Chevy Sonic: Running Lean P0171 - Part I
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Date: 2020-08-05
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Comments and reviews: 10
Kevin
Not to be overly nitpicky, but when you rev up the engine under no load, the reason the fuel trims improve when there is a vacuum leak is because the overall airflow into the engine increases while the amount of air getting in through a vacuum leak stays roughly the same, therefore the fuel trims improve because the vacuum leak is causing a lower percentage of error in the MAFS reading. For example, lets say you have a vacuum leak that is allowing 1 g/s of unmetered air in on a 2. 0L engine at 20 inHg, where we might expect to see 2 g/s of airflow through the MAFS under normal conditions, the vacuum leak will be causing a 50% error in the MAFS reading, so we might expect to see total fuel trims around 50%. If we increase the engine speed off idle to 3000rpm, we may see the airflow through the MAFS increase the 5 g/s under normal conditions, but because the manifold vacuum is not significantly changed by revving the engine under no load (I'd expect almost no change once the engine speed levels off, it may even increase manifold vacuum due to the higher engine speed) the vacuum leak will still only be allowing in about 1 g/s of air. So, that 1 g/s of air caused a 50% error in the MAFS at idle when the engine is ingesting 2 g/s of air, but its only a 20% error when at 3000rpm under no load when the engine is ingesting 5 g/s of air, so the total fuel trims should improve to around 20%.
Great job with the video and teaching a new tech how to properly diagnose a car.
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Not to be overly nitpicky, but when you rev up the engine under no load, the reason the fuel trims improve when there is a vacuum leak is because the overall airflow into the engine increases while the amount of air getting in through a vacuum leak stays roughly the same, therefore the fuel trims improve because the vacuum leak is causing a lower percentage of error in the MAFS reading. For example, lets say you have a vacuum leak that is allowing 1 g/s of unmetered air in on a 2. 0L engine at 20 inHg, where we might expect to see 2 g/s of airflow through the MAFS under normal conditions, the vacuum leak will be causing a 50% error in the MAFS reading, so we might expect to see total fuel trims around 50%. If we increase the engine speed off idle to 3000rpm, we may see the airflow through the MAFS increase the 5 g/s under normal conditions, but because the manifold vacuum is not significantly changed by revving the engine under no load (I'd expect almost no change once the engine speed levels off, it may even increase manifold vacuum due to the higher engine speed) the vacuum leak will still only be allowing in about 1 g/s of air. So, that 1 g/s of air caused a 50% error in the MAFS at idle when the engine is ingesting 2 g/s of air, but its only a 20% error when at 3000rpm under no load when the engine is ingesting 5 g/s of air, so the total fuel trims should improve to around 20%.
Great job with the video and teaching a new tech how to properly diagnose a car.
reply
Frank
Good for you! When my Wife's Chevvy Cruze threw the same code, the dead giveaway was the ton of air hissing into the valve cover. The hole is a port that opens the back side of the diaphragm to the atmosphere so it can move as the valve position changes. When the diaphragm ruptures then you have the open hole into the engine that bypasses the mass flow sensor.
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Good for you! When my Wife's Chevvy Cruze threw the same code, the dead giveaway was the ton of air hissing into the valve cover. The hole is a port that opens the back side of the diaphragm to the atmosphere so it can move as the valve position changes. When the diaphragm ruptures then you have the open hole into the engine that bypasses the mass flow sensor.
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Budget
my 10 homemade smoke machine is literally just a soldering gun wrapped in old sock with mineral oil and it became my favorite trick of thr trade, Not only for boost/vacuum leaks but quickly used it for all sorts of things like checking coolant leaks/blown hg/clogged cats/valve problems its so easy to make one and so cheap I love em.
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my 10 homemade smoke machine is literally just a soldering gun wrapped in old sock with mineral oil and it became my favorite trick of thr trade, Not only for boost/vacuum leaks but quickly used it for all sorts of things like checking coolant leaks/blown hg/clogged cats/valve problems its so easy to make one and so cheap I love em.
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Camby
Great job. Keep it going, You will gain an enormous amount of knownedge from youtube Good & Bad, It's knowing which is which that is key. You have a thirst for knowledge, and Eric has it all to share, as long as you have the hunger to learn, Eric will keep you on the right track. Thank You.
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Great job. Keep it going, You will gain an enormous amount of knownedge from youtube Good & Bad, It's knowing which is which that is key. You have a thirst for knowledge, and Eric has it all to share, as long as you have the hunger to learn, Eric will keep you on the right track. Thank You.
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GNX157
My concern locking at the initial live data was the coolant temp. She said it was okay but it was only 109F. How long had it been running? Did it ever warm up to operating temp? I would of preferred to let it warm up fully before looking at the fuel numbers.
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My concern locking at the initial live data was the coolant temp. She said it was okay but it was only 109F. How long had it been running? Did it ever warm up to operating temp? I would of preferred to let it warm up fully before looking at the fuel numbers.
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Papi4l2
I gotta say, i am super impressed. The whole beginning where maries using the scan tool. how ling did u practice that? I mean she was flying through that scan tool not skippin a beat like shes been doin it for years. Great job marie, keep on learning
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I gotta say, i am super impressed. The whole beginning where maries using the scan tool. how ling did u practice that? I mean she was flying through that scan tool not skippin a beat like shes been doin it for years. Great job marie, keep on learning
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bigdaddymak1439
After the racoon deserved to die video I didn't think it could get any better! I was wrong absolutely love the family atmosphere of your shop and humor! And absolutely love Ms. Marie's accent! Keep on learning Marie!
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After the racoon deserved to die video I didn't think it could get any better! I was wrong absolutely love the family atmosphere of your shop and humor! And absolutely love Ms. Marie's accent! Keep on learning Marie!
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ecleveland1
You seem to have a good pupil that really is excited about learning. You, sir, are doing an excellent job teaching. I really liked the after diagnosis conversation it showed an understanding of troubleshooting.
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You seem to have a good pupil that really is excited about learning. You, sir, are doing an excellent job teaching. I really liked the after diagnosis conversation it showed an understanding of troubleshooting.
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PastBlast
Great video you two! Really nice to see the next generation coming along. Marie shows wonderful potential. Kudos to you Eric for training your replacement. Everyone wants to retire someday.
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Great video you two! Really nice to see the next generation coming along. Marie shows wonderful potential. Kudos to you Eric for training your replacement. Everyone wants to retire someday.
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Daniel
Thank you both. Learned something new again! Testing vacuum via dip stick! Oh and I think this is the first time I've seen Eric wearing his wedding band.
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Thank you both. Learned something new again! Testing vacuum via dip stick! Oh and I think this is the first time I've seen Eric wearing his wedding band.
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