Hi guys. About a week ago i noticed that i made 30-40% less on a full tank than normally i make(10-12 mpg). I started digging all over the forum to find some answers. I definitely found answers on what to check(clean maf, coils, spark plugs, cat, vacuum line leaks etc...). As of now i was not able to find leaks in the vacuum lines, I cleaned the maf and i am going to work on coils and spark plugs this weekend.
My problem is that i checked the OBD readings yesterday and behavior of the MAF and STFT(short term fuel trim looked pretty weird to me). Unfortunately i forgot to take a picture so it's kinda hard to explain.

Can anybody who has an OBD post a picture of your diagram for STFT and MAF? If you can post other readings(temperatures etc) it will be just awesome.
If I will see some big difference i will know which way to dig Thanks in advance.

It's 2004 manual with 56k miles. Was taking care of this car a lot. I feel like i was servicing it more than i was driving it

 

you already mentioned coils and plugs ... just start there, no need to dive into fuel trim numbers yet (not even sure why you would want those numbers as they would just reinforce that there is a problem, not show where the problem is)

also check your cat to make sure it isn't clogged up.

 

The easiest way to check fuel trim accuracy is while idling and steady state cruising (cruise control on flat ground is best). The LTFT will be steady, and could be just about anywhere. The STFT should be 0%, plus or minus 1%. If it's any farther off than this, it means that the engine is pulling air that isn't going past the MAF, so it isn't adding fuel for that air, but then the O2 sensor is seeing the lean condition, so it's commanding the STFT to add fuel to compensate. You would have a vacuum leak at that point.

 

Thanks for you advises.
paimon.soror, I didn't ask for numbers, but for visual diagram. I want to see how the "healthy" one looks. So if anybody could post a picture of healthy diagram i will appreciate it a lot

Mine behaves like there is a leak(MAF malfunction) but I was not able to find a leak. I am not that experienced in that so I could miss some spots which should be checked.

What's the easiest way to check if the cat is clogged? Do i necessarily have to take it off or there is some way to check the exhaust output pressure?

 

Taking a picture of "STFT: 0%" is largely the same as just typing "STFT: 0%"

Not sure what else you are referring to as a "diagram". It's a variable number that changes based on the commanded equivalence ratio (or AFR) that the front O2 sensor is seeing at that moment in time.

 

When i was checking yesterday my STFT there was 2 options. FIrst one is to look on the numbers and second one is to click on it and see the line graph of real time changes. It will show the changes within 10-15 seconds. I want to see how the healthy car changes look like.

So it's supposed to look like that - http://cdn.2carpros.com/question_ima...jpg?1327922847

The way it changes in real time looks on mine weird to me that's why i am asking for the pic. Not for a number. If i will be able to do that again today I will post a pic of mine. Sorry maybe my bad English is a reason of misunderstanding here

 

Ah, makes a bit more sense. Many ODB2 tools don't have a graph over time option, and usually just display the number. The graph over time is just showing the number on the Y axis and the time on the X axis, so it's largely the same thing, just need to look at it in a different perspective.

The STFT should be a flat line when the engine is in a steady state. The graph will change with peaks and valleys if you start changing throttle inputs, and it's going to be entirely based on what you are doing.

If that is what your graph over time looks like when at idle, you have one or more things wrong. It should be a flat line.


Edit:
Most ODB tools have a display that looks more like this:

Note: This image is just grabbed from the web. No idea what car that is, so these numbers should not be looked at as important to you at all. Just an example of the display.

 

Mine was changing when idling. It was not a flat line. The engine was almost at working temperatures. It was hot, but then i spent like 10-15 minutes in the store and after it i checked. WHen i pressed the pedal stft was going just flat, but when idling it was going all over between -4 to +4 and once it jumped to 12.5 for just a moment. I finally found which resistance i should have on the MAF, so today i will test if it's faulty. If not, than next thing to do it coils, spark plugs and more researching for vacuum leaks.

Do you know if there is any way to test clogged cat without taking it off the car?

 

That definitely sounds like a vacuum leak. Possible to be the MAF, but usually a vacuum leak when the STFT is anything but flat and also staying between -1% and +1%.

Some of the common vacuum leak points are:
- Lower Intake Manifold service nipples have rubber caps that can fall off
- vacuum lines from the upper intake manifold crack from age at one end or the other, or rub against something creating a hole in the line



A cat clog is a restriction against exhaust flow. Since exhaust flow is needed for power (air + fuel = power, more power = more air, more air = more exhaust), a clogged cat will act as a "cap" to the amount of power you can make. It is exceedingly rate to manifest at idle, as idle only pulls about 5 grams of air per second, vs full throttle pulls more like 200 grams of air per second. A clogged cat affects MPG in that it makes you tend to push the pedal down more to maintain the same speed (basically trying to pressurize the exhaust to force it through faster). A nearly completely clogged cat will drop you to about 9mpg by the time it's limiting you to ~55mph. A completely clogged cat won't let you drive. It can go from partially clogged to completely clogged in a few hours of driving. I don't think a clog is your problem here, but it's still possible.

There are plenty of other easier items to check.

 

I don't think it's cat either. I don't have to press harder to maintain normal speed. I just figured out that the warranty for the cat expires within couple month. So i will take a look and if it's bad i can replace it under warranty Thank for the notification about rubber cups. DO you have a picture of their location?

 

Not on hand, but I know there are several around.

Just find the MAF on the accordion. Follow it to the throttle body, then down the black plastic upper intake manifold (UIM), and as it curves down next to the engine, it mates to the lower intake manifold (LIM), which is metal. On the 2 main runners visible on the LIM, there are 2 metal ports that stick out toward the fender with the caps on them. Only about half an inch long.

 

Thanks a lot for help. i definitely missed that spot yesterday. Will keep you guys updated. I saw so many threads about various fuel consumption issues, but nobody actually posted the resolution of their problems So if i will be able to find it myself and fix i will definitely post some pics and info on it

 

So far i have 85-90 miles on a new tank and the gauge is showing a slightly more than a half. My gas meter is pretty unaccurate. Normally first half of the tank goes really fast and the second half lasts much longer. I don't know for sure yet, but it seems like cleaning the MAF was the only thing i had to do. I'll have more accurate results pretty soon, but as of now i do really believe i will make my average 210-220 miles on it

Even if the problem disappears it's good that i had it. I learned a lot of things about maintenance and finally i've got blutooth OBD No more driving to autozone to check the codes

 

So my problem disappeared after cleaning the maf sensor. Then i drove couple tanks with normal consumption and all of a sudden i noticed that the last one was like 14-15 mpg(not 10-12 like it was the first time). The only thing coming to my mind is that first time my problem appeared when it was 80-90 outside. WHen i thought it's fixed by cleaning the MAF, it was 60-70. Right now it jumped to 80-90 again and the problem is here again.

Do you guys know if there can be any component which can mess up the injection because of hot temps?

 

Sub for later use. Ive noticed my AFR's run from 14.7-13.8 sometimes while
In cruise control. I'm thinking it's my front O2... :/

 

Forgot to post an update guys. There was 2 reasons for my low mpg:

1. MAF sensor needed to be cleaned
2. Front rotor trailing spark plug was dead. Changed all of them plus coils...
https://i.imgur.com/K4PER.jpg