I have a 2000 Blazer that is still running lean bank 1 and bank 2. Both the STFT and LTFT values are very high (+20's for LT and approximately +5 to +6 for ST). However, my o2 sensors show that the exhaust is rich (consistently .6 volts to .8). They hover in that range way more consistently than the lower, more lean range. A vacuum leak would create a .1 to .5 volt pattern and the o2's don't even come close to showing that trend. My vacuum at the intake reads 18-19 inHG and steady-- engine warm and at 650 RPM idle. An exhaust leak prior to the o2's would show high oxygen and would also register low o2 voltage, thus increasing the fuel trims, but again my o2 voltage is high. I am currently getting 19-20 MPG with a check engine light illuminated (P0171 and P0174). The computer doesn't seem to be able to lean out the A/F despite the rich o2 values (little oxygen remaining). The STFT's never go below -3 and, when they do, it's only for a very brief period of time; the LTFT have never been negative. What an I missing? Could the previous owner have tried to increase the fuel mileage and mess with the stock fuel maps? My brain hurts these days... haha

 

Could just be bad o2 sensors. Does the sensor's voltage vary up & down on the scan tool?

 

1. An exhaust leak wont always add oxygen. Happens sometimes but depends where it is.

2. How do your plugs read? Do they show running lean?

3. Forget what the V reading is... the FT's are saying it has to add fuel based on O2 sense output.
- Old {dirty} O2's dont wear out by changing where they see rich/lean, they get slower.

Seems to me a classic case of dirty MAF sensor

Added: Or else one of those stupid ebay chips that mess with your IAT.

 

Smitty Smithsonite... thanks for your response...
Yes, both o2 sensors respond and they do actually cross the .45 volt threshhold. However, they certainly hang around the high voltage areas. Both bank o2s have been replaced with Denso about 2 months and 3000 miles ago. If the o2 reading are indeed rich, which I have to suspect is accurate, then one of my concerns is whether the computer has the ability to lean out the fuel trims to compensate for the high voltage readings. My vacuum readings with a gauge (19inHG) match my reported MAP values (34kPa=19inHG) with Map=Barometer-Vacuum; so an intake leak seems very unlikely. The o2 readings just do not hang out in the low voltage area so I'm not sure why the computer is justified in continuing to add fuel to the maximum amount without leaning it back when consistent high voltage readings are encountered.

 

Thank you for your input on this issue. My #1 plug looked rather black/rich when I pulled it to verify TDC when adjusting my Cam Retard from -5 to -2 (the closest I could get to 0). The MAF was replaced just prior to the o2 sensors and it reads 4.95 g/sec at idle and it seems to track with rpm and tps values when graphed. Now, please correct me if I'm wrong, but don't the o2 voltage reading back to the computer actually dictate the fuel trims?... not vice-versa? In other words, aren't the fuel trims an indication of and immediate response to what the o2 sensors voltages are telling the computer. As far as an exhaust leak, wouldn't anything prior to the o2 sensors show increased oxygen-- not decreased oxygen? Please don't get me wrong, I'm just making sure that my logic is in the right direction as my brain is hurting trying to keep all this straight... Oh yea, my IAT is new because I noticed the old one consistently read upwards of 10 degress warmer on a cold engine (sitting over 8 hours outdoors) than the ambient temperature.

 

So.. you've repleced the MAF AND the O2 sensorsYes, and that's what I said.. inartfully.. I should have made clear the ECM had to trim the fuel input positively {adding fuel} based on the feedback from the O2's.
- The map they run on from start to warmed up is trimmed. No. There's a net positive pressure in the exhaust. If air is drawn in, it's because of the volume of exhaust passing over the leaky spot 'siphoning in' outside air. A variation of venturi effect. Typical location for this is the manifold/head port junction when the head's exhaust port is smaller than the matching manifold. But in most cases you will hear the leak as well.
I've never seen anyone supply so much info. Since you are a whiz at it.. extrapolate the effect of that 10 deg change {cooler, thus denser air charge} on your old fuel map. What should the trim differences be?
Also, after all those changes, if the LTFT is held over they are useless.. except for curiosity's sake.

I also assume your vacuum readings are taken from the MAP? Have you compared them to a mechanical guage? From what I recall, and buttressed by a little research, you man vac is pretty much dead on.. assuming you're at about 500 to 1000 foot altitude.

After changing all those sensors you DID reset the ECM to default factory, right?

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The more I think about it, all else equal and assuming you manually verify ECT, MAP, IAT and that your MAF is working correctly.. the more it does look as if someone screwed with the ECM mapping.
ESP if all plugs look as if they are running rich.
- I tried to figure out the effect of two leaking injectors on STFT once and got a migraine.. so it IS a good idea to read all your plugs.

 

The factory-setting LTFT seems to start out at +7.95 when in open loop, if my memory is correct, and it's supposed to be somewhat rich until it goes into closed loop. Then, at engine shut off, the LTFT values stay at an adjusted value based on previous readings. However, I can only do the reset after disconnecting the battery due to an apparent software issue with my scantool and I have not done that since replacing the o2 sensors and MAF. I figured that if my problem was corrected the LTFT's would naturally decrease as noticably as they increased and end up +-10%.
As far as the IAT, I mentioned it based on your statement of a "cheap ebay chip messing with it". I assumed that an IAT reading too cold would affect the fuel trims by making the mixture richer in fuel and too warm of a reading would lean the fuel out too much. I, perhaps incorrectly, figured that my IAT reading might affect the fuel trims but I wasn't too sure in which direction; the +10 degree reading just seemed 'wrong' to me and I ruled it out at minimum expense and effort.
My vacuum reading were taken from the PCV hose coming from the manifold and measured using a mechanical vacuum gauge; reinforcing the results with the MAP reading consistency. I'm at +594 Ft and the Baro, not adjusted for elevation, is 99Kpa. That's why I think my mechanical vacuum readings are reinforced by my MAP values. No apparent vacuum leaks, am I correct?
Are you recommending that I do a reset to factory by disconnecting the battery or is there another way; perhaps by fuse?
"I've never seen anyone supply so much info. Since you are a whiz at it.. extrapolate the effect of that 10 deg change {cooler, thus denser air charge} on your old fuel map. What should the trim differences be?"
My intent is not to be a know-it-all and I'm hoping not to be quized. I'm just trying to be accurate and somewhat intelligent in my descriptions...

 

Well, I wasnt trying to be either.* In fact I had to read the OP several times to see your point but I DO see your point.

Snap reading of the O2's indicate they are running rich yet the STFT implies the opposite.
- but here's the problem.. the trims, from what I have just read elsewhere, are affected by more than just O2 feedback. Also by variable from the MAF and IAT. That COULD be wrong info, but I aint discounting anything.

And like I said.. when I tried to visualize what a couple leaky injectors would do to the trims, it made my head hurt because the effect on the sensors would be highly variable.. and the bottom line is that the cyls with good injectors would tend to run lean. I think.

As I amended, .. IF.. all your plugs read rich or tend to rich, I think you'd be well served to get yourself to a salvage yard to find a 'virgin' ECM for your truck. Cuz it sure does look screwy. Also it would be fun to see on an Oscilloscope what the output of the O2's look like in real time.

*Although I'm sorta puffed that I guessed your approx altitude {over 500 ft} based on dead reckoning.

 

pettyfog... thanks you for clarification and your input-- really. I didn't think anything negative was coming from you, it's just my mind is tired and confused right now and I don't want to sound like a moron.
HaHa, you were indeed spot on with the altitude! All of these sensors and how they interact can certainly cause one's brain to spaz! That is why I'm here, to check out my logic on others besides driving myself 'stupid' with trying to compute the logic behind the 'systems' solely on my own. I feel like I'm capable of following and understanding the logic but when something like this comes out I find myself thinking in circles. I let it go for a few weeks and, as the problem continues, I revisit it with hopefully a fresh mind. However, just like in the past few months, my sensor readings don't seem logical and I find myself asking, "What did the previous owner do? The computer shouldn't be behaving this way. The other sensors show their data via my scantool and they all seem in spec. Why won't the o2 sensors, and their voltage, make the computer respond in the way that I 'think' I understand they should? Lean out the fuel stupid!; that's what I think! Haha"