There's a couple of things to consider, but not this fault code.
P0171 - System too Lean (Bank 1)
That will not cause stalling. A shorted oxygen sensor shorted in the way it can cause stalling is very rare, and then it is almost always a dead short. The engine will not run at times. The mass air flow sensor was a good guess as those did have a history of being intermittent on GM engines, but you should have gotten a fault code related to that.
Instead, the most common cause of intermittent stalling on any brand of car is the crankshaft position sensor or the camshaft position sensor, and they commonly fail by becoming heat-sensitive. They tend to remain working as long as you keep driving, due to natural air flow keeping them cool. When you stop for a short time with a hot engine, as in when stopping for gas, "hot soak" lets engine heat migrate up to those sensors leading to a heat-related failure. To add to the insult, they often do not set a diagnostic fault code just from cranking the engine. Often they need more time to be detected, as in when a stalled engine is coasting to a stop. When there is no fault code related to the stalling, you need a scanner to view live data and see what the Engine Computer is seeing. In this case we're looking for a missing signal from one of those sensors.
Another good suspect for the symptoms you described is a plugged or collapsing strainer inside the gas tank. It's shown in the photo. This is a really elusive thing to diagnose. The first clue for fuel-injected engines with the pressure regulator on the fuel rail on the engine is the stalling will occur when the highest volume of fuel is being pumped, which is during coasting. The first time this happened to me, the engine ran fine for over 200 miles on the highway, then it stalled when I coasted off my off-ramp.
To prove this is causing the stalling, connect a fuel pressure gauge and clip it to the radio antenna or under the right wiper arm so you can watch it when the problem occurs. Normal fuel pressure for my Caravan is 45 -50 psi, but the stalling didn't occur until it fell below 20 psi. GM engines aren't so tolerant. Often they will stall by the time fuel pressure drops as little as five to ten pounds.
The next clue is the engine will restart after you've been sitting on the side of the road for about five minutes. That gives the collapsed strainer time to stretch out enough to let fuel flow again. Crank and cam sensors will also work again after cooling down, but that typically takes about an hour.
Most auto parts stores rent or borrow fuel pressure test kits with a variety of adapters, so you don't have to buy one. In my city, they make you pay for the tool, then you get a full refund when you bring it back. If you choose to keep a tool, you still return it, then they give you or order you a new one. For the benefit of others researching this topic, here's a link to a guide on using the fuel pump tester, if you need it:
https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator
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Friday, June 12th, 2020 AT 5:09 PM
