Fuel Trim Question- Poor Mileage

Hello, I have a 2005 Toyota Tundra 4.7 v8. I've owned the truck for two years, and it has consistently gotten 460-480 kms per tank (about 16.5 l\100km). Out of the blue, over the last three tanks, I can barely get 380 kms per tank. That works out to be a 20% drop in mileage. That being said, the truck is running perfectly. Does not miss, stumble, nothing. No CEL displayed either. I am not hauling or towing anything either. Same route to work as well. I simply can't explain it.

To diagnose, I connected a Blue Driver scanner and checked live data compared to what the trucks service manual specifies. The O2 sensors all look fine as well as the MAF sensor. The only thing I did notice, was that the LTFT figures were fairly low at idle and just above idle. Once driving they return to normal. The attached screenshot shows that bank 1 and bank 2 long term fuel trim are showing -24.2% and -22.7% respectively at idle.

What I have read mentions that this would show that the truck is running rich and the fuel trim is trying to compensate, but only at idle.

Where should I start in trying to determine the issue?

That is cutting A LOT of fuel... I want to see both 02 sensors though. If it is cutting that much fuel, your 02 should be reporting rich. Since it is on both banks, I wouldn’t really put much time into thinking it’s an 02 problem. Other PIDS that I think are important would be MAP, Baro, ECT and IAT.

What kind of fuel system is it?

It can be getting to much fuel pressure, or have a leaking injector? A simple fuel pressure test can give us more direction.


A stuck open EGR could cause it to run rich but if this was the case would likely idle rough so I don’t find it that likely in this case.

fuel trims are all over the place... negative LTFT's and positive STFT's at idle. If i'm reading your scantool correctly, STFT's are showing -20.3 thru +10.9 on bank1 and -14.8 thru +8.6 on bank2.

Neg LTFT's that get better with RPM points towards a fuel delivery problem. As John Curtis says, more info is needed... with engine at full operating temp get this info at idle.
STFT both banks
LTFT both banks
02v B1S1
02v B2S1
Fuel Pressure KOEO (engine not running)
Fuel Pressure KOER (at idle)
Fuel Pressure at idle (remove vacuum from pressure regulator)
Fuel Pressure as you raise the RPM to 2500

Ask and ye shall receive.

For some reason scan tool doesnt show fuel pressure in this truck. I'll get a manual reading tomorrow.

The rear oxygen sensor reads dead lean it looks like at .210 while both the front air fuel ratio sensors are reading 3.3 roughly. If it’s truly running that lean I’d expect the voltages for the front oxygen sensors to be around 4 volts or more. (Higher voltage is lean lower voltage is rich) try forcing them rich and lean. The mass airflow looks like it’s reporting kind of a little high at idle slightly

What immediately sticks out to me is that the ECM is cutting a lot of fuel while at the same time, the sensors are reporting lean (which is contradictory).
The first thing that comes to mind, is fuel. If it has the correct pressure, does it have the correct volume? Is the fuel quality off? (Do you use the same gas station every time?)

The reason why fuel quality comes to mind is because I seen high negative fuel trims and lean reading 02s on a vehicle that had the wrong type of fuel. Many shops were having cars with no start issues as well and it was all sourced back to the same gas station.

Waiting for those fuel pressure readings. LTFT's have compensated for the rich condition, STFT's are at 0. LTFT's get better with RPM and no codes. B1S2 is a bit off but could be a secondary factor.

Thank you ScannerJohn. Apologies, busy time of year, I haven't gotten the fuel pressure readings yet. However, I did run a full report while driving to work this morning. The attached Excel files gives all the data the Bluedriver reports. I have highlighted the times where the truck was idling. Maybe this will help?

Hi, would you be able to post another log file, with the same drive patterns putting the engine through its paces, especially a WOT run, BUT, save the file as an Excel (.xls) instead of the .pdf?

You can change that in your Bluedriver settings.

Then, I'll post some pretty graphs for everyone to analyze!

Sorry, was not pushing you to get data... I was responding to John Curtis post. I was saying I was waiting to see the results of the pressure test before making a diagnosis.

It's Christmas time... MERRY CHRISTMAS ALL.

I actually have it saved as xlsx already but the site would not allow me to attach that file type. Had to be pdf.

Ah, that's to bad, I remember analyzing BD files here before!

Maybe upload to a cloud server and post a share link?

Merry Christmas!

There are paste-bin sites, like pastebin.com that allow you to upload files and then post a link to us here

UPDATE.... I have run a tank of Super 91 Octane and have seen a small increase in mileage. (Again, the truck has consistently gotten between 460-480 kms per tank (i track with fuelly.com). The last few tanks of 89 octane have been resulting in no more than 380 kms.) The latest tank of Super resulted in 440 km per tank. A small increase, so I am still not sure if the higher octane fuel is compensating for the winter blend gas (if that is the issue).

I have also uploaded the Blue Driver data to the site below in Excel format. I haven't been able to retrieve the fuel pressure, as the truck has no port to connect a fuel pressure gauge. I need to buy a special banjo fitting to do so.

BlueDriverData