CEL (MIL) P0171

[cooyarpjm]

So my lancer has been idling roughly and when increasing speed after stopping at lights etc and changing gears upon depressing the clutch and removing my foot off of the accelertor instead of the revs dropping the engine would surge. Today the ecu threw a P0171 error code, i've looked up what the code means (System too lean) and the mostly likely cause is faulty O2 sensor. But there are several other possible causes, I had already cleaned the MAF senor and put in a new air filter, any suggestions as to where to start would be appreciated.

[himynameisdaniel]

inspect and clean throttle body. mine was really dirty after not touching it for 150k. not enough to throw a code but enough to stall occasionally after a period of rough idle.
actually come to think of it, I'm pretty sure the PCV dump goes to the intake after the air filter somewhere so that would explain why the TB and MAF would become filthy over time.

also do you have any intake mods or exhaust mods?

[cooyarpjm]

No intake and exhaust are stock.

[wallaw819]

Maybe can try running with MAF unplugged and see the results.

[Quismiff]

Maybe can try running with MAF unplugged and see the results.

car won't run without MAF, it'll start then die out...

[wallaw819]

the car should still run with MAF unplug.

Try it once when changing to open pod and forget to connect the MAF, ran fine.

Did some googling and saw it will run on stock map or close loop or something.

[cooyarpjm]

So cleaned throttle body this arvo which made no difference, so will replace pre cat oxygen sensor next Wednesday hopefully that'll do the trick if not I will continue to investigate.

[cooyarpjm]

So i have an update,

Over the last 7-8 months i have replaced every sensor (including O2 sensors), PCV, fuel return solenoid all to no avail car still idles rough. So i came to the conclusion that it could only be one of three things.
1. crack in the manifold or manifold gaskets
2. ECU issue.
3. Cam cover gasket

So i had previously purchased the gaskets needed ready to pull off the manifold and cam cover to replace the gaskets. Suspecting that the previous owners incompetence having seen the way the front of the car had been put back together and recently having a bolt fall out of the fuel rail bracket when checking the oil. (taking off the engine cover i broke the mounts on the manifold they had been tightened so hard). So i purchased a new used manifold from the wreckers last week.

Today i removed the manifold (not as hard as you'd think and didn't even have to remove everything Mitsubishi says you have to). The previous owner (or someone he knows) had tightened the manifold SO TIGHT it had destroyed the gaskets, one metal Sleeve(?) from the bolt holes was missing. And evidence shows air was being sucked in on at least 3 cylinders in multiple spots.

So now with new gaskets, new injector O-rings, assembled to correct torque.................

Start engine.....
Warm up...... (rough idle mainly occurred once the engine was at temp)

RESULT..

NICE SMOOTH IDLE. Yippy

All in all it took only 5 hours to complete.

[lancerdarryl]

Excellent result and well done for you tenacity.

[cooyarpjm]

Here's a photo of the removed manifold and gaskets.

[cooyarpjm]

So am hoping that this will cure the CEL. All in all it should.

[bumblebee]

Excellent work, very well done and thanks for sharing.
So glad you got to the bottom of it

Word of advise for O2 sensor replacement
Put a new sensor up front before the cat
The rear one , get it off a wreck
Why?
Old the O2 sensor gets the slower it gets
The ECU looks for a voltage difference between front and rear, the bigger difference the better.
If the voltages are too close , it will cause all kinds of CEL.codes


The other way to see if your inlet manifold gasket is kaput, if you let the car idle and squirt say something like "start ya bastard" round near the gasket on the head and the revs take off , that's where your leak is

[cooyarpjm]

Haha yeah that's what i did, new sensor in the front and a used one off of a lancer with only 40000kms in the back.

Yeah after replacing the sensors and such i knew it pretty much had to be an air leak just couldn't find it, the things that were just adding to the confusion was with the air conditioner running the engines idle was pretty much spot on, and as i said in the previous msg the guy/s who put the car back together after the accident did a shoddy job. Then with the bolts falling out, i had pretty much decided to remove the manifold to check it for cracks but not being able to spot any had pretty much come to the conclusion that in one way or another suspected the gaskets and had purchased them a couple of months ago but hadn't had the time over Christmas with work to do anything about it.

But very happy with the result, because if this didn't fix the problem i was pretty much already discussing with my dad about replacing the engine.

[bumblebee]

Did you hang on to the O2 sensors you removed?
They might come in handy down the track

[cooyarpjm]

Yeah mate still have every part i removed