LTFT/STFT Problems
 

https://cimg1.ibsrv.net/gimg/www.cam...49cdd473f9.jpg
Sorry to hijack the tread but I think I'm having a similar problem. The other day before work while warming up my car I noticed the o2 left side voltage was stuck at 56 until it slowly went open loop. Also I noticed my long term fuel trim stay at 160, doesn't even move. This is a a 95 T/A w/ a mild cam (Cc502) with little boltons and a tune by Solomon.
https://cimg2.ibsrv.net/gimg/www.cam...769bb2ef0a.jpg

Re: LT and ST fuel trim reads high in data log
 

Do you still have a working AIR pump? Was it running when you saw the 53mV reading in open loop? That can happen in open loop with the AIR pump running. And, in open loop the PCM isn’t using the O2 readings. A more important question is in closed loop, do the O2 sensors move rapidly up and down over a range of less than 100mV to more than 900mV.

If a LTFT stays at 160 (which Cell were you looking at) it means the exhaust is indicating a lean condition, the PCM has raised the LTFT to 160 to add more fuel. But 160 is the highest the LTFT’s can go, so now you have to look at the STFT's. to see if the PCM is using them to add even more fuel.

Snapshots like you posted are really not very useful. You need a data log that shows a lot more, and how it is changing with time.

Re: LT and ST fuel trim reads high in data log
 

No my air pump is disable in the tune. There times the left o2 sensor stays in one range 885 - 945 for a couple sec then when I press the gas it drop to like 34 then back to normal. Right o2 sensor bounces everywhere like it should.

Yeah I smell gas at times and rpm be jumpy (Alitte rough) in idle. But no codes yet. When warming up my STFT both side will be at 128 and it will stay there. When in close loop right bank be 127-126 and Left bank he in the 128-132 range. Most of the time in the low 130s

Sorry I don't have a laptop, all I got is a scanner. I did purchase a better scanner tool (Tech 2) Should arrive this week

Re: LTFT/STFT Problems
 

If the left O2 sensor was sticking at 885 and higher, the PCM would drop the LTFT's to reduce fuel, not increase them to 160. These readings - O2 volts, LTFT's and STFT's are meaningless, without knowing which fuel trim cell they are in. When you "press the gas", the PCM will move to a different fuel correction Cell, based on MAP and RPM. We need to know which cell. You need a data log that captures all this data in one record, and covers an extended period of engine operation. The PCM updates these values almost 10 times per second. With a single screen shot you aren't seeing enough to know what's really happening. A data log would have told me whether the AIR pump was operating.

The STFT's stay at 128 in open loop because the PCM is not using the O2 sensor feedback. Again, in closed loop, they are changing up to 10 times per second.

I really can't help you without a decent log.

Re: LTFT/STFT Problems
 

It's weird cause that the left o2 sensor be behaving oddly vs the right o2 sensor.

Do you mean LTFT CL? If so it always say "16" if that what your talking about "Cell"

So what's the best way to give you info without a laptop?

Re: LTFT/STFT Problems
 

Cell 16 is the cell for "idle" (closed throttle, 0 MPH). There are 19 different cells, most of them (00 - 15) based on combinations of engine load (MAP) and RPM. Looking at your second photo it shows the LTFT;s, but is doesn't indicate which Cell they are in.

I don't know the capabilities of the Tech-2 with regard to data logging, and producing a log in .csv format. No one has ever sent me one from a Tech-2, and I've probably looked at over 1,000 logs in the past 20 years. Not aware of anything other than a Windows laptop plus the right software (Scan9495 preferred, and free download). If you limited the log to idle, you can do it with a desktop PC and a long cable. But that would just tell us about how it idles, plus allowing us to look at the complete startup process - how it starts, what it does in open loop, how long it takes to transition to closed loop, and how it idles in closed loop. But that's all.

Re: LTFT/STFT Problems
 

The tech 2 scanner has a lot features for gm we'll see when it gets here Saturday. Until then I'm gonna do some more scan time and see if I can get some at least clues for you. This actron scanner is not that great, maybe there's a update. Is there anything I can do for you to get on the right path to figuring out this problem for me?

Re: LTFT/STFT Problems
 

Will loose exhaust manifold bolts cause this issue?

Re: LTFT/STFT Problems
 

I inspected the ex manifold and found one loose bolt (driver side) at the end. It was so loose I can turn it with my hand. Idles better, but scanner still sees 160 for Long term fuel trim L, It drops to 150-145 when I press on gas.

Re: LTFT/STFT Problems
 

An exhaust leak BEFORE the O2 sensor can screw up the readings and cause elevated LTFT's. The degree of impact varies with the load on the engine, and the RPM. Your LTFT's change when you "press on the gas" because you are moving the PCM to a different LTFT Cell.

The system is correcting for what it perceives is a lean condition. There are "true" leans, caused by something like a faulty/dirty MAF, air bypassing the MAF (torn inlet bellows), vacuum leaks, low fuel pressure. Then there are "false" leans caused by exhaust leaks before the O2 sensor, misfires, and faulty O2 sensors.

If the LTFT's are "off" by the same degree, it's a problem affecting both banks - MAF or low fuel pressure would be examples. If the LTFT's are only off on one bank, its more likely to be a vacuum leak on that side, an exhaust leak, misfires, or O2 sensor on that side of the engine.

Re: LTFT/STFT Problems
 

Hm I'm gonna change out both o2 sensor and see what happens and the maf is brand new (Ac Delco brand) it's weird cause when I pulled the plug on the maf to see if that was bad, no ses light came on. But when i pulled the ECT sensor on the wp it came on. I didn't see no rip or anything in the intake, and the intake manifold gasket is brand new from the cam swap. I've been thinking to replace the map sensor, there time we're it stay stuck around 2.45 at idle. I don't know if that the right range for that sensor though.

Re: LTFT/STFT Problems
 

MAP at idle, and how steady it is depends to some extent on the cam, how good the tune for it is, the valve adjustment and elevation relative to sea level. Misfires will also affect MAP. Looks like you had a reading of 2.45 volts, which corresponds to a manifold absolute pressure (MAP) of 16.2”Hg. Think of MAP as the opposite of vacuum. At sea level 16.2”Hg MAP corresponds to about 13.8”Hg vacuum. That's probably a bit low for a mild cam lhe CC502.

My guide to LT1 scanning values:

https://www.camaroz28.com/forums/com...-guide-886891/

Re: LTFT/STFT Problems
 

The scanner always sees BARO 26.80 to 28.30 Hg I live in San Francisco, and out here San Francisco is 46.80 above sea level. I don't know if that's accurate.

Re: LTFT/STFT Problems
 

Standard barometer (BAR) at sea level is 29.92”Hg. A barometer of 26.8”Hg would be typical at 3,000-ft above sea level (or a huge hurricane in the Caribbean). Your local weather, Accuweather, etc. should have the barometer reading available for you to check your MAP sensor. BAR vs. elevation, and a conversion from volts to “ of mercury, to kiloPascals is included in the scan guide I linked.

Your readings sound too low. The GM MAP sensors are usually very accurate and reliable. The pressure does change with the weather, and can go above or below 29.92”Hg, but not down to 26.8” at a few feet above sea level.

Re: LTFT/STFT Problems
 

It's actually 52 ft above sea level over here, so that what mean the map sensor voltage is off and giving bad readings. Today when I drove it to work barometer read 27.38 hg and map volts was 1.90 in idle.