Hey all,

Looking for a little help if anyone has experience that would help me turn this information into something useful.

I have 2003 Mazda Protege that is running rough sometimes and stalling.

The problem happens at regular intervals when idling - it will be idling fine, and then all of a sudden start stumbling (if it was in gear it would stall and freak out my wife)

There check engine light is on and the code is P0171 which has these possible causes:http://engine-codes.com/p0171_mazda.html

Using an OBDII reader with the Torque app on my Android device, I have observed the following information: at idle:

MAF 1.6g/s (stable)
Load 15.3%
STFT1 25% (THIS DROPS TO 0% WHEN THE ENGINE STUMBLES)
Timing Adv 4.0Deg (SPARK ADVANCE GOES NUTS DURING THE STUMBLING)
F/T 1x1 25% (THIS IS THE SAME AS STFT1? IT DROPS TO 0% WHEN THE ENGINE STUMBLES)
02 1x1 0.0V (Could this mean one o2 sensor is shot?)
02 1x2 0.2V
LTFT1 -5.5%

Fuel status goes from "Closed Loop, using 02 sensor for fuel mix" to "Open loop due to system failure" when the engine stumbles.

Ones it does that, it will stay there and run rough - as soon as I rev the engine a little, it reverts - the STFT1 and F/T 1x1 go back to some value, and the fuel system status goes back to closed loop / using 02. Then, after a short time idling, the engine stumbles and round and round we go.

Any advice would be greatly appreciated.

Thanks

The car is 12 years old, try this old school suggestion.

In my day the first questions/solutions were, when was the fuel tank last drained ? Try fuel cleaner/meths in the tank to deal with the water, drain the tank, fill the tank with super, drain the float bowls in the carb too cus that all gives the same symptoms. Just no error code.

(Yes, new cars don't have a carb !, so how munted are the injector nozzles ?)

considering it seems to idle fine for the most part id guess its some sensor error

but at least the first issue from the list is easy to check
get a spray can with some form of alcohol in it and spray it over the intake air pipes
if the revs go up youve found your leak

looks like an error of the O2 sensor. Once the ECU loses its signal it goes into limp mode to safe the engine, runs mega rich and stumbles as you call it ('open loop' in general means running without sensor feedback and control)

Could be an all out sensor error, or just a lose or corroded cable. The O2 sensors sit pretty exposed to nature (water and salt, dirt and stones)

Thanks guys, that helps. The car doesn't get driven far very often, mostly used to drive kids to school in the winter, which is close. So I am wondering if the first O2 sensor is very fouled or just failed.

If the system runs mega rich in safe mode how come the error is that the A/F ratio is lean?

Edit: also wondering why it would come back when some throttle is applied? Could that indicate said O2 sensor is dirty but gets enough exhaust moving across it to get a reading or something?

I used to drive a BA Protegé a while back and as far as I remember from others' experience the MAF sensor could occasionally fail and cause all sorts of weird behavior. One of suggested diagnostic tricks was to disconnect the MAF altogeher. The ECU would then switch to some fallback maps which should work better than when the ECU is fed with bogus data from the faulty MAF.

EDIT:
I did some Googling on the issue and I came up with two articles on the topic, especially the second one seems interesting. The upshot from the articles is that a vaccuum leak is probably the cause since the car is having problem at idle and not at higher revs.

http://www.obd-codes.com/faq/fuel-trims.php
http://wiki.ross-tech.com/wiki/index.php/Fuel_Trim_Info

As noone replied here in a while, I guess it would be alright to borrow this thread.

So, here's the deal. My father's Citroen C4 suffered a fuel sensor (let's just call it that ) failure after a traffic accident. It isn't completely dead, but it shows 10-15L more fuel than there actually is in the car. My friend tried plugging an OBDII reader in the car but it did not show any errors concerning the fuel sensor. The mechanics at the Citroen service say that the sensor itself is okay (I later found out that these guys suck, so they may be wrong) and that the issue is related to the car's electronics.
(If I had to bet, I'd say that the mechanics suck and that the sensor itself actually is faulty. (Some people will now say that I should let them do their job since they know more than me, but I saw what they did to the car, so I actually have a good reason to say that. I also met a former colleague of theirs who says they suck. Bad luck Brian: I met him after the car came back from the service.))

So, to get to the question(s). If the issue really is electronics related, shouldn't the OBDII reader find it? What application (may be Win or Android app) would you recommend, to get best scan results. Are there any tips and tricks to get more info out of the OBD, since it doesn't show an error that is obviously there.

Quote from matijapkc :

So, to get to the question(s). If the issue really is electronics related, shouldn't the OBDII reader find it? What application (may be Win or Android app) would you recommend, to get best scan results. Are there any tips and tricks to get more info out of the OBD, since it doesn't show an error that is obviously there.

Dealer level (mark specific) diagnostic equipment is in a whole nother league compared to a standard OBD reader.

My car shows live, wheel independent speeds as determined from the ABS sensors. If one sensor goes bad, you might not get any kind of warning on the dash and no error on any kind of basic OBD reader. But using the right equipment you would.

Oh, I get it.
Any thoughts about that fuel sensor? Should I recheck the sensor itself, or should I go with the electronics? (I know you can't be sure, but I'd just like to hear your opinion on that.)

Quote from Ball Bearing Turbo :

Thanks guys, that helps. The car doesn't get driven far very often, mostly used to drive kids to school in the winter, which is close. So I am wondering if the first O2 sensor is very fouled or just failed.

If the system runs mega rich in safe mode how come the error is that the A/F ratio is lean?

Edit: also wondering why it would come back when some throttle is applied? Could that indicate said O2 sensor is dirty but gets enough exhaust moving across it to get a reading or something?

The o2 sensor reads during the closed loop and once rpm's rise, the maf sensor reads during the open loop.

Try cleaning the maf sensor near the airbox with "contact cleaner" otherwise it's time for a new maf sensor.

Perfect, I have some contact cleaner at home thus will give it a shot.

Thanks for that, and thanks MadcatX for the above advice as well.

I've done exactly nothing else yet due to time constraints but that shouldn't take very long :P

Update:

Fixed.

There was a hole in the intake hose which runs between the air filter and intake manifold. By hole, I mean it was basically destroyed, so it's been sucking in plenty of unfiltered air for who knows how long :/

Popped the hood to have a serious look today, noticed that in about 30 seconds, made two phone calls and was out and back with the new hose installed all in less than one hour from start to finish.

Thanks again for the input in this thread.