Faulty Fuel Pump? Please Help!

For learning purposes and trying to help others figure out the infamous P0171 I figured I would update everyone with how replacing the fuel pump on my truck did in fact clear the P0171. The truck was having starting issues after being ran for awhile, shut off, and then would struggle to turn back over. I replaced the fuel pump with a new denso pump and strainer. I have not had any issues since with starting after running the truck for awhile. Feels good to diagnose the right part the first time! However, as soon as the P0171 code cleared, a new one was present for the O2 sensor after the cat. Guess I will be replacing this in the near future, although I am guessing this O2 sensor is just reporting whether the exhaust gas is meeting emission levels. So, does it really matter? Thoughts?
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Original Post:

Hello,

So, I have been dealing with the P0171 code (System too lean bank 1) for awhile now, and it seems to be an infamous one. Did a lot of research and I understand that it can be related to a malfunctioning/dirty MAF, any leak between the intake and MAF, O2 sensor, lack of fuel (e.g. bad pump or clogged injectors). The truck will sometimes have some serious trouble getting started, and when I finally do get it started by giving it some gas it will want to idle rough and shut down if I don't continue to give it gas. I believe I have a bad fuel pump, but just wanted to post here to see if anybody else has had a very similar problem, and agrees with my diagnosis. I have been methodically going through and trying to check all these things. I have done the following...

1. Disconnected battery, pulled MAF and checked resistance with multimeter to confirm it was working, and also confirmed while driving with an OBD scanner. I also cleaned the MAF before putting back in. Then, when the battery was reconnected, I pulled the EFI fuse and put back in because I've read that it will reset the fueling values? (Figured it was worth a try?) Check engine light went away for about 200 miles, but then came back

2. Changed the air filter just to make sure the MAF wasn't getting dirty and it was about that time as regular maintenance. I do not use the K&N reusable filters because I've heard bad things about those getting the MAF dirty. Use FRAM replaceable ones.

3. Found very small (about the size of a pin head) hole in the airbox, and put electrical tape over outside.

4. Tried checking resistance of the pre-cat (bank1 sensor 1) O2 sensor, but unfortunately couldn't enough of my hands on it enough to get it apart. If anybody has any suggestions on how that thing comes apart let me know. I don't have a lift or floor jack. Just the one that comes with the truck, so it's difficult for a large guy (6'4" 250lbs) like me to get under. I believe it was replaced at approximately 130K miles anyways, so I don't think it would be bad yet.

I have a long 10 hour drive coming up soon, and it seems like the truck typically has issues starting after driving for awhile, turning off, and then turning back on in a short period of time. The truck has 178k on it, so I figure it's about time to replace the fuel pump anyways, but want to make sure I'm actually fixing the issue. Battery is fresh and has 750CCA. I run sea foam through after every 4k oil change, so I don't think it's injectors, but obviously a possibility.

Thanks for any help.

 

You hit every point except for the one you need to troubleshoot the pump haha If you think the pump is going bad you need to check it for output/maintained fuel pressure at the outlet on pump to the fuel rail feeding the injectors also fuel filter not sure on specs

 

did you change the fuel filter?
I had problems with that code on my 98 tacoma and it was the pump, I had rust in the baffle area of the tank that had clogged the pump screen.
Had the same code on my 02 tundra except it was for both banks (p0171 and p0174) i replced the pump and the filter both at the same time, no cel since.

 

Make sure the EGR valve isn't stuck open or closed.

 

It's your fuel pump for sure. I had all the exact same symptoms and it took me months to figure it out. I cleaned and cleaned my MAF, replaced my TPS, air filter, fuel filter, cleaned my throttle body. Turned out it was the fuel pump all along. Had it replaced and she's run like a dream ever since.

 

Thanks for the replies. I forgot to mention that I did change the fuel filter as soon as this problem started happening. Sounds like it is the fuel pump or clogged screen. Not sure if I'm mechanically inclined enough to check the fuel pressure supply myself. I found one "how to" that looks like it's out of a Toyota service manual, but I don't know if I'll have time to get that done myself this week before leaving for 10 hour trip on Friday. Might just take it to a mechanic with the parts and hope for the best. New to the area I'm in, so I don't know who's reputable.

 

This is the reply I was hoping for! Haha. Thank you I appreciate it!

Did you replace it with a Denso, Bosch....?

 

For shits and giggles I'd try a new (or another) fuel pump relay, first. You never know.

 

I didn't think about that. Thanks

 

Happy to help. As for the brand, I don't remember. I always go OEM if possible on this kind of stuff. So probably Denso?

 

Spend the $50 to have a mechanic check it over. I had the same code and changed out 10x that worth in parts before taking it in. Mine ended up being the fuel pump, but 5 other threads it was O2 sensors, 5 more it was fuel injectors, 5 more it was MAF, ..........

As far as the fuel pump goes. Go through rock auto. I got a Denso pump for cheaper then any auto part store brand.
I did it by taking off the bed instead of dropping the tank. I've dropped tanks before, but felt this way was easier.

 

Thanks for the input. I think in my case I have it narrowed down to fuel pump or injectors because I checked the MAF & O2 sensor with a multimeter to confirm proper resistance specs and then used OBD reader to check operating values. I also changed the fuel filter. I already bought the Denso fuel pump & strainer from RockAuto because they were definitely the cheapest. I believe Denso is the OEM? Parts will arrive Wednesday, and should be installed by Thursday evening. I will post an update to let everyone know if this resolves the problem. I feel like I should do a write up on troubleshooting this issue after digging for all the MAF, O2 sensor, and fuel pressure specs. So-much-googling

 

For learning purposes and trying to help others figure out the infamous P0171 I figured I would update everyone with how replacing the fuel pump on my truck did in fact clear the P0171. The truck was having starting issues after being ran for awhile, shut off, and then would struggle to turn back over. I replaced the fuel pump with a new denso pump and strainer. I have not had any issues since with starting after running the truck for awhile. Feels good to diagnose the right part the first time! However, as soon as the P0171 code cleared, a new one was present for the O2 sensor after the cat. Guess I will be replacing this in the near future, although I am guessing this O2 sensor is just reporting whether the exhaust gas is meeting emission levels. So, does it really matter? Thoughts?

 

Glad you got it fixed. Bummer you didn't see my thread earlier.

http://www.tacomaworld.com/forum/1st-gen-tacomas/330233-p0171-solved-finally.html