PO171 AND PO174

I have a 2005 tocoma prerunner 2wd 4.0L,130k miles. I have had the dealer diagnose the codes 3 times.
The 1st time they said it was the MAF which I replaced with a denso MAF.
The codes came back within 100 miles.
The 2nd time the dealer said it was the upsteam O2 sensors which I replaced with denso sensors.
The codes came back within 100 miles.
The 3rd time the dealer claimed that I would need to buy toyota denso sensors from them(claimed a part update)or the codes would never go away.Strange the originals lasted 130k.?
I paid them to diagnose and I replaced parts.
All the while I have replaced the PCV valve,used 6 or 7 different injector cleaners in the gas,took the airbox off searching for air leaks(even ran black silicone around manufacture seam of airbox),replaced o-ring on dipstick,checked seal of gas cap,checked all vacuum hoses,searched intake and re-torqued,sprayed carb cleaner and starter fluid on every hose and gasket listening for an idle change to no avail.
The engine has a very slight throttle pulse or modulation under normal acceleration and none when you accelerate harder.
I asked the dealer each time to check the fuel pressure but don't know if they did.
HELP!!

 

Where is a common area that a vacuum leak is found?

 

so its showing lean on both banks right? sounds like your on the right track looking for leaks. . . that is kind of a generic pair of codes. what else could make the truck run lean? i would verify adequate fuel pressure, and wait for someone to chime in with some more obdII experience lol. sorry im not much help with this but heres a bump

 

I would say do a fuel pressure and flow rate test. Also, a map sensor can cause this issue. Do you have access to a scanner or higher end code reader that you can read your PID's off to me.with?
Edit: I'm pretty sure you can get a fuel rate tester to rent at o'reilleys

 

No map sensor. The likely hood of both AF sensors going bad at once to throw off both banks is unlikely but possible. I too would search for a vacuum leak, then I would perform a fuel pressure and volume test. Toyota fuel rails suck and I don't recall us having a port to even connect a loaner fuel pressure gauge. I usually use my high dollar set up to put in line with the factory connectors.

Generally with a weak fuel pump on the Toyota Lexus trucks, you will notice a lack of power during a higher power band or just when accelerating up to highway speeds. Any of that happening?

 

With the mileage you have on the truck, I would be either suspicious of the cheesy o-ring style gaskets that seal the plenum to the lower intake, and the cheesy o-ring style gasket that seals the throttle body to the plenum. Spray those really well.

There is also a really old bulletin from 2005 for updated AF sensors. The Toyota part number is the same as the old sensors, but that doesn't always mean the part was or was not changed. Maybe that is why they wanted to put the new AF senors in.

There is an additional bulletin for the same codes that requires replacement of the fuel pump for no start in cold weather and or lean codes present.

 

Yes,it does have a slight lack of power when accelerating up to highway speeds.

 

As mentioned, P0171 and P0174 are both lean codes. you mentioned spraying cleaner around the intake and such and listening for an idle change, however if the leak is not a large leak, your idle will not change. In some cases, a small leak that can set a lean code will actually seal up a tiny bit when a liquid hits the leak area, and will not get a full "pull" of the added spray. I have dealt with these codes since OBDII came out. What you need to do is get a small bottle of propane (like the camping type) and a torch that screws into that type of bottle. Then connect a scanner that can show you fuel trim numbers. Have someone watch the fuel trim while you open the valve on the propane torch and move it around the intake areas. DO NOT LIGHT THE TORCH of course. Just let the raw propane come out. A leaking intake, even a small leak, will draw this vapor in and you will see the fuel trim climb towards the rich end.

 

Yes, I did read that you had to cut a seal and attach inline to check fuel pressure.
I have a friend who owns a shop and is a ASE master car and truck tech that I could have to get to check fuel pressure and flow. He could also scan for the PID's for values.
If I can't find a vacuum leak I will need a good tech to do the scan and fuel tests.
I now believe its not the building(Dealer)but the tech thats performing the diagnostics.

 

Thanks everybody I fixed the problem!!!!!!!!!
It was the fuel pump and screen/filter.
I have never had a pump problem unless it just quit.
I think the screen was clogged because the pump never quit.
The screen/filter is attached to the pump and very small.
The tank was clean as a whistle.
The dealer ran tests twice and insisted it was the fuel ratio sensors and they were wrong.
Each time I asked them to do a fuel pressure test but they did not.
The computer tried to richen the mixture to the point of setting the codes.
Starving for fuel.

 

Glad to hear you got the problem corrected. All suggestions here were good ones.

 

Hello Everyone, I know this is an old thread and I'm not sure if I'll get a reply but it's worth a shot I guess. I have a PO171 & PO174 code on my 2015 Toyota Tacoma (4.0 V-6). I've replaced the MAF, had the engine tested for pressure (which showed to be all good), and filled the system with smoke as to find any potential vacuum leaks. I didn't find any leaks in the system and the replacing the MAF didn't do anything. I also tested the ignition coils (no issue found), cleaned the throttle body, and fuel injectors which seemed to help for a few miles but then the truck went back to driving sluggish. It seems like my Tacoma drives great at certain times but other times will shake as if it's about to shut off similarly as if a vehicle is running out of fuel. The mechanic who tested the fuel pressure for me said it couldn't be the fuel pump because the engine pressure was good. I'd also like to mention I had additional codes which may have relation to the ones mentioned: 01604 Startability Malfunction, P1605 Rough Idling, P219A Bank 1 air-fuel ratio imbalance.

From reading everyone's comments it seems like a possible solution is replacing the fuel rails, replacing the fuel pressure regulator, and eventually the screen filter/ Fuel pump.

Pls help.

 

I should have checked the most obvious fault first. The air intake boot had slipped off the throttle body. Fortunately I did not put a lot of miles on before finding the issue but not before buying a new MAF sensor.
https://drive.google.com/file/d/1x8KrThv27CHMML67VN7XEu8oAW3H0xwO/view?usp=sharing