Fuel trim question

[hemi steve]

I got a PPI done on a car I'm looking at out of state so I wasn't able to be there. One issue showed up. The long term fuel trim numbers are high. (1-5) -7% and (6 -10) -14%. Are these numbers way out of wack? Are there some common maintenance issues that can drive those numbers? The car is an '01 GTS with 19 k on the clock and no maintenance records. The inspector also reported a "rough idle". Does the stock '02 cam produce a rough idle? I wasn't there so I don't know weather it was rough compared to a Toyota or unusually rough for a viper. Anyway, there were no misfire codes reported either.

Steve

[My98RT10]

I once recorded the fuel trims during idle and created the graph below. As can be seen, also here a difference between both banks although smaller numbers.... But I am not an expert on fuel trims so I cannot say what is still normal and what not.... I am sure someone wiht more knowldege will chime in !?

[uvbnbit]

I am the least of knowledgeable on this topic, but, if they are out of whack, depending age of upstream O2's, I'd think replacing those and a PCM reset ought to resolve any issue/concern one might have....

Next...

[dave6666]

I am the least of knowledgeable on this topic, but, if they are out of whack, depending age of upstream O2's, I'd think replacing those and a PCM reset ought to resolve any issue/concern one might have....

Next...

Idle should not be rough for an '01. Those trim numbers are not whacked albeit also not "perfect." I would also recommend new primary O2 sensors. Highly recommend doing genuine OEM NTK brand not junky aftermarket Walker or Bosch like many of the cheap asses do around here. I mean, the sensors are the thing tuning your car, and when people replace them, their tune is usually whacked.

Other maintenance items would be plugs and wires. Use the Gen 3 plugs and stay away from aftermarket wires claiming to be quality. They are overpriced junk. Use the Mopar wires.

The fuel could also be old.

[Bill W]

Idle should not be rough for an '01. Those trim numbers are not whacked albeit also not "perfect." I would also recommend new primary O2 sensors. Highly recommend doing genuine OEM NTK brand not junky aftermarket Walker or Bosch like many of the cheap asses do around here. I mean, the sensors are the thing tuning your car, and when people replace them, their tune is usually whacked.

Other maintenance items would be plugs and wires. Use the Gen 3 plugs and stay away from aftermarket wires claiming to be quality. They are overpriced junk. Use the Mopar wires.

The fuel could also be old.

Just put Gen 3 plugs and Mopar wires in my Viper. Expected little to maybe fairly noticeable throttle response. Could not believe the difference it made. Huge! Seat of the pants feels like double the power. Extremely happy with the outcome.

[ellowviper]

Normal trims for today's ethanol blend. Remember fuel trims are calibrated by OEM using non-ethanol fuel to ensure consistency across SAE guidelines...all vehicles, all manufacturers. "This fuel contains up to 10% ethanol blend" will cause your trims to be off (as the PCM modifies fuel to achieve expected O2 sensor AFR readings). If you really want to experiment, try a tank or two of non-ethanol and report back.

[99RT10]

Normal trims for today's ethanol blend. Remember fuel trims are calibrated by OEM using non-ethanol fuel to ensure consistency across SAE guidelines...all vehicles, all manufacturers. "This fuel contains up to 10% ethanol blend" will cause your trims to be off (as the PCM modifies fuel to achieve expected O2 sensor AFR readings). If you really want to experiment, try a tank or two of non-ethanol and report back.

After doing New plugs(Champ 438s) and wires.

[hemi steve]

I'm not a subject matter expert here but it seems to me that the ethanol would make the engine want to run a little lean. That would drive the fuel trim value in the positive direction. This car shows negative trim values that would be dealing with a rich mixture.

Steve

[Steve M]

As long as the fuel trims are within the bounds of what the PCM is capable of add/subtracting, they're fine. Yes, you'd want them closer to zero, but being negative isn't a bad thing, at least from a longevity standpoint. It just means the car would otherwise be running rich. When you got WOT, the PCM doesn't reference trims since it goes open loop...if the trims are negative going into a WOT condition, it won't pull any additional fueling at WOT. The only thing that controls the fuel trims are the primary O2 sensors as dave6666 already pointed out, so trying new ones would be my first go-to if you really suspect a problem.

You would ideally look at both the LTFTs and STFTs throughout an entire drive, and see how they all average out. Just looking at them at idle won't really tell you much of anything. Winter vs. summer blend gas, conditions on that day, etc. can all affect your trims. That's why they exist...too many variables across the world to calibrate a car to run perfectly in every corner of this planet along with everywhere in between.

[Dan Cragin]

Normal, they often differ from left to right bank.

[hemi steve]

I would have thought that the numbers would be lower and more matched. If the issue is the 02 sensors then I guess I can understand the differences and the deviation from zero. Overall, it doesn't seem like this is a show stopper.
Steve