Oxygen Sensor Question.

[PAUL161]

I posted this on the tool forum by mistake. What happens if the oxygen sensor goes bad? What happens if it's disconnected? My pickup with a 350 in it is starting to use more fuel than normal. Someone told me to see if the oxygen sensor was bad. This truck is in top mechanical condition and always kept in tune. The sensor was replaced about a year ago. Any clues?

 

[DeltaAir423]

Is the truck a post '97? With the sensor connected and installed, are you getting a check engine light?

 

[PAUL161]

The truck is a 93 Chev. Choo Choo custom with a 350 engine and duel exhausts. I get no check engine light. Everything seems normal with the exception of increased fuel consumption.

 

[DrEntropy]

IIRC, sensor failure makes the "brainbox" send a full rich instruction to the injectors. There are other things which could contribute to the fat setting but the sensor would be the first thing to change if you can't pull codes from the ECU.

You should be able to have it checked for free at an AutoZone, Pep Boys or such. I'm not familiar with V-8 Chevy fuel management but I bet THOSE guys are. :wink:

Other thing may be a pressure regulator problem.

 

[AweMan]

Paul:
The Oxygen sensor tells the Computer in your vehicle if there is a rich or lean condition. The Computer then responds accordingly adding more or reducing the amount of fuel delivered to the T.B.I or Injectors or whatever the case may be. A bad oxy sensor can rob your engine of power and/or cause it to use excessive fuel. I would check the connections at the sensor, make sure they are secure and the concetivity is good.

Typicaly these sensors are designed to go for around 70,000 miles {some go a lot more} But that doesn`t mean yours hasn`t gone bad.

As far as Leads go there are four different sensors
a one lead, a two lead, a three lead, and a four lead,
One, two and Three lead sensors use the sensor body as earth
Two and three lead sensors have pre heaters and the extra leads are for that purpose { a two lead uses the body for preheat earth,One lead for the preheater and one for the actual sensor. A three lead sensor uses one wire for Preheater Voltage and one for earth, the third lead is the actual sensor lead.
The four lead sensor uses one lead for earth through the wireing harness, One for sensor voltage, One for preheater earth and one for preheater voltage.

There is a home test you can do but typicaly it is a hit and miss type thing at best.

Typicaly the home test goes something like this, connect your dcvm to the sensor lead {good luck sorting out which lead it actually is} and a suitable ground, start the engine, you should read 0.1-0.3v {until your engine comes up to op temp} once up to normal op temp accelerate the engine up to around 2,500 rpm, the reading should fluctuate between 0.5 and 0.8v {some say 0.9v} {<span style="text-decoration: line-through">Lower</span> Higher Voltage indicates a rich condition,}



You really need an O2 sensor Monitor to get an accurate accounting of whats going on with it. {it could be what is termed "Lazy" or "slow" or not working at all or it could be the preheater portion not functioning if so equiped.

The Oxy sensor isn`t the only sensor that can cause excessive fuel consumption there is a M.A.F. sensor {Mass Air Flow} Normaly located in the intake system and the T.P.S. {Throttle Position Sensor} Normaly located on the throttle body somewhere {near where the butterfly valve is}.
One or both of these sensors can be feeding the computer faulty readings also.

Another thing is to ensure there are NO VACUUM LEAKS, they too can cause faulty sensor reading and increased fuel consumption.

In My Opinion, your best bet is a diagnostics reading by a reputable shop before you go willy nilly replacing sensors that may or may not be bad. The diagnostic test will tell you excatly what and where the problem is if any.
I hope this gives you a basic understanding of how Oxygen sensors work.
F.W.I.W.
Kerry

 

[martx-5]

Typicaly the home test goes something like this, connect your dcvm to the sensor lead {good luck sorting out which lead it actually is} and a suitable ground, start the engine, you should read 0.1-0.3v {indicating a rich condition until your engine comes up to op temp} onece up to normal op temp accelerate the engine up to around 2,500 rpm, the reading should fluctuate between 0.5 and 0.8v { Lower Voltage indicates a rich condition,}

I believe you have that backwards. A higher voltage reading indicates a rich condition.

 

[AweMan]

My Bad! You are correct! Higher reading is richer!

For the sake of clarification Here is a link to some information on Oxygen sensors

 

[PAUL161]

Thanks guys for your input. I think the best thing for me to do is have a diagnostics check run on it. Hopefully that will pinpoint the problem.

 

[PAUL161]

Update! Had the truck checked and found out that one spark plug was firing off the side of the electrode and creating a flash on the side of the ceramic. Crazy, I've never heard of that. These plugs are only ten thousand miles old and are the ones GM recommends. Also it was recommended to step up the heat range by one number. Seems the exhaust was running a little colder than normal, meaning the wrong signal was being sent to the computer. After checking the old plugs, the one that was messing up has a metallic burn mark about a 1/16th of an inch wide down the side of the ceramic along side of the electrode.

 

[mailbox]

Could have cracked it when it was put in. I would also replace the rotor cap and plug wires. Make sure the distributor cap doesn't have any burn thru either. Just to be safe.

 

[PAUL161]

Could have cracked it when it was put in. I would also replace the rotor cap and plug wires. Make sure the distributor cap doesn't have any burn thru either. Just to be safe.

That's the crazy part of it all, the plug is not cracked! When I insert a plug, it's stuck in the end of a rubber hose and the threads are lubed with anti seize. I am careful not to bump the plugs so as not to change the gap setting. The plug wires, rotor and cap are new, two months old. All premium grade.
This truck, one that I bought new in 93, is meticulously maintained. The engine bay still looks like new. That's how my vehicles are maintained. I don't feel so bad about the situation, as it has some GM experts baffled. An expert race engine mechanic I know said he only saw a plug do this one time. It was a new iridium plug and had a leak around the ceramic under high compression. Not bad for a 16 dollar plug. (Oh, it was replaced for free)! Even though they did loose the race.