LTFT question

[wardssix]

Hi, questioning what to do first.

 

Spit a 420 code the other day on the highway about 20 miles into a trip - no driving abnormallities other than being the dohc but when I pulled the freeze frame data it was normal except long term fuel trim was high I think:

 

Fuel system = S1 closed loop

CTS = 197.6 deg

Fuel trim B1 LT = 12.5% <====<

Calc. load = 16.9%

Fuel trim B1 ST = 0.8%

Intake man. pressure = 12.11 inHg

RPM = 2875

Veh. speed = 63.4 mph

gotta love that low 5th gear!

 

I reset the code to see if it comes back and was thinking I'll start with the rear O2 sensor first if it does - what do you guys think?

98 obw orig 2.5MT DOHC POS with 119k

I don't think it's the front O2 but what do I know?

Any input would be greatly appreciated.

MPGs have not changed to my knowledge other that winter driving stuff.

thanks

bob

[OB99W]

First, let me say that I'm impressed with posting of the freeze frame data (which doesn't happen nearly frequently enough in my opinion). However, two points off for not giving us some basic maintenance history, if you happen to know it.

 

Ideally LTFT would be 0.0%, but even though 12.5% isn't terrible, in the real world under 5% would be more acceptable. Since the trim is in the positive direction (assuming a minus sign didn't get lost), it indicates that the ECU ''thinks'' that the mixture is averaging lean, and therefore the injectors are being given a somewhat widened pulse to compensate.

 

If the ECU is ''wrong'' about the A/F ratio (because it's being fed bad sensor data), it might be enriching it unnecessarily, and that could trigger the P0420. If that's what's going on, and you're inclined to change an O2 sensor, the front one is a more likely culprit than the rear. Since you have a scan tool, monitoring the O2's voltage swings might provide a clue.

 

Of course, air metering and other factors can affect the mixture, and it's possible that the catalytic converter is actually bad. A pretty good article that might help with understanding fuel trim is at http://www.motor.com/magazine/pdfs/092006_11.pdf . Ones that cover causes of the P0420 code are http://www.motor.com/magazine/pdfs/102006_09.pdf and http://www.catalyticconverter.org/news/news_page.cfm?Key=catalytic_converter-&News=120 .

[wardssix]

I'll watch the O2 voltages from startup to closed loop and see what happens. The car was new to me 1.5 years ago and exhaustwise just a new muffler by me. I thought the rear first but I'll see - may "back it out of the stream" to see if it acts differently. I thought the front sensor would screw up the Short term?

thanks for the quick reply

bob

[OB99W]

[...] I thought the front sensor would screw up the Short term?

Not necessarily. ''Short'' and ''long'' are relative terms. It doesn't take much time for the ECU to sense a trend in the STFT and transfer it to the LTFT. That allows the STFT to not run out of range, and if compensation is nearly sufficient due to LTFT, then STFT won't be much off of 0.0%.