Lean condition caused by exhaust leak?????

[Oldman]

I have a 171 code indicating a lean condition. I was told that this could be caused by an exhaust leak that I have after a CAT delete. Is this a possibility? I have replaced every vacuum line and have checked everything I can think of and have been unable to find a vacuum leak. I have checked everything on the fuel system and pressure and volume of the fuel system looks great.
i can't understand how an exhaust leak would cause a lean condition.
Can someone let me know if this is a possible reason for the code?

Thanks, I am anxious to hear an explanation for this!

[01bonneSC]

Did you have the PCM tuned for a cat delete? Or have an emmulator plugged in?

And you should fix the leak anyways, can't sound too great.

[redzmonte]

A cat delete would not cause a lean condition. you would have a cat failure code but not a lean code. I would look closer at your vac lines, exhaust gaskets, intake gaskets, ect...

S

[Oldman]

I am not asking if the cat delete would cause the code. I have removed my cat because I couldn't afford to replace it. I am just asking if there's any way that an exhaust leak would cause a lean condition.

Sorry if I wasn't clear. Can an exhause leak cause the 171 lean condition code to appear?

I do have a code for the cat removal, but I know about that and understand the reason for it. One of the possible causes for the lean condition that showed up on the print out that they give you at O'Reilley Auto Parts was an exhaust leak.

[Mechanical Mike]

An exhaust leak is listed as one of the possible causes of P0171 in the factory service manual. I would think the exhaust system would be under a slight amount of pressure & some exhaust would simply exit the leak. It seems to me that you'd have to have fresh air entering the leak to cause the code. It also seems to me that the leak would have have to be upstream from the front O2 sensor but the manual doesn't state where the leak would have to be.

Vacuum leaks, bad MAF sensors, modified intakes & bad O2 sensors are the most
likely suspects.

[01bonneSC]

I am not asking if the cat delete would cause the code. I have removed my cat because I couldn't afford to replace it. I am just asking if there's any way that an exhaust leak would cause a lean condition.

And Im asking if you had a rear 02 emmulator plugged into the rear 02 connector or if you had the the PCM tuned so a CAT efficiency code wouldnt show.

And an exhaust leak could cause a lean code. more than likely it would have to be upstream of the front 02. What does your fuel pressure look like?

[00Beast]

Likely the exhaust leak would have to be between the exhaust ports and the front O2, so looking at the manifold gaskets and donut on the crossover would be my first place to look. Would be fooling the PCM into thinking it's burning a different amount of fuel than it is, so it's screwing it all up.

[Oldman]

My exhaust leak is downstream from all O2 sensors. I cut the pipe downstream of the cat, gutted the cat and reinstalled the cat, but have a leak where the pipe was put back together with a poorly fitting sleeve.

I have no other exhaust leaks, vacuum leaks or intake gasket leaks.

Thanks for your help, I will just ignore the code.

[harofreak00]

A lean condition isn't a code that you just want to ignore. Lean = blown engine

[01bonneSC]

Are you having any stumbling problems? Im thinking fuel related.

[Oldman]

I have no performance issues. Car runs great, gas mileage is as good as it has ever been. All is good other than the code.
Am becoming disappointed with feedback on posts these days.
Seems that this site used to be much more active and helpful in the past.

Anyway, thanks for your suggestions.

Moderator - "don't ignore the code"
That's the extent of your participation?
Thanks,

[harofreak00]

You've received help/suggestions on the problem from several members, not really sure what more you expect out of a forum.

Moderator = He monitors the interchange of contributors and makes decisions regarding content and the direction of threads. Moving discussions from one section to another to keep topics organized is also a common job for a forum moderator.

Moderator ≠ providing answers to everyones problems.

I don't enjoy helping people with an attitude...

http://bonnevillecarparts.com/files/P01 ... 20Lean.pdf

[jedwards83]

The suggestion not to ignore the code is the best advise given here, IMO. Running lean under boost will quickly destroy an engine. The PCM will compensate for the lean condition by increasing injector pulse width to maintain a complete burn, but it can only do so much.

Even if the car runs fine, your fuel trims are all out of whack, that is what that code is telling you. Fuel trim is the PCM adding or subtracting fuel based on input from the oxygen sensor. Normally the PCM determines the amount of fuel by comparing sensor inputs (throttle position, coolant temp, mass airflow, among others) and it gets the amount of fuel from a table and uses that. In closed loop operation, the oxygen sensor then tells the PCM if the result is lean or rich.

If lean, the PCM "adds fuel" which results in a positive fuel trim. If the PCM is subtracting fuel to maintain the desired air/fuel ratio, you wind up with a negative fuel trim.

The code you have means the PCM has had a positive trim over a predefined level (20%?) for X period of time. (not sure what the exact specs are for GM). But in a nutshell, you PCM is having to add fuel when it shouldn't be if everything was working correctly.

The cause can be many things. A bad sensor, typically the MAF. If the MAF tells the PCM that less air is coming in than actually is, the PCM injects too little fuel, and the oxygen sensor tells the PCM it's lean, and the PCM compensates. Think of the O2 sensor as the feedback device.

A vacuum leak before the MAF can cause a lean condition, but this usually occurs at idle when vaccum is highest, and tends to ago away under load. This is because vacuum pulls in air that isn't metered by the MAF. More air needs more fuel and the PCM isn't seeing the extra air.

A week fuel pump may not be able to maintain the correct pressure. Low pressure means a longer injector pulse width is required to dispense the proper amount of fuel. The PCM does not monitor fuel pressure. It only knows to open the injector X number of milliseconds. Therefore it's only means to correct for low fuel pressure is the O2 sensor telling it that the burn is lean, and it adds fuel to compensate, resulting in high fuel trims. If it gets bad enough, the lean code sets.

Bad fuel pressure regulator manifests itself in a similar manner as the above.

My suggestion to you is to check the fuel pressure. The pumps weaken as they age and even though they work and the car drives fine, the PCM has to keep adding more and more fuel (resulting in the high fuel trims I described) to correct for the weak pump. Someone correct me if I'm wrong but pressure should be 52 PSI with the vacuum hose unplugged, and it should build with boost at about a 1:1 ratio, approx 1 PSI for each pound of boost. At idle it runs about -46 PSI (??). Someone please verify this.

I got that code several months ago, and my fix was a new fuel pump. I had no driveability issues either. Having a scanner helps confirm this suspicion because you can read the fuel trims at various RPM and loads. A vacuum leak will typically only have a high trim at idle or under vaccum. A bad fuel pump or FPR and you'll see the high trims all over, especially under load when the fuel pressure should be increasing.

[01bonneSC]

I suggest you fix your known exhaust leak. Cars are goofy sometimes. It MAY be the cause of the code.

[Oldman]

As I mentioned, the fuel pump is fine. I arrived at this determination by testing the pressure with and without the vacuum line disconnected.
My question was is there any way an exhaust leak could cause the "lean condition" code.
While you have impressed me with a terribly in-depth response - and I appreciate this very much - an answer to my question would have been preferred.
I am not attempting to have an attitude. I am not attempting to offend anyone. I simply was looking for an answer to my question.

Let me re-state:
I have checked fuel pressure, replaced the MAF sensor, replaced all vacuum lines and still have the code. When I went to O'reilleys Auto Parts, they supplied me with a print out of the code and all possible causes. About 6 months ago, I removed my CAT because i was getting symptoms and codes that indicated that it was restricting my exhaust and I was not able to replace it at the time. When putting the exhause back together without the CAT I developed a slight exhaust leak that is obvious when the car is cold but lessens when the car warms up.
Is it possible for this exhaust leak to cause the code?
Is it possible for the exhaust leak to triger a sensor and cause the code?
Is it possible for the exhaust leak to cause an actual lean condition?

I appreciate your responses and anxiously await an answer to my question.

[jedwards83]

If the exhaust leak is before the O2 sensor or in the immediate vicinity, absolutely. If it's downstream of the O2 sensor, it won't affect anything.

Did you test the fuel pressure under load? Testing at idle won't tell you anything other than that the pump can keep up under light duty, which is the least likely condition that it will exhibit any symptoms. I'm not holding a gun to your head insisting it's the fuel pump (please take that into consideration in your response!), only pointing out that this is a possibility that still exists.

Do you have access to a scan tool? This will help you immensely. If you have an Android powered phone or tablet, a $25 bluetooth OBD-II adapter and $5 torque app and you've got a powerful diagnostic tool that should help you get to the bottom of things, and it will give us a lot more information to go by before we make suggestions. You have to understand that with the information you've given, we're taking a stab in the dark... There's a lot of possibilities and you'll have to eliminate them one by one.

[dpnewfie]

I know you are focused on the exhaust leak as a possibility, and it may very well be your cause, but I have to go back to fuel pressure. You may have done this already but have you checked the FPR? How about, while checking the fuel pressure, removing the vacuum line that is conencted to the FPR and verify if the fuel pressure goes up. Doing this simulates the system under load.

[jedwards83]

I know you are focused on the exhaust leak as a possibility, and it may very well be your cause, but I have to go back to fuel pressure. You may have done this already but have you checked the FPR? How about, while checking the fuel pressure, removing the vacuum line that is conencted to the FPR and verify if the fuel pressure goes up. Doing this simulates the system under load.

By the rash responses he's given, it seems he's done this. The problem is that removing the vacuum line and testing fuel pressure only tells so much. The comprehensive test would be under load, the fuel pressure should build with boost pressure, something you can't tell at idle.

My bad fuel pump tested fine both ways at idle, with and without the vacuum hose attached. But under load was a different story, and it explained why I had both the lean code and horrendous KR under boost.