MAF over reporting by 4-5%

[icetraxx]

After having my tuner look at a log file from my 08 5.3 because of the p0420 code, he says that my MAF is over reporting by 4-5%. What could I check to see why this is happening? i'm running stock air box/filter, straight intake tube, dynatech headers with their cats. Would this cause me to run a little rich?

I haven't heard back from my tuner in a couple of days which is the reason I am posting here. I'm still in the learning mode.
thanks,
Larry

[swingtan]

If the MAF was "over reading" then during CL mode, the O2 trims would pull the mixture back to stoich. This would also cause the long term fuel trims (LTFTs ) to learn the negative correction and would also pull our some mixture during OL mode. The overall effect would be to make the normal running mixture correct. It's not uncommon for the MAF to need some calibration, and 5% out is not a big amount when you are still running CL corrections. In fact, running 5% rich is better than running 5% lean

A P0420 is more likely to be triggered due to relocating the O2 sensors further away from the engine, or if the CAT was not working as well as the original stock unit. For example, if the new CAT's are 100CPI units, they probably flow better than the stock ones, but don't have as big an effect on the exhaust gases. You can see how well the CAT is working by logging the NB O2 voltages for pre and post CAT sensors. It has very little to do with the MAF readings at all. The easiest fix is to turn off the DTC all together as lond as you know everything is working as expected.

Simon

[joecar]

P0420/P0430:

After the correct conditions are met, the PCM manipulates the commanded EQR and looks at the response of the rear O2S wrt the response of the front OS2...

the PCM measures the time delay, if it is too short then the PCM sets the DTC.

Look at this chart... look at EQR transition and at HO2S11/H02S12 responses... the marker is where the PCM set P0420 and illuminated MIL.

[joecar]

In that chart you see:

First, PCM drives commanded fuel lean, HO2S12 responds in a short time, but PCM ignores this because it doesn't have a reference point from HO2S11;

Then, PCM drives commanded fuel rich, HO2S11 responds immediately, HO2S12 responds a short time later.

The PCM times the HO2S12 transition wrt the HO2S11 transition, and if it's shorter than the minimum it sets DTC P0420.

If the rear O2 is slow the test will always pass, if the front O2 is slow, the test may fail, but in both cases you will have DTC's for insufficient O2S activity.

[gmh308]

What is the approx lagtime required before the DTC pops Joe?

[joecar]

Ian,

I measured 1.5-1.9s between front and rear O2 transitions in that log, and then the DTC pops about 0.5s or less later.

The TIS Mode $06 document (2002file6.pdf) for 2002 LS1/LS6/LQ4/L31/etc. says:

OSC Time Difference = OSC Worst Pass Thresh - OSC Compensation Factor * (Post Cat O2 Resp Time - Pre Cat O2 Resp Time)

It says OSC Worst Pass Thresh is:
bank1: 3.1s
bank2: 2.1s

It doesn't say what OSC Compensation Factor is.

Reading Mode $06 data for 0C60 and 0C70 (for 12212156) I see that the OSC Time Difference thresholds are:
bank1: 0.407s
bank2: 0.302s

If OSC Time Difference goes above those thresholds the test fails.

A brand new OEM cat produces OSC Time Difference below -5s so it passes by a large margin.

OSC Time Difference looks like it is the amount of time that the response time delta (multiplied by the factor) exceeds the worst pass threshold.

[icetraxx]

Still have not heard back from my tuner. What I am seeing is while cruising or idle the LFT's can be different by 1-4%. would this be a sign of a bad o2?

[mr.prick]

LTFTs move around, it's normal.
There will also be a difference between both banks, they rarely are the same.
Bank-1 = 0% & Bank-2 = 1.6%