High fuel trims, and drivability problems
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I went ahead and checked the air filter. It's pretty clean. Looks like it was replaced fairly recently. I put the MAF I just cleaned on, and the numbers went slightly lower again, down to about 1.6lb/min. Out of curiosity I pulled the intake tube so the MAF would have zero restriction, but the numbers still averaged 1.7lb/min at 1700.
I'll check the power and ground tomorrow, and if my oscilloscope will work I'll graph the frequency.
I'll check the power and ground tomorrow, and if my oscilloscope will work I'll graph the frequency.
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From: North Central Indiana
https://blazerforum.com/forum/2nd-ge...eadings-97498/
https://blazerforum.com/forum/diagno...plained-35154/
If you want to quickly check the MAF sensor, this is part of the P0101 factory diagnostic info for a 2001 Blazer:
A wide open throttle (WOT) acceleration from a stop should cause the MAF sensor g/s display on the scan tool to increase rapidly. This increase should be from 6-12 g/s at idle to 130 g/s or more at the time of the 1-2 shift. If the increase is not observed, inspect for a restriction in the induction system or the exhaust system.
IMHO need to figure out why it is intermittently running so lean to the point that it misfires. Intermittent problem with fuel pump? Or did the P0171 and P0174 codes get set from jacking around with the different sensors? I know of no testing that is effectively done by unplugging a sensor and watching what an engine does. How do you trust LTFT (average over time) when different sensors have been unplugged and the engine run as a test?
https://blazerforum.com/forum/diagno...plained-35154/
If you want to quickly check the MAF sensor, this is part of the P0101 factory diagnostic info for a 2001 Blazer:
A wide open throttle (WOT) acceleration from a stop should cause the MAF sensor g/s display on the scan tool to increase rapidly. This increase should be from 6-12 g/s at idle to 130 g/s or more at the time of the 1-2 shift. If the increase is not observed, inspect for a restriction in the induction system or the exhaust system.
IMHO need to figure out why it is intermittently running so lean to the point that it misfires. Intermittent problem with fuel pump? Or did the P0171 and P0174 codes get set from jacking around with the different sensors? I know of no testing that is effectively done by unplugging a sensor and watching what an engine does. How do you trust LTFT (average over time) when different sensors have been unplugged and the engine run as a test?
Last edited by LesMyer; Dec 20, 2020 at 10:09 AM.
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Joined: Jan 2007
Posts: 4,628
From: Florida
I forgot the IAT sensor for your basic sensor test, engine cold and off. Is it close to ambient?
The first set of MAF testing things We have discussed will only see an obvious failure. Many MAF problems are under reporting or high speed glitches that are less obvious and require further analysis.
Les has given you the next level look at the sensor which is a good test. Following on Les’ post, if your scan tool can graph PID,s, the most comprehensive way to get a look at what is happening with your truck is to track a set of PIDs on a test drive that includes a few conditions, one being wot to the shift points like Les said. You look at things like rpm, MAF reading, iat, fuel trims, front O2 sensors, etc. You do a volumetric efficiency calculation at the shift point and look at the O2 sensors and fuel trims. This tells us if your engine is breathing properly at full power, has a MAF reporting issue, a restriction, a fuel delivery problem, etc. Did the O2 sensors peg at full high or low output, what was the MAF value, etc. Les has given you a good next step to see the MAF reading at the wot shift points but if it’s low you still won’t know why and if to condemn the MAF. I don’t normally get this detailed and nerdy but you seem to have an interest in going farther than most. If my prior suggestions and Les’ input don’t solve your problem and you want to get really sophisticated in analyzing your problem, let me know and We can go deaper into this with you. Most people here don’t get this far but We can fill your brain up with plenty of techy stuff and keep you up at night. LOL.
George
The first set of MAF testing things We have discussed will only see an obvious failure. Many MAF problems are under reporting or high speed glitches that are less obvious and require further analysis.
Les has given you the next level look at the sensor which is a good test. Following on Les’ post, if your scan tool can graph PID,s, the most comprehensive way to get a look at what is happening with your truck is to track a set of PIDs on a test drive that includes a few conditions, one being wot to the shift points like Les said. You look at things like rpm, MAF reading, iat, fuel trims, front O2 sensors, etc. You do a volumetric efficiency calculation at the shift point and look at the O2 sensors and fuel trims. This tells us if your engine is breathing properly at full power, has a MAF reporting issue, a restriction, a fuel delivery problem, etc. Did the O2 sensors peg at full high or low output, what was the MAF value, etc. Les has given you a good next step to see the MAF reading at the wot shift points but if it’s low you still won’t know why and if to condemn the MAF. I don’t normally get this detailed and nerdy but you seem to have an interest in going farther than most. If my prior suggestions and Les’ input don’t solve your problem and you want to get really sophisticated in analyzing your problem, let me know and We can go deaper into this with you. Most people here don’t get this far but We can fill your brain up with plenty of techy stuff and keep you up at night. LOL.
George
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The IAT is reading 42F. According to the weather channel, and an app I use, it's 37F outside, and feels like 27. My scanner can graph, I just can't have more than 2 graphs open at a time during playback. So just to be clear, I should graph RPM, MAF, IAT, Fuel Trims, and upstream O2's on a WOT up to the 1-2 shift after the system has gone into closed loop. This sound correct?
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[QUOTE=LesMyer;730102
IMHO need to figure out why it is intermittently running so lean to the point that it misfires. Intermittent problem with fuel pump? [/QUOTE]
That could be. I was thinking maybe that, or a poppet getting stuck, and freeing itself up when giving it full throttle. When it happens, it's always after starting, and during the initial take off. Once it clears up, it doesn't do it again while driving. It was still having a stumble from 3/4 to 7/8 throttle until I changed out the parts I did. Now when it does it, it'll run fine at all throttle positions after it clears up.
IMHO need to figure out why it is intermittently running so lean to the point that it misfires. Intermittent problem with fuel pump? [/QUOTE]
That could be. I was thinking maybe that, or a poppet getting stuck, and freeing itself up when giving it full throttle. When it happens, it's always after starting, and during the initial take off. Once it clears up, it doesn't do it again while driving. It was still having a stumble from 3/4 to 7/8 throttle until I changed out the parts I did. Now when it does it, it'll run fine at all throttle positions after it clears up.
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Just did a wot run up to the 1-2 shift. I'll have to go back and upload the graphs but here's some numbers.
Short trims were at 0% throughout the run.
The MAF started at 73g/s and went to 139.4g/s, then ended at 178g/s at the time of shift.
O2 #1 started at .9v and went to .925, .94, and was at .945 at the time of shift.
O2 #2 started at .86 and went to .87, .915, and was at .935 at the shift.
Short trims were at 0% throughout the run.
The MAF started at 73g/s and went to 139.4g/s, then ended at 178g/s at the time of shift.
O2 #1 started at .9v and went to .925, .94, and was at .945 at the time of shift.
O2 #2 started at .86 and went to .87, .915, and was at .935 at the shift.
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Posts: 4,628
From: Florida
Sounds like your game to go the full Monty so here it is.
The first thing we need is your VE or volumetric efficiency which must be measured at full throttle near peak torque or the shift point. This is a measure of the amount of air the engine is consuming compared to the theoretical amount under those conditions. The engine has restrictions and flow losses so the answer should be between 75 and 90% for a naturally aspirated engine. Intake and exhaust tuning can get you close to or slightly above 100% and turbo/super chargers even higher. One of the challenges is that we rely on the MAF to tell us what the volume of air is so we need other data to complete the picture. Here is a VE calculator so you see what you need. The data must be captured at peak torque shortly before the shift.
https://atgtraining.com/atg-volumetr...cy-calculator/
To complete the picture your also going to need:
fuel trims: (always both ST and LT on a given bank with sign). We need to see if the sensors, computer and O2 sensors think that the mix is rich or lean and how that’s changing. These numbers must always be added together because what the computer has done is calculate the average fuel trim needed over the long haul and is commanding the fuel system that way full time with LT. As conditions change in the short term such that this LT trim is not appropriate then a ST adjustment is made and the resulting combination of LT and ST is what is commanded to the injectors. Why bother having both values then if the computer just adds them together? Because we often need to see the instantaneous result of changes which we see in the ST values. LT should be more stable and change slowly. If say a fault is corrected, then ST will make an immediate move away from LT to net near zero and both numbers will start walking towards zero in synch.
O2 sensors: What you should see are the upstream sensors successfully transitioning from a square wave to pegging high at a flatline around 0.85V. This tells us that the computer commanded a rich mixture for the full throttle event and that the rich mixture was possible at max power. Flatlining low would demonstrate for instance the the fuel system was not up to the job.
You will need all of this also at idle and cruising speed as a baseline.
In summary, your sensing and/or running lean and commanding rich. We need to find out if it’s asensor problem or an actual lean condition and if lean, why?
George
The first thing we need is your VE or volumetric efficiency which must be measured at full throttle near peak torque or the shift point. This is a measure of the amount of air the engine is consuming compared to the theoretical amount under those conditions. The engine has restrictions and flow losses so the answer should be between 75 and 90% for a naturally aspirated engine. Intake and exhaust tuning can get you close to or slightly above 100% and turbo/super chargers even higher. One of the challenges is that we rely on the MAF to tell us what the volume of air is so we need other data to complete the picture. Here is a VE calculator so you see what you need. The data must be captured at peak torque shortly before the shift.
https://atgtraining.com/atg-volumetr...cy-calculator/
To complete the picture your also going to need:
fuel trims: (always both ST and LT on a given bank with sign). We need to see if the sensors, computer and O2 sensors think that the mix is rich or lean and how that’s changing. These numbers must always be added together because what the computer has done is calculate the average fuel trim needed over the long haul and is commanding the fuel system that way full time with LT. As conditions change in the short term such that this LT trim is not appropriate then a ST adjustment is made and the resulting combination of LT and ST is what is commanded to the injectors. Why bother having both values then if the computer just adds them together? Because we often need to see the instantaneous result of changes which we see in the ST values. LT should be more stable and change slowly. If say a fault is corrected, then ST will make an immediate move away from LT to net near zero and both numbers will start walking towards zero in synch.
O2 sensors: What you should see are the upstream sensors successfully transitioning from a square wave to pegging high at a flatline around 0.85V. This tells us that the computer commanded a rich mixture for the full throttle event and that the rich mixture was possible at max power. Flatlining low would demonstrate for instance the the fuel system was not up to the job.
You will need all of this also at idle and cruising speed as a baseline.
In summary, your sensing and/or running lean and commanding rich. We need to find out if it’s asensor problem or an actual lean condition and if lean, why?
George
