Ok I have 2 codes p171 and p174 but my truck runs fine. The gas mpg has droped to 12 mpg. My thoughr is one of the oxygen sensors has gone bad

Quest 1. Where are the sensors
2. how to test
3. Am I thinking the right way.

Im searching the threads all i found was trucks runing badly

Thanks Phil

 

Intake problem, MAF possibly, first thought for me is also O2 sensors. What year is your truck?

 

98 4.0 auto, but it runs good

 

Well it's not written in stone it'll run bad. It runs bad as the condition gets extreme.
Check out this link:
http://www.mr2.com/TEXT/O2_Sensor.html

 

Possible intake leak or MAF like stated before. Coolest way to check for intake leak is with a smoke machine. quick and simple, but not available to everyone unfortunately.

 

When DTCs P0171 and P0174 suddenly appear together, it's hardly ever attributable to faulty 02 sensors. For one thing, the odds of both upstream sensors going out at the same time are pretty slim. Secondly, those codes, while related to the fuel management system, do not tell you that there is anything wrong with the 02 sensor units or circuits; they only indicate that the sensors are encountering and/or reporting a lean condition in both banks for which the PCM can no longer compensate (i.e., long-term fuel trim limits have been exceeded).

It is wise, therefore, to approach the problem from the perspective of a lean condition first, before condemning the 02 sensors or their circuits. Lean condition = too much air or not enough fuel, or a combination of both. Hence, you want to look out for the usual lean suspects, some of which have already been noted: upper or lower intake gasket leaks, leaks at the vacuum and air hoses/connections that run to and from the upper intake, PVC system leak, EGR system leak (or system malfunction), dirty MAF sensor, and also fuel-delivery issues such as clogged filter/injectors, weak fuel pump, faulty regulator, etc. A fuel pressure check would be a nice thing to do if possible.

 

I like Rockledges summation, but rellik I'm a bit confused by your saying it still runs good, but the gas mileage has dropped off to 12 mpg.

Seems to me if you had a vacuum leak so bad that it leaned the mixture to the point that mpg droped to 12, you'd be having & hearing spark knock!!!!

So do you hear any spark knock at all????

 

Well, I could atribute the 12 mpg to idle time it has been cold her, i like to let it run 4 or 5 min before i go. The morning it started it was 11 below zero. No it runs fine.

In checking it out to day i found the oil fill cap lose.
I cleaned the mas air flow sensor.
checked for vacumm leaks, every thing looks good.

I will update the mpg in a couple of days,

My volt meter died today and will check the mass air flow and o2 sensors in a few days

 

Check out this thread...same problem.https://www.ford-trucks.com/forums/s...7&goto=newpost

 

Same problem runs great, used a whole can of carb cleaner checking for vac leaks. (cleaned the MAF/ and PVC) No luck. Cleared the codes, came back the next day.
Now yapping at someone about this he mentioned that since this truck gasses up between NY and PA that the differance in gas may be the problem. Any thoughts?

 

Welcome to FTE.

Are you saying you have codes P0171 & P0174 too???

If so, good idea to look for a vacuum leak, it could still be in the PCV valve mechancaly sticking open, or not sealing well, so if it's time, just replace it.
Could be it's gromet, or hose fit is loose, or split, or maybe you have a vacuum leak in the vapor recovery system, brake booster diaphragm, or intake manafold gasket.

Cleaning the MAF sensor was a good idea, did you use a non residual spray cleaner, like CRC MAF sensnor spray cleaner????

Or maybe you have low fuel pressure????
Or maybe lazy O2 sensors, or some combination there-of.

Not likely that the gasolines from those two states would cause a lean code imo.

More thoughts to ponder.

 

Its probally not the gas. Its possible that the maf might be bad.

However if swapping the maf doesnt work you might be in for some fun. My 98 4.0 had the same problem and it turned out to the the lower intake gaskets.

What year, engine and mileage?

 

99 4.0 136000

 

With that mileage, the PCV valve is due for replacement & the O2 sensors switching speed should be on your suspect list, as most folks replace them between about 75K-100K miles, because as they age their switching speed gets slower, so they are said to be "lazy", sorta like pawpaw!!!! lol

So if you can get a scantool that'll read PID's, hooked up to the diagnostic port & give us some PID readings like the O2 sensors switching speed & voltage switching range, short & long term fuel trim numbers, MAF sensor PID to the computer & post ALL KOEO & KOER trouble code numbers you may have, that are set, or are pending, we may be able to make better gusses on how best to proceed.

More thoughts for pondering, let us know what you find.

 

OK a little more info
99 Mazda B4000 SE 4.0
136000
2nd owner.
I have had it now for 5 years and have done very little work because it runs great with very little problems.
While putzing around yesterday looking for vac leaks and such i found this,

Still connected but not plugged into anything. Any ideas?