Advanced help with fuel trims

[Blue02WS6]

I have LT's on the car and I have the maf dialed in pretty close using a aem WB, now with the NB's and back to CL (cl enable set at 169 to get the NB's hot) and the LTFT's off and I know to re-set the fuel trims before every new scan, I have around 15 scans and most of the scans the nb's read good, avg. within 0-10 either way...close enough I think, but every now and then they start out of wack, one scan started pos 10-15 before taking several minutes to settle down and read good and one scan started neg 20-30 before taking about 10 minutes to settle down and read about 0...what is going on? Does the computer in the car get lost sometimes or is the HP readout off or something else happening? I want to put the car back to CL and be done with it, but is this a real problem? is going neg 30 on the STIT's going to damage the engine? Does anyone know how much fuel this is pulling? I need to know how to proceed.
Thanks

[knyght4]

Are you using a MAF only tune? If not, did you get your VE table tuned in as well? IMO, if you are going to be using CL, you are better off tuning of the STFT instead. And, you shouldn't you the WB for the partial throttle cells.

However, you should keep in mind the your WB and NB will never report equal readings.

[Blue02WS6]

Are you using a MAF only tune? If not, did you get your VE table tuned in as well? IMO, if you are going to be using CL, you are better off tuning of the STFT instead. And, you shouldn't you the WB for the partial throttle cells.

However, you should keep in mind the your WB and NB will never report equal readings.

I have the VE table tuned with the WB.

I don't understand what else you are saying...but my final VE and MAF tables done with the WB DO match what the NB's say most of the time, it's just ever now and then that the stft's are out of wack for some reason.

[pontisteve]

Perhaps they're getting fooled by exhaust leaks? Or the O2s are old? Are both O2s out of whack to the same amount at the same time?

Narrowband O2s and STFT's are never really happy. They will always bobble around 1 - 4% just by nature. The PCM trims up and down all the time, waiting for the O2s to see that, and follow. This is how the PCM knows the O2s are good. How fast the O2s follow is representative of how fresh they are. Old ones are slow to react to changes in A/F.

A good wideband, and the car in open loop, should allow you to tune to your commanded a/f ratio exactly, without the computer bumping the commanded a/f up and down. You should tune with the wideband, in open loop, with CAT disabled, in steady state, one system (VE/MAF) at a time. No transients.

If all that is good, and the car only reads weird when it's cold, check for vacuum or intake leaks. They're more pronounced when coolant temp is cold, and at idle. Once the engine warms up, gaskets swell up some, and the leaks are reduced.

[smithers]

There are several other tables that would introduce enrichment besides COT, ECT is one big one.

such Open Loop F/A vs ECT vs MAP
Any adjustments should be made at optimum ECT's or having these tables contain a multiplier of 1.00.

DFCO is another item you must have disabled as this would skew any logging.

Transients is key for the best tune as well, noting that RPM is a 'transient', so ideally, a steady state dyno is your best tool and the edge over a street tune.

[Blue02WS6]

Perhaps they're getting fooled by exhaust leaks? Or the O2s are old? Are both O2s out of whack to the same amount at the same time?

Narrowband O2s and STFT's are never really happy. They will always bobble around 1 - 4% just by nature. The PCM trims up and down all the time, waiting for the O2s to see that, and follow. This is how the PCM knows the O2s are good. How fast the O2s follow is representative of how fresh they are. Old ones are slow to react to changes in A/F.

A good wideband, and the car in open loop, should allow you to tune to your commanded a/f ratio exactly, without the computer bumping the commanded a/f up and down. You should tune with the wideband, in open loop, with CAT disabled, in steady state, one system (VE/MAF) at a time. No transients.

If all that is good, and the car only reads weird when it's cold, check for vacuum or intake leaks. They're more pronounced when coolant temp is cold, and at idle. Once the engine warms up, gaskets swell up some, and the leaks are reduced.

I don't think it's a exhaust leak...but I could be wrong. I have CL enable at like 160 deg. to warm up the o2's and they are fairly new, less than 6000 miles on them and my old one do the same thing. And the 1 - 4% I understand and that makes sense, and 9 out 10 scans my stft's read somewhere between pos. 5 to neg 10 with an avg. of about neg. 3 to 4, right where I want to be, but there is the scan that the trims go nuts for a while and I have seen pos as much as 10 to 20 and another scan go neg. as much as 20 to 30 ...do you know how much fuel it's pulling at neg. 30? I have not seen the info on how much equals how much, the car runs real good even when going out of range, I just don't want to be doing damage when this happens.

[pontisteve]

The car shouldn't run good if the O2s are inconsistently pulling 20-30%. If consistent, the car may run good, but something in the tune is way off. If fuel pressure is varying erratically, that might explain it. Or if the injectors are inconsistent in their flow rate.

The O2 trims represent an actual % of fuel being pulled out of the tune, so 20% is 20%. At part throttle or idle, pulling this much fuel won't likely hurt the motor, since load is low. If it's doing wacky stuff at full throttle, you had better get a handle on it before doing any more racing.

The car running good is odd. I've seen guys accidentally switch the O2 harnesses from side to side and get wacked out trims like that, but usually the car runs just awful when that happens.

You might try using the VCM scanner to enable and disable short and long term trims, reset trims, etc. This could show you how the car runs without help from the O2s.

MAF and VE tables should be tuned individually, with the other table being disabled while tuning whichever one.

Exhaust leaks near the O2 sensor, or in the headers anywhere, could cause this. I've been fooled by a seemingly leak-free exhaust before. Best bet is to rig up some tailpipe caps, and pump compressed air into the exhaust system when it's still cold. Leaks show up real quick that way, and there's no guessing involved.

[Blue02WS6]

The car shouldn't run good if the O2s are inconsistently pulling 20-30%. If consistent, the car may run good, but something in the tune is way off. If fuel pressure is varying erratically, that might explain it. Or if the injectors are inconsistent in their flow rate.

The O2 trims represent an actual % of fuel being pulled out of the tune, so 20% is 20%. At part throttle or idle, pulling this much fuel won't likely hurt the motor, since load is low. If it's doing wacky stuff at full throttle, you had better get a handle on it before doing any more racing.

The car running good is odd. I've seen guys accidentally switch the O2 harnesses from side to side and get wacked out trims like that, but usually the car runs just awful when that happens.

You might try using the VCM scanner to enable and disable short and long term trims, reset trims, etc. This could show you how the car runs without help from the O2s.

MAF and VE tables should be tuned individually, with the other table being disabled while tuning whichever one.

Exhaust leaks near the O2 sensor, or in the headers anywhere, could cause this. I've been fooled by a seemingly leak-free exhaust before. Best bet is to rig up some tailpipe caps, and pump compressed air into the exhaust system when it's still cold. Leaks show up real quick that way, and there's no guessing involved.

Yes it's strange the car runs well when and when not doing this...

I have the LTFT's disabled and re-set the trims before each scan.

OL it runs fine but by feel I would say it actually runs a hair smoother and better in CL, that's what gets me is how smooth it's running these days and every now and then I get these funked up STFT's with the car running nice and smooth...if I had not been doing all these scans and catch it on the scanner I would have not known.

[pontisteve]

Hook a wideband up and see what she's really doing. And test thoroughly for exhaust leaks.

A pinhole in the weld where the O2 bung was installed in the header, for the wideband, would be enough to skew things. A blown gasket or slip header tube certainly would do it.

With a 30% trim happening, something's gotta give. That's huge, and you should be able to feel it.

[jimmyblue]

LTFTs follow whatever the sensor tells them. Spend
a lot of time at low load, low RPM and watch your
idle / low-cruise LTFTs creep positive, due to sensor
heat shortage. This is bogus history, to be driven out
over time when/if the sensor gets hot and becomes
accurate.

[Blue02WS6]

LTFTs follow whatever the sensor tells them. Spend
a lot of time at low load, low RPM and watch your
idle / low-cruise LTFTs creep positive, due to sensor
heat shortage. This is bogus history, to be driven out
over time when/if the sensor gets hot and becomes
accurate.

I was kinda wondering if this wasn't an equipment error, like the o2's not being hot enough, as it usually happens at the beginning of my scan, even though I have cl enable at 169 I have wondered if it could be the o2's or a small leak that seals when hot...just wonder why the o2's have read the afr so rich, that doesn't make sense.

[pontisteve]

How far back are the O2s? maybe if they were way back in the exhaust sytem, like at the rear of an X-pipe or something, they can't maintain enough heat. I think the heater helps them warm up, but they still rely on at least some exhaust heat, if I'm not mistaken.

[jimmyblue]

I run 'vette rears, in the Y maybe 1 foot back of the
collector-Y joints, and they are still slow at best (not
anything like stock reswitch rates).

Heater may keep them on "life support" but they aren't
gonna get up and dance until you get a decent EGT
and flow rate to beat all the thermal losses.

[Blue02WS6]

Mine are in the collector tubes, I would think they would get blasted!
And when they do misbehave they settle down with the drive and
with LTFT's off I wonder if they have to get that hot to behave?

[jimmyblue]

You can tell this by how it behaves when you drop
to idle after running it hard. If it idles fine for a few
seconds and then slowly turns rough, that says it's
cooling off too much.

[pontisteve]

Does HP Tuners have tables for adjusting primary O2 heater voltage?