Gas Mileage Issues

I am having issues with my 99 3.4 5 speed Tacoma. Started in the middle of winter gas mileage suddenly dropped and has not changed with summer gas. No check engine lights. Spark plugs/wires replaced, maf sensor cleaned, new 02 sensors, cleaned throttle body/IACV, no vacuum leaks found when smoke tested the intake manifold, new catalytic converter. I have no idea what is the issue with truck but would like to fix it. I have access to live engine data and the tachometer seems to be off. The bore of the throttle body is also worn a little. Gets only about 70 km on the first quarter tank of gas it used to get about 150. Gas has become very expensive up here in Canada open to suggestions. thanks for reading

 

fuel trims can tell a lot. Should be around 0. Long Term fuel Trim (LTFT) + or - short term fuel trim (STFT) gives the overall fuel trim. If you OBDII reader can read the engine coolant temps and the intake air temps that is an indicator too. Sitting here thinking... check to see if there are any dragging brakes too. I have researched and have yet to find if the programming for Toyotas allows for the resetting of the barometric sensor in the system. On other manufacturers this is done by 2 short wide open throttle runs. I have heard of people complaining that the vehicle ran great on our trip and now that we are back from the mountains it runs like crap... just spitballing the easy and free things first.

 

On another vehicle I had a cracked exhaust manifold do this. The venturi effect pulls fresh air in through the crack, tells the o2 sensor the engine is lean, and causes the computer to add more fuel. I think it happened because the manifold got drenched when hot and cooled unevenly.

 

As far as drive train no brakes dragging, hangar bearing is making noise so i am changing that the alignment is off but i was planning on getting Light racing uca's and bilstien 5100's so after i install those the alignment will get fixed. As far as fuel trims go they seem to be normal without any definitive numbers for my truck specifically. I have not looked at the manifolds recently but i will test them to see if they leak. Thanks for the help any other suggestions would be appreciated.

 

the other suggestion that I can make is to do a leak down test on each cylinder. You can pick up a decent leak down tester at harbor freight and with a small air compressor you can do this test yourself. This is a check of how well the motor is sealing internally. If there is leakage she just might be worn a bit... does she burn any oil between changes? You never mentioned the last time the valves were checked/adjusted... Do the P/S first it is the easy side. The thing that I am struggling with is the suddenness of the issue. Tire air pressures/new tires, different oil? As a long shot wag (wild ass guess) try a different gas station. I have only run Chevron in all of my vehicles since the early 80's. I was a Porsche/Ferrari/Audi tech and back then, it made a BIG difference in the amount of carbon deposits, especially on the turbo cars. Chevron, Shell and others are what is referred to as Tier 1 fuel. Again, just spitballing here.

thinking about this some more... I dont know that I would trust using the gas gauge as a monitor of mpg, the resisitance of the sending unit could change as the float wiper wears. Do the mileage and liters calculation over a period of time.

 

93 vs 43 (converted km to ~miles) on 1/4 tank
my recent mileage issues have not gone this far down
I am back to over 300miles per tank with ease
few months ago it struggled to get to 270

 

My next item that i am doing is checking valve clearances and timing. I have tried different gas stations and no help i am going to leak check all exhaust components and what not i do not have access to an air compressor so i cannot do a leak down test but i will keep posted. Any other ideas would be appreciated it was no that sudden it dropped over 2 months or so.

 

I had an speedo/odo failure (in another vehicle) once that first appeared as not a full failure, but wonky ODO recording. At the time I was traveling for work to non-routine locations and I wasn't catching the discrepancy. One day I drove a good distance of a known mileage and noted the ODO was way low, making my MPG way off.

These days that's easy to check with phones or GPS units. Then, not so much.

And you didn't happen to have a tall tire change around the time your MPG fell did you? That can throw a several % error in the mix.

 

I would never use the fuel gauge to try and calculate mileage

My mileage is pretty constant between 10 and 12mpg since I bought the truck

 

I did change over from 31's to 33's after the mileage issue had begun already but i have checked the speedometer with a gps and it says that it is ony about 2km/hr higher. Is there any over to check mpg ?