Hey Scotty!
I've got a 2002 Chevy Suburban 5.3L Flex Fuel, 295,000 miles. It's running very well but recently the CEL came on. My cheap scan tool gave codes P0172 and P0175. The live data is showing long term fuel trim to hover around negative 15%. Short term is close to zero.
Air filter and MAF are both new. The fuel pressure regulator is not leaking. I don't hear any obvious vacuum leaks.
Could it be a problem with the alcohol sensor? Any suggestions on what I should do or check or replace?
Thanks!
Vacuum leaks usually don't cause a rich running condition. They lean the mixture out and the O2 sensors will tell the ECU to add fuel, not subtract it. Did you replace the MAF sensor with the ACDelco OEM sensor? Cheap knock-off Chinese parts don't typically work correctly. Have you run fuel injector cleaner? I would start there, and possibly run a tank of Shell V-Power through it, then go from there.
I bought the Duralast brand MAF, which should still be in warranty. Maybe I could try a warranty exchange and if that doesn't work then buy ACDelco.
I'm hesitant to believe the injectors are the problem. I've been using injector cleaner pretty regularly, plus it is rich in both banks. Seems unlikely fuel injectors would go bad on both sides at the same time.
Look at your live data, your airflow rate at idle with a fully warmed up engine should be the engine's displacement in grams, meaning your 5.3L V8 should be pulling 5.3 grams of air per second.
Thanks for the tip. I checked it out and the data shows 5.4 - 5.5 grams per second, which is about 4% off target. Do you think that is enough to throw off my air/fuel mix?
I also discovered that the intake temp is quite a bit off. It's about 35 degrees F outside right now, but the sensor was showing 68 degrees. Is it possible the engine bay is heating up the air that much near the headlight where it pulls air? It also didn't change when I hit the gas, which I'd imagine if air is sucking faster there would be less time for the air to be heated up by the engine bay. Could that throw off LTFT by neg 15%?
5.4-5.5 sounds close enough to me. I'm currently exploring an issue with my Ranger where the long term trim is -6% to 9% and jumps around at idle. It's almost 0% when I drive. My MAF sensor is reading 2x displacement or more, I have no CEL, but I'm going to try MAF cleaner and go from there. The intake temperature sensor isn't usually up at the airbox, it's usually closer to the throttle body. Check what it says when it's cold and you just start it. A cold engine in cold weather will need more gas, so if the IAT sensor is reading too low, it will cause the mixture to be overly rich.
So I just did a warranty exchange at AutoZone, they swapped out the Duralast MAF. No luck. LTFT still neg 15% or so, neg 10% at idle, both banks. I think I'm gonna swap out the fuel pressure regulator since it's cheap and is 100% mechanical (I'd imagine that bad sensors would throw their own code.)
How's it going with your Ranger?