My short term fuel trim is 20%
 

I got P0171 short term fuel trim lean CEL code about a month ago. I got a lot of good suggestion and since did seafoam thinking may be a fuel injector problem. But, I am still getting P0171. The car runs ok, just a little rough idle and few hastiation. 2003 TL with 56K.

Today I got a obd-ii USB PC interface. What jumps out is my short term fuel trim is always close to 20%. Long term is 7%. The O2 sensor is jumping between 0.1v to 0.9v. Thanks in advance for all your advise.

Today my long term fuel trim went up to 10.61%. Anyone know what's intake manifold pressure? It read 30 Kpa at idle, from what I read the vacuum pressure should be at least 54-70 Kpa? I think I have a vacuum leak, will do the leak test this weekend.

My long term fuel trim went up to 17.7% tonight and the CEL is on with P0171. It seems no matter how much short term fuel trim try to compensate, the ECU keep asking for more fuel. The O2 sensor is going up and down between 01v to 09v and seems to be working. I now suspect the MAP is sending wrong reading so it always think there is more air in the intake, unless I have a huge vacuum leak?

i suggest replacing the 02 sensor

Wow I've got quite a bit to learn about my car. How are you testing these things? Cause I've noticed some hesitation in my car also...but I think it's just my foot not wanting to push things too hard. I'm not one for wasting my tires.

I got this from ebay http://cgi.ebay.com/ebaymotors/ws/eB...2BSI%26otn%3D4

Is pretty amazing and comparable with most all cars. The easyobd2(free) software is good. But I am still trying other software to get more data from my car. Now I just wish I can open up the engine cover and work on my problem :)

First of all, can you verify your fuel system is in closed-loop when you take your measurements?

Your O2 sensor operates as a switch...if the voltage read is > 0.5V, means you have a small amount of oxygen, so more oxygen is needed (which corresponds to reducing the amount of fuel). The PCM thinks you're running rich and as such reduces the amount of fuel (a negative STFT should result).

On the other hand..if the voltage read is < 0.5V, means you have too much oxygen, so more fuel is needed (which corresponds to increasing the amount of fuel). The PCM thinks you're running lean and as such increases the amount of fuel (a positive STFT should result).

This switching happens many times per second so that the PCM can "close-in" on the optimal amount of fuel that yields the best air/fuel ratio...that error eventually yields the long term fuel trim. For example, if with what the PCM is throwing at it, the O2 sensor is indicating 'too much O2', the PCM gradually increases the amount of fuel it throws until the O2 sensor now reads 'not enough O2'. Then the process repeats over & over to close-in on the 'perfect' amount of fuel to throw at the injectors (actually how long the injectors should stay open to be PC).

Assuming your O2 sensor (and wiring to it) is not faulty, when the PCM makes its corrections based on the O2 readings, and stuff doesn't change "as it should", that's where you have a problem.

Now, if you're getting positive long term fuel trims (PCM thinks it needs to dump more fuel), this means one of two things (assuming your O2 sensor is putting out good values)
1) Too much air, or
2) Not enough fuel

So for #1, you would check
- Vacuum leaks (MAP too low...e.g., it's not "sucking" good enough)
- MAF sensor faulty (not measuring airflow correctly)
- and other stuff related to air flow

For #2, you would check
- Gas system leaks and/or low fuel pressure
- Injectors blocked
- or other stuff that could lead to not enough gas ending up where it needs to be given steady conditions

Those are my thoughts. Keep in mind I'm not a tech or even a mechanic, so these are just educated guesses of mine; experienced mechanics can probably guess flat-out what to do but then there's the rest of us. I'm just someone who's had lots of problems too but likes solving stuff.

Nice to hear from you newperson!! I read your long thread about your problem extensively. It helps me a lot and give me confidence on finding the problem myself. Your diagnose is very accurate. I already clear and checked the fuel system, so today I will work on the vacuum leak etc as you suggested on #1 above. I will report back once the problem is fixed. Thank you!

Newperson.. We dont have MAFs on these cars.

My bad...I'm mad I sold my service manual.

slam_w2k, good luck; let us know!

I just checked the MAP and vacuum leak. The MAP looks normal and clean, it's just a sensor sitting on top of the TB. My Intake Manifold Pressure is 100 Kpa at idle, if I disconnect the MAP, the pressure went up to 171Kpa. When the car is running the kpa drop to around 30 Kpa. My friend's ford focus show the same data, so can I assume my MAP is good?

I used a propane to check all the hoses, but didn't find any leaks.

So, my fuel trim is still sky high (code P0171). I cleaned the fuel and intake twice already. Could there be a fuel pump problem? My O2 sensor didn't seems to go up and down as fast as my friends ford, but it does go up and down and I don't get any code about bad O2 sendor. The next thing to do is get a coil pack and look at the spark plugs....

I got rid of my 0171 code by clearing the intake. But mostly just by using techron. I was monitor my fuel trim and CEL using the software. It really help knowing what the ECU is thinking as I putting fixes in the car.

BTW, my long term fuel trim is now at less the 3%. I plan to use another bottle of techron after couple more tanks. I think they said you can't use techron at every fill up.

actually chevron gas has a maitenance does of techron in every tank
read bottle for cleaning dose - its 3 bottle iirc
To fully clean the fuel system, I would run Redline SI-1 fuel injector cleaner at 1 bottle to 1/2 tank
and repeat 1 can seafoam thru manifold vac port
Check front and rear motor mounts- some years are vac assisited and broken mount will lose vac
Clean the IACV under the TB assembly
Pull the manifold and clean EGR ports just to be sure
Codes are clues not diagnosis!!

Why a coil pack? you dont get any misfire codes!

are you resetting the ECU after each fix attempt?

I know that is a very old topic. Do you remeber the solution?

The OP is long gone.

Are you having fuel trim problems? Let us know, maybe we can come up with a solution.

Rares; all problems are unique, although there are some common symptoms....one would need to troubleshoot your exact problem.
what kind of problems are you having with the car?

My car is a hond aaccord european, k24a3 with
-sparkplugs changed
-timing valves adjusted
-fuel filter and checked fuel pressure 3.6 bar
-deleteed front mount engine
-o2 sensor changed and donut gasket on the exaust changed
-verified for vaccum leaks on intake tube, brake booster, pcv valve and vapor/purge valve NO problem

Next to check:
-sealing of TB but it's DBW
-cleaning injectors.

Still positive short and long trim like 10% for stft and 7-8% ltft

Any FT difference between idle and say 3000RPM?

Hello Everyone,

I would like to joint this topic as I have similar problem and similar attempts of fixing them.

I have Fiat Stilo 1.6 16V 103 HP, also with STFT/LTFT problems (STFT -10..30, LTFT 30..40), P0171 (running lean) and multiple missfires erros on just one cylinder.

I have:
- cleaned the throttle,
- checked MAP and intake pressure - seems ok,
- checked DOx sensor voltage - jumping 0..1 V but it looks like it's reacting properly to STFT; it looks like it's overshooting valve time,
- cleaned throttle - was a bit dirty,
- checked for vaccum leaks with pressurized air - no leaks.

What I'm going to do next:
- clean injectors,
- adjust timing valves.

Have a nice day!