I have an 2004 Sentra 1.8S, I had a P1278?, and I changed the O2 Sensor in the front. Erased the code, and it didn't come back, but a P0171 came back instead. I changed the O2 sensor on the exhaust pipe, the MAF Sensor. I did a fuel pressure test, and as soon as I turned off the key, all the fuel pressure drained out within several minutes, and I discovered that my gas mileage was down 3/4. I replaced the fuel pump, and now the car sounds really great, but the fuel trims didn't improve. Before the new fuel pump was installed, the Short F/T switched back and forth between 0 and 25, staying on 25 most of the time. Long F/T was at zero. After the fuel pump was replaced, Short is on 25 most of the time, and flashed back to 0 every so many minutes. The Long is 13.3 steady. By the way, I forgot to mention that I did smoke the intake with a hand made smoker, which didn't go very well, but I didn't find any vacuum leak. Anyone have any idea where I should look now? What are these Fuel Trims indicating to you? I thing I have a vacuum leak somewhere, maybe an exhaust donut leak? I don't think it is fuel injectors, because it's not running rough at all, especially with the new fuel pump, nor is it missing.

By far the most likely causes of P0171 on an older Sentra are 1) a cracked intake boot or 2) a dirty or bad MAF.

Thanks! When I did smoke the intake, I suspected the intake boot may have had a small leak, but I didn't confirm that. I think the next thing I will do is take that intake boot off, and examine it closely. I did drive the car yesterday nearly 14 miles, and the light hasn't come back on. I have to go back to town today, so I will see if it comes back on. Thanks again.

How can I get access to the Service Manuals again? I had access before I joined, and suddenly I no longer could access them. I thought I would be able to access them after I joined?

Wadeh62 wrote: ↑

Wed Feb 16, 2022 12:33 pm

Thanks! When I did smoke the intake, I suspected the intake boot may have had a small leak, but I didn't confirm that. I think the next thing I will do is take that intake boot off, and examine it closely. I did drive the car yesterday nearly 14 miles, and the light hasn't come back on. I have to go back to town today, so I will see if it comes back on. Thanks again.

What happens on the older Sentras is that the accordion boots crack, usually inside one of the pleats where it isn't visible. Then as the engine rocks in the mounts under accel or decel, the crack opens up and a gush of unmetered air causes a code. When the engine returns to center in the cradle, it seems fine because little or no air is leaking. You thus get "mystery codes" that seem to pop up randomly and vanish when erased.

Wadeh62 wrote: ↑

Wed Feb 16, 2022 12:39 pm

How can I get access to the Service Manuals again? I had access before I joined, and suddenly I no longer could access them. I thought I would be able to access them after I joined?

AZhitman wrote: ↑

Tue Feb 15, 2022 8:40 pm

Almost there, should be back up within 48 hours. Thanks for your patience!

VStar650CL wrote: ↑

Wed Feb 16, 2022 1:01 pm

Wadeh62 wrote: ↑

Wed Feb 16, 2022 12:33 pm

Thanks! When I did smoke the intake, I suspected the intake boot may have had a small leak, but I didn't confirm that. I think the next thing I will do is take that intake boot off, and examine it closely. I did drive the car yesterday nearly 14 miles, and the light hasn't come back on. I have to go back to town today, so I will see if it comes back on. Thanks again.

What happens on the older Sentras is that the accordion boots crack, usually inside one of the pleats where it isn't visible. Then as the engine rocks in the mounts under accel or decel, the crack opens up and a gush of unmetered air causes a code. When the engine returns to center in the cradle, it seems fine because little or no air is leaking. You thus get "mystery codes" that seem to pop up randomly and vanish when erased.

Thanks for the information. Yes, those boot cracks can be hard to find based upon videos I've watched, and now your comment. Tomorrow will be a beautiful warm day, and I'm going to remove it all the way off and check it thoroughly for cracks. By the way, my check engine light hasn't come on again, which is great, but those fuel trims haven't changed, and that's not great. I'm doing a mpg test to see if my mpg improves with the new pump.

VStar650CL wrote: ↑

Wed Feb 16, 2022 1:04 pm

Wadeh62 wrote: ↑

Wed Feb 16, 2022 12:39 pm

How can I get access to the Service Manuals again? I had access before I joined, and suddenly I no longer could access them. I thought I would be able to access them after I joined?

AZhitman wrote: ↑

Tue Feb 15, 2022 8:40 pm

Almost there, should be back up within 48 hours. Thanks for your patience!

Thanks!!!:^)