Tuning and O2 Sensors

[Black*Death]

I am having a low end stumble from 1000 RPM to 2000 RPM. I mechanic guy(who didn't tune it) states he thinks it's a problem with the fuel trims?

I data logged my O2 sensors and the fronts were always reading the same values.

HOWEVER. the 2 back ones were different. I had no reading on one sensor and a choppy reading on the other. When the stumble occurred during acceleration, the back O2 sensor that was reading would bounce all aroung.

My mechanic guy states the back sensors don't figure very much into the tune.

Finally my question(s)

Is it normal for one O2 sensor to be off and one on?

Is the starnge reading a possible cause of the stumble or a result of the stumble?

Any insight would be appreciated.



Lastly changed TPS, IAC, New plugs and fuel filter.

The stumble is less in cold weather and worse with hot weather???

 

[Janstang]

Shut the rear 02's off. Log your short-term and long-term fuel trims and see what they are doing. Let us know what they show.

 

[Black*Death]

Shut the rear 02's off. Log your short-term and long-term fuel trims and see what they are doing. Let us know what they show.

I will give it a shot. I will datalog and send to my tuner. Can we turn off the o2 sensors?

 

[Janstang]

Yep. Turn off only the rear 02's.

 

[Janstang]

I see you are in California. I'm not sure how your emissions people will be with you turning off the rear 02's permanently. Try shutting them off to see if it helps you with your issue. I'm pretty sure it won't. Sounds like an issue in the tune.

 

[01yellercobra]

They won't know if you turned them off. I went to a referee station with the rear O2's off and the flag for the P1000 code turned off. They never said a word.

That being said....when you say the front O2's are the same, do you mean they have the same value, but are still moving around? Or do they level out at something like 1.25 and stay there? The rear O2's are there for making sure the cats are working. They have nothing to do with the fuel trims. If there isn't enough of a difference in the readings between the front and rear O2's the computer thinks the cats are bad. That's why some high flow cats set off the CEL.

 

[Black*Death]

They won't know if you turned them off. I went to a referee station with the rear O2's off and the flag for the P1000 code turned off. They never said a word.

That being said....when you say the front O2's are the same, do you mean they have the same value, but are still moving around? Or do they level out at something like 1.25 and stay there? The rear O2's are there for making sure the cats are working. They have nothing to do with the fuel trims. If there isn't enough of a difference in the readings between the front and rear O2's the computer thinks the cats are bad. That's why some high flow cats set off the CEL.

Sorry don't remember values but fronts were basically dead on with each other.

One rear had no reading and the other had a reading different down low RPM's(Under 2K) then the fronts.

I would feel the stumble and see the rear O2 jumping around in values? Again I am unsure if the rear o2 sensor is a cause or symptom.

May be low end fuel trims? The car seems to be perfectly fine above 2K without any stumble at all.

I am leaning toward a crappy low end tune...Basically no tuning under 2 K?

 

[01yellercobra]

Sorry don't remember values but fronts were basically dead on with each other.

One rear had no reading and the other had a reading different down low RPM's(Under 2K) then the fronts.

I would feel the stumble and see the rear O2 jumping around in values? Again I am unsure if the rear o2 sensor is a cause or symptom.

May be low end fuel trims? The car seems to be perfectly fine above 2K without any stumble at all.

I am leaning toward a crappy low end tune...Basically no tuning under 2 K?

If it hasn't been tuned very well under 2K then that's your problem. If the MAF transfer isn't right then the car won't drive very well. First I would turn off the rear O2's just to get them out of the equation. Then concentrate on tuning the low end. Make sure you have the adaptive turned off while you do this. Then you just have to watch the STFT's.

 

[FerstlerSlim]

How do you turn off the rear 02 sensors?

Dave

 

[Janstang]

Do you have the Pro Racer Package?

 

[Black*Death]

Do you have the Pro Racer Package?

I do not have the pro racer Package(if you were talking to me?).

I had local tuner create my 91 octane tune...of course it is locked except

some end-user features...gear ratio, top speed, spark..etc.

 

[ZeroSVT]

How are you logging your data? If you are using LiveLink, post a datalog.

 

[Black*Death]

How are you logging your data? If you are using LiveLink, post a datalog.

I do have a data log. unsure how to post it though?

 

[ZeroSVT]

Upload it to medafire then post the link. Then we can have a peak and offer some suggestions

 

[Black*Death]

Upload it to medafire then post the link. Then we can have a peak and offer some suggestions

Will do that tonight after work

 

[01yellercobra]

How do you turn off the rear 02 sensors?

Dave

It has to be turned off in the tune. If you have PRP you can do it. Otherwise you need to talk to your tuner.

 

[Black*Death]

Copied the files for anyone interested. In "99 Cobra Datalogs" at Media Fire. 4 total datalogs.

Click link below...let me know what you find

Free File Hosting Made Simple - MediaFire

 

[ZeroSVT]

Do another Datalog

Log the following in that are in CAPS;

STFT 1&2
LTFT
MAF COUNTS
FUEL DUTY CYCLE
SPARK ADVANCE
SPARK SOURCE
RPM
TPS ABSOLUTE
IAC DUTY CYCLE
LOAD
Fuel pressure drop

Please do one at idle for 5 Min. Then one at a slow acceleration in 2nd gear to 4k. Do not go past half peddle.

I'm already suspecting a vacuum leak but i need to see the other values to give you a good guess.

What kind of fuel system do you have? What is your PCM code or Strategy (Prefered). Do you have the PRP or a tune? Do you have a wideband? What kind of MAF? Blow thru or Draw?

When did this start? Just out of the blue? How long did you drive it and it was ok?

-Matt

 

[Black*Death]

Do another Datalog

Log the following in that are in CAPS;

STFT 1&2
LTFT
MAF COUNTS
FUEL DUTY CYCLE
SPARK ADVANCE
SPARK SOURCE
RPM
TPS ABSOLUTE
IAC DUTY CYCLE
LOAD
Fuel pressure drop

Please do one at idle for 5 Min. Then one at a slow acceleration in 2nd gear to 4k. Do not go past half peddle.

I'm already suspecting a vacuum leak but i need to see the other values to give you a good guess.

What kind of fuel system do you have? What is your PCM code or Strategy (Prefered). Do you have the PRP or a tune? Do you have a wideband? What kind of MAF? Blow thru or Draw?

When did this start? Just out of the blue? How long did you drive it and it was ok?

-Matt

Fuel-42 pound injectors, walboro FP, KB BAP

Tune from JBA in San Diego

Blow through

Wideband? Not sure

MAF-90mm? with a Diablo Sport

PCM? Unsure where to get that from? Data log?

This has been going on for a while. Worsened with JBA tune so I have been leaning that way

I have changed spark plugs, new TPS, newer IAC. Almost feels like a "MISS" down low but COP all check out electrically

 

[Black*Death]

New datalogs per ZeroSVT's request at link below

Logs are there..Not the ones in the file..those are older ones