensor on early TJs with just two 02 sensors has no effect on the fuel mixture. It only verifies that the cat is functioning properly.
From what the guys are saying, the before cat O2 controls the mixture and the after one just verifies the cat is working, so no need to change the after sensor as long as I don't have a CEL?
Correct, at least on early TJs which only have two 02 sensors..
This may not be entirely true., I believe it's primary function is to monitor the CAT though In the description and operation section of service manuals all the way back to 1996 it states that ...

"The downstream heated oxygen sensor input is used to detect catalytic convertor deterioration and provide fuel adjustment information.

As the convertor deteriorates, the input from the downstream sensor begins to match the upstream sensor input except for a slight time delay By comparing the downstream heated oxygen sensor input to the input from the upstream sensor, the PCM calculates catalytic convertor efficiency

When the catalytic converter efficiency drops below emission standards, the PCM stores a diagnostic trouble code and illuminates the Malfunction Indicator (MIL) (or Check Engine) lamp. For more information, refer Emission Control Systems.

The downstream sensor input is also used to adjust the upstream O2S goal voltage. This allows a better control of the exhaust gases as the catalytic convertor ages."


The section which states that the downstream sensor is used to adjust the upstream sensors goal voltage suggests that there is at least some influence to the air/fuel ratio coming from the downstream sensor, but how much influence I am not sure of.

For some time I've wanted to isolate the signal wire from the downstream sensor and ground it and/or feed it 1 volt and monitor the upstream sensor and fuel trim data to see what effect it has. If I ever do I'll be sure to share my findings.
 
#12 ·
So no more black soot? I too have a '97 SE, it does it a little. I guess I'll change mine. How far can you get on a tank now? From what the guys are saying, the before cat O2 controls the mixture and the after one just verifies the cat is working, so no need to change the after sensor as long as I don't have a CEL?
 
#19 ·
Well if a catalytic converter became less active and given that the downstream sensor is just a heated narrow band oxygen sensor, then a tired converter oxides less resulting in sensor seeing more oxygen. So what be be a useful modulation of fuel trim for a tired cat
Enrich mixture to drop downstream oxygen that would be bad for net emissions
Lean mixture more to account for less post combustion oxygenation of exhaust that over lean would decrease power and be bad for the engine

Just cannot see any practical use of post cat narrow band sensor info on fuel trim

Plus we know the FSM lies or at least grossly misrepresents emissions facts like where it says it does not go closed loop till temp sensor shows engine has warmed up

A test of a good cat and o2 sensor vs a simulator would be interesting but substituting 0 or high sensor output voltage would trip error codes and may show more how PCM reacts to error code than how it reacts to output from a downstream sensor that it thinks is functioning

I seriously doubt the trim is dependent on downstream reading unless and until it gets so far out that an error code is set but welcome additional test results
 
#20 ·
Well if a catalytic converter became less active and given that the downstream sensor is just a heated narrow band oxygen sensor, then a tired converter oxides less resulting in sensor seeing more oxygen. So what be be a useful modulation of fuel trim for a tired cat
Enrich mixture to drop downstream oxygen that would be bad for net emissions
Lean mixture more to account for less post combustion oxygenation of exhaust that over lean would decrease power and be bad for the engine

Just cannot see any practical use of post cat narrow band sensor info on fuel trim

Plus we know the FSM lies or at least grossly misrepresents emissions facts like where it says it does not go closed loop till temp sensor shows engine has warmed up

A test of a good cat and o2 sensor vs a simulator would be interesting but substituting 0 or high sensor output voltage would trip error codes and may show more how PCM reacts to error code than how it reacts to output from a downstream sensor that it thinks is functioning

I seriously doubt the trim is dependent on downstream reading unless and until it gets so far out that an error code is set but welcome additional test results
Different manufacturers setup engine management in different ways so I am not certain that early OBD II Jeeps use the downstream O2 for fuel trim adjustments, the FSM suggest they do but how much I don't know. That said I am positive that Toyota as well as some other manufacturers do, which is why I am interested to find out if Jeep does as well.

I have diagnosed on a number of occasions P0171 (lean) codes set but the engine was actually running very rich and blowing black smoke. The fault being a bad downstream O2 stuck at .1 volts or less. The ECM sees the downstream O2 at .1 volts and thinks the engine is running lean so it adds fuel but the O2 voltage doesn't move so more fuel is added, trims keep raising positive, and the code sets. The engine was running with a stoich A/F but the ECM was tricked from the stuck O2 and is now running very rich with a lean code. All that from a downstream O2 sensor.

I don't know exactly why the engineers depend on the downstream O2 sensors for the A/F mixture in some vehicles, I just know that they do. I have not seen this behavior in a Jeep, but I have also not seen a stuck downstream O2 sensor in a Jeep that I can recall.
 
#23 ·
That is 12 volts which is way outside of the normal sensor max so once that happens you are way outside of range of what might happen with just a tired cat or o2 sensor so doubt very seriously that would be in program to do running adjustments as referenced to in FSM

Like I said before an min 80s fuel injection system on a vehicle I observed would shut down as soon as it went closed loop with a shorted O2 sensor getting 12 volts as the high voltage made ecu basically shut of injection to try to try to increase exhaust o2 and thereby decrease sensor voltage output

Would stay running fine if you unplugged cylinder head them sensor and kept it in open loop

But that system only had one o2 sensor guess we would call it upstream
 
#25 ·
That is 12 volts which is way outside of the normal sensor max so once that happens you are way outside of range of what might happen with just a tired cat or o2 sensor so doubt very seriously that would be in program to do running adjustments as referenced to in FSM
I agree and in fact there are specific codes that state 'O2 signal short to power/ground'. That said, in theory it is possible that there is just the right amount of resistance in a short that the full 12 volts does not bleed through and you end up with a stuck signal voltage that is within the 'normal' range of operation.
 
#34 ·
I have better luck getting REALLY stuck 02 sensors out with an open-end 7/8" wrench. That type of wrench puts the force down low right next to the bung the sensor threads into. The only times I have luck using my special 02 sensor socket wrench that has the wiring slot cut into it is when they're not stuck. I consider that tool more of an installation tool than one to try to get a stuck sensor out with.

I've had some 02 sensors so stuck over the years that I had to weld an extension onto the 7/8" box-end wrench to gain enough leverage to get a couple of them out.
 
#39 ·
So, I got my sensor from Rockauto already, they ship super fast! I didn't have a wrench to fit it, and it was raining and I was feeling lazy, so I stopped by a local muffler shop that charged me $25 to change it. I watched him do it and it appeared to come out really easy. When I got back home I reset the ECM using my scanner, don't know if I needed to or not, but when I filled up I calculated mileage using the miles driven/gallons method which showed 16 mpg. I know that might not be accurate because of my tire size and whatnot, it I'm going to do it the same way for this tank and compare the results. Thanks guys for all the help and advice.
 
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