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Jake, Nissan Technician
Category: Nissan
Satisfied Customers: 3652
Experience:  Experienced collision repair technician, Pulaski Tech graduate and DSM hobbyist.
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I have Nissan Almera 2002 Sedan 1.8 Automatic. Problem,

Customer Question

Hi, I have Nissan Almera 2002 Sedan 1.8 Automatic. Problem, Fault code P0171 which is in theory indicating lean condition, but if I read the live stream data it is as following: Engine idling: LTFT 35.9 constant, STFT going repeatedly from 0 to -25% like
a saw, O2 sensor 0.900V most of the time, If I rev to above 2500rpm the LTFT starts to fall to 12, O2 sensor starts to fall as well. Additionally while idling, the engine rpm falling repeatedly. If this is happening the STFT goes back to 0 and again keeps
falling till next loosing of the rpm. Repeatedly like a saw graph. I have checked the fuel pressure reg on the common rail for leaks but didn't see any. But after pulling out the vacuum line from fuel pressure regulator the engine rpm while idling became steady.
But no changes noticed to LTFT or o2 sensor. Also, car is eating fuel like mad, around 20 litres per 100km. I can send some scan screenshots. It doesn't make any sense for me. Regards, Rad
Submitted: 12 months ago.
Category: Nissan
Expert:  Jake replied 12 months ago.
were any recent repairs done prior to this problem? The STFT is what's really beneficial for getting accurate indications of whats going on. The code indicates lean, but the A/F sensor is actually leaning out the fuel trims because is it detecting a rich condition. So yea that doesn't make alot of sense. Normally when i see the STFT switch from 25% to 0 rapidly like that, its because the o2 sensor was installed incorrectly (wired, not direct fit) or the bank 1 and bank 2 connectors got switched on accident which is why i asked if any recent repairs were done. Other than that you want to check all the normal causes for rich conditions like your experiencing. One being the MAF and the other being exhaust leaks and A/F sensor function. You can let the engine idle with the MAF unplugged and see if the STFT level out at 25%.
Customer: replied 11 months ago.
Hi Jake,There was the change of a MAF, A/F sensor, cam shaft sensor not too loong time ago because car didn't pass NCT for emissions. About half a year ago. After that emissions passed, but car still was eating fuel like mad. I didn't pay attention that time to live data due to simple lack of time and my wife doesn't make lots of miles.Now. I did what you said about the disconnecting the MAF sensor for a very short while. I had it connected back quickly as the engine rpm started to rise. After connecting back the rpm did stay around 1500. So I switched the engine off and on. Rpm came back to normal. Any way you wouldn't believe what happened. After that everything came back to normal. LTFT around 0, A/F sensor started to oscillate normal. STFT behavior normal. System working in a closed loop all the time. RPM flat like a table. Like a new engine!!! Did I reset something accidentally or it's a bad MAF connection? I can't understand what just happened.
Please see the file attached. The little black line at the top graph is the moment I did messing with MAF.
Customer: replied 11 months ago.
And sorry for my bad English. I got excited little bit.
Expert:  Jake replied 11 months ago.
You may have had a bad MAF connection indeed. what is the STFT at now? is it within +or- 7%
Customer: replied 11 months ago.
Hi,I got excited to fast. I got a spin and now if I press accel. pedal gently the car speeds up, but if I go to WOT it has not power at all. Something seriously wrong.
After a spin you can see the end of a graph attached.
Expert:  Jake replied 11 months ago.
yea, your running really rich. Your STFT when the engine is warm should be NO MORE than 7% in either direction.
Customer: replied 11 months ago.
Should I change MAF? What flow MAF should be showing on the FOT?
Expert:  Jake replied 11 months ago.
the g/ms of the MAF should be a consistant graphical increase as throttle increases.
Expert:  Jake replied 11 months ago.
usually if the vehicle will run normally when you unplug the MAF then it's a good sign the MAF is out of spec and causing it to run rich. For what it's worth, usually a good indication of MAF failure is when you have fuel trim issues and then unplug it and the trims revert to 25% exactly. When they revert to a lower number like 17% or something of that nature then it can be an air leak causing it. MAF's when they start to fail can be hard to diagnose so we use these tricks when there are no other obvious issues causing the trims to be off. If you have no exhaust leaks in front of the upstream sensor and no strange readings from it, then it's most likely the MAF at this point.
Customer: replied 11 months ago.
I did what you said and the STFT changed to -25 constant. So the conclusion is to change the MAF sensor, is it?
Expert:  Jake replied 11 months ago.
Yes, that's where i would start if it was me!
Customer: replied 11 months ago.
OK, That's what I will do. Hopefully this will solve the problem. What make would you suggest? I know that Nissan car are very moody about the exchange sensors. Something that is still very good quality but reasonable price. Also I know that there are two types of connectors. My one is kind of more square and thicker. If you give me the part number I will be very satisfied. :)
Expert:  Jake replied 11 months ago.
whats the chassis #
Customer: replied 11 months ago.
Expert:  Jake replied 11 months ago.
I can't get that frame# ***** run for some reason. 226805U400 (this should be the part#, but i can't verify it because it's not verifying that frame# ***** some reason)
Customer: replied 11 months ago.
Ok, I will find something I hope. Don't close this question yet so I will let you know what's the final result.
What makes of the sensors are good ?
Expert:  Jake replied 11 months ago.
I don't close any of my questions with customers manually. I always allow free follow up after ratings. The only thing that will make the question close is if you go like 2 weeks without replying.

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