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Lean Code CEL Issues

Dybz

Banned
Before getting into it some specs on car: APR Stage 2, 42DD DP, Modshack intake, 60k miles

Issue: Lean code, idle searches sometimes between 750-850 rpm but often times is steady, no other issues detectable during driving. CEL code is

1 Fault Found:
000369 - Fuel Trim; Bank 1
P0171 - 000 - System Too Lean - MIL ON
Readiness: 0010 0101

Ran boost, fuel rail, some other logs and everything seems ok. I did switch from the modshack to the stock airbox after noticing the MAF air straightener grill was covered in a black sooty like substance. Also, cleaned the MAF with MAF Cleaner from CRC on sunday morning. Drove around for about an hour throughout the day and was fine. CEL popped back up this morning on the way to work. Same code and all. I have already searched for leaks, but can't find anything and since boost logs are ok I'm guessing it's not a leak.

So now the question is, what's left? Can the MAF be bad? Bad O2 sensor? Any way to test these without buying new ones?
 

hummvee2134

New member
A bad PCV valve is a common root cause to those symptoms on our cars. It creates a huge vaccum leak and a lean condition.

Pull the PCV line, plug the hole in the intake manifold (don't leave it like that for too long) and see if that smooths the idle out. If it does, PCV is your issue.
 

shortydh

Ready to race!
Mmm... Bad maf sensor seal. Make sure you're cop are on in all the way
 

hummvee2134

New member
Ok I'll take a look. So best way to test it is to unhook pcv hose and block it off along with the intake manifold hole?

Just block off the hole in the intake manifold so there's no vacuum leak. You can leave the PCV open temporarily.

How noticeable is the bad idle, if it's pretty rough, i would say just pull the line off and block the hole in the intake manifold with your hand while the car is running.

what shortydh said is a possibility too. 90% of the time, a lean code is caused by a vaccum leak of some sort.
 

Dybz

Banned
Just block off the hole in the intake manifold so there's no vacuum leak. You can leave the PCV open temporarily.

How noticeable is the bad idle, if it's pretty rough, i would say just pull the line off and block the hole in the intake manifold with your hand while the car is running.

what shortydh said is a possibility too. 90% of the time, a lean code is caused by a vaccum leak of some sort.

The idle is noticeable when it happens, but it doesn't always happen. Idles fine 95% of the time then randomly just goes to 850, sits there for a bit, goes back down to 750, sits for a bit, back to 850. I will try and find something to block the hole with for a bit.

I ran with the MAF unplugged yesterday and didn't give me a code. Tried cleaning it again, so hopefully helped.
 

Dybz

Banned
Does anyone understand the lambda values and their difference between blocks 31 and 32? 31 displays as a number value less than or greater than 1.00 while 32 displays as a %. For 31 I see mine go down to 0.85-0.87 often while idling. I'm guessing this is bad as this means the ecu is making the mixture richer during idle, correct? Would this imply a leak since the o2 sensor sees that the actual combustion mixture is leaner than the maf says it should be, lowers lambda thereby increasing fuel going in, making it run richer?

Not sure if I'm understanding lambda values correctly here.
 

Dybz

Banned
Replaced pcv... And now it's worse. Now the car is going crazy. During warmup it misfires on all cylinders at idle, once warmed up the idle surges to like 1100, if I step on the gas a bit it revs up fine, then coming back down it hangs badly at about 1500 and slowly goes down to 750-800 idle and then seems to idle ok
 

hummvee2134

New member
Replaced pcv... And now it's worse. Now the car is going crazy. During warmup it misfires on all cylinders at idle, once warmed up the idle surges to like 1100, if I step on the gas a bit it revs up fine, then coming back down it hangs badly at about 1500 and slowly goes down to 750-800 idle and then seems to idle ok

Did you try the test i suggested before replacing the PCV? Can you try it now and see if it makes a difference?

-If your old PCV was bad, you should have seen better symptoms after you replaced it.
-If your old PCV was good, you shouldn't have seen a difference in the symptoms after you replaced it.
-If you replaced your PCV and the symptoms are worse, you either got a bad PCV or there's an issue with the way you installed it.

Disconnect your PVC hose to the intake, block off the intake with your hand and see if your misfires and high idle go away (this should literally take 30 seconds to try). If the symptoms do go away, check your installation. If your installation is good, we may have to look into other things.

Try my steps and report back
 

RaV$

Ready to race!
could it be the maf itself?
 

TheMKVI

Go Kart Champion
Also the actual lambda on block 31 goes to 1.25 while specified sits at 1.00.

I believe a 1.00 is stoichiometric for lambda or 14.7 AFR with gasoline. A 1.25 would be an 18.4.

Your car should go lean (lambda above 1.00) during engine decel and max out the sensor (probably has a max of 1.25). If it is reading 1.25 and that is the max while idling then it would follow the sensor getting air from somewhere (If 1.25 is indeed the max).
 
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