The Oxygen Sensor "Puzzler"

[nascar24jd]

The person that can answer these questions is clearly the smartest person on our fine board...

My 1993 3.4L Firebird was getting very poor fuel mileage, averaging 15mpg. I decided to change the O2 sensors about 1 month ago. Well, the results were even worse. Fuel mileage dropped to 12mpg.

#1. Why would the fuel mileage get worse with brand new O2 sensors?

Being completely out of ideas, I decided to disconnect the O2 sensors all together. The car runs fine, and the fuel mileage increased to 16mpg.

#2. Why would disconnecting the O2 sensors improve the fuel mileage?

Bonus Question - Can I do any harm to my engine by running it with the O2 sensors unplugged?


Thanks in advance for your help.

[tkoforpresident]

Re: The Oxygen Sensor "Puzzler"

wow, thats really odd. I don't know why you would get worse mpg with the new o2's, but perhaps you are getting better mpg's with them unplugged cuz the pcm runs off a set table for fuel delivery even w/out the o2's plugged in.

there might be something wrong with your pcm, thats all i can think of.

[silvachris1]

Re: The Oxygen Sensor "Puzzler"

Maybe the car isn't running right at all. With bad sensors/no sensors, it fuels based on the set table in the PCM...... I dunno much about OBDI and the older PCMs though. My guess is that once the car started getting good readings with the new 02 sensors, it's adding fuel because of some other problems with the car (the original cause of the bad mileage).

Seafoam/Injector cleaner, plugs, wires, fuel filter, air filter, pcv valve, clogged cat.

[nascar24jd]

Re: The Oxygen Sensor "Puzzler"

Maybe the car isn't running right at all. With bad sensors/no sensors, it fuels based on the set table in the PCM...... I dunno much about OBDI and the older PCMs though. My guess is that once the car started getting good readings with the new 02 sensors, it's adding fuel because of some other problems with the car (the original cause of the bad mileage).

Seafoam/Injector cleaner, plugs, wires, fuel filter, air filter, pcv valve, clogged cat.

I do appreciate any suggestions, so keep them coming.

Coils packs, wires, & plugs are all new.
All filters are new.
PCV is new.
No cat, so it is not clogged.

Car runs perfect, just rich. Does not burn any oil, plugs looked good when I pulled them out to change them. They just had normal wear.

If 15 to 16mpg is all I am going to get then so be it. I am thinking what Max said, that the car has a bad PCM.

Can any OBD1 experts please chime in and explain to me what the PCM is doing when the O2 sensors are unplugged. Is it now running in "open loop"?

[nascar24jd]

Re: The Oxygen Sensor "Puzzler"

wow, thats really odd. I don't know why you would get worse mpg with the new o2's, but perhaps you are getting better mpg's with them unplugged cuz the pcm runs off a set table for fuel delivery even w/out the o2's plugged in.

there might be something wrong with your pcm, thats all i can think of.

Thanks Max. Hey, tell me again, what is your MPG on your 3.4L?

Thanks...

[bigbrian442]

Re: The Oxygen Sensor "Puzzler"

The person that can answer these questions is clearly the smartest person on our fine board...

My 1993 3.4L Firebird was getting very poor fuel mileage, averaging 15mpg. I decided to change the O2 sensors about 1 month ago. Well, the results were even worse. Fuel mileage dropped to 12mpg.

#1. Why would the fuel mileage get worse with brand new O2 sensors?

Being completely out of ideas, I decided to disconnect the O2 sensors all together. The car runs fine, and the fuel mileage increased to 16mpg.

#2. Why would disconnecting the O2 sensors improve the fuel mileage?

Bonus Question - Can I do any harm to my engine by running it with the O2 sensors unplugged?


Thanks in advance for your help.

1 The new O2 could be bad. Have you used a scanner and checked the mV and cross counts it's reporting? As well as any other sensors that can go bad and not through a code. TPS,MAP. You may even have a leaking injector.

2 Apparently the O2 is seeing a lean condition and telling the PCM to add fuel.

3 Yes, if it runs too lean. By unplugging the O2, the PCM looks at a predetermined fuel map and doesn't adjust fuel trims.

[nascar24jd]

Re: The Oxygen Sensor "Puzzler"

1 The new O2 could be bad. Have you used a scanner and checked the mV and cross counts it's reporting? As well as any other sensors that can go bad and not through a code. TPS,MAP. You may even have a leaking injector.

2 Apparently the O2 is seeing a lean condition and telling the PCM to add fuel.

3 Yes, if it runs too lean. By unplugging the O2, the PCM looks at a predetermined fuel map and doesn't adjust fuel trims.

So I guess it is now running on the predetermined fuel map in open loop. It can't be running too lean with it getting 16 MPG, right??? I have not checked the mv or cross counts with a scanner. The TPS is relatively new, and the only code I throw is the EGR after a long highway run (the one that everyone with a 3.4L gets).

So if I keep getting 16mpg and I do not plug the sensors back in do you see any negative long term effects?

[nascar24jd]

Re: The Oxygen Sensor "Puzzler"

Here is what I am thinking...

For whatever reason the old O2 sensors were "dead" and the car was running in open loop getting 15-16mpg.

By changing the sensors the car went into closed loop but there is an issue in the PCM which is causing it to use too much fuel and resulting in 12mpg.

Now by unplugging the sensors I have put it back into open loop basically the way it was before I touched it, getting about 16mpg.

Does that make sense???

[bigbrian442]

Re: The Oxygen Sensor "Puzzler"

If melting pistons isn't a problem then it will be fine. There is a problem somewhere causing the bad MPG while running in closed loop. If one injector is not putting in enough fuel the PCM compensates adding fuel to all injectors. I would run some injector cleaner in the next few tanks just as a precaution and then try to plug the O2 back in.

[nascar24jd]

Re: The Oxygen Sensor "Puzzler"

Makes sense. I usually use Chevron Techron injector cleaner. Do you have any better product suggestions?

[bigbrian442]

Re: The Oxygen Sensor "Puzzler"

That should work fine. Just run it through several tanks.

[tkoforpresident]

Re: The Oxygen Sensor "Puzzler"

hmmm, leaking injector sounds like a good shot. It wouldn't throw a code either.

btw, my 3.4 gets like 17mpg and thats with new plugs/wires/coils/cai/exhaust/ you name it!

just the nature of the beast.

[ssms5411]

Re: The Oxygen Sensor "Puzzler"

I would try and fix your problem rather than unpluging sensors , but it is your engine. I'll look for my engine blew up thread in the future.

[nascar24jd]

Re: The Oxygen Sensor "Puzzler"

I would try and fix your problem rather than unpluging sensors , but it is your engine. I'll look for my engine blew up thread in the future.

What??? I have never, ever blown an engine in any of my cars, and I do not plan to start now. 19 years and counting...

First off, the sensors are now plugged back in. I would love to fix the problem, if I knew what it was.

With the sensors plugged in, I hooked the car up to an OBD1 scanner and it will not flash the SES light to give me the codes. According to the scanner manual this means that the PCM is stuck in open loop.

So, why is it stuck in open loop. I have owned this car for 2 years and it has always ran rich. I also tried to scan it in the past and it was stuck in open loop back then as well.

Bad PCM???

[bigbrian442]

Re: The Oxygen Sensor "Puzzler"

Did you check too see if the light bulb for the SES is good?