Tell me what you think...

[GmX5]

My check engine light has been on for a couple of weeks (I've been driving my trusty civic). Anyway, I'm tired of driving the civic so I ordered a new gas cap thinking this might be the issue, since gas cap was looking kinda sad anyway. I go to autozone to get the code(s) read and they inform me that auto stores cant read codes in California anymore . So i go down the street to a shop they recommend and make an appointment for 1 1/2 hour later. I put the new gas cap on and drive around for an hour..then go to my appointment. They tell me it read code P0171 (bank one system one lean). I turn on the car and light is gone. I ask the guy if he reset it and he swore he didnt. I know this post is getting lengthy...so here it is...1) could the gas cap have been the problem? 2.) if the problem is still there shouldn't the light come right back on, if so how soon? What are your thoughts? And yes, I did use the search tab..just thought i had a unique situation

Thanks--Gm

[Weasel]

The gas cap would not cause a lean mixture fault, but a vacuum leak or any other form of unmetered air would. If the fault was not detected for so many drive cycles (55 I think?) the light will turn off. And it has to see the fault for at least a couple drive cycles first before turning the light on.

May be time to search down a leak. I forgot, you got a 3.0i, or a v8?

[nikip]

Good it is a very good question i hope you gate ans.......

[GmX5]

I have an '01 4.4. If I hadnt just replaced the pcv vent hoses i know that would be a good place to start. So the light will turn off if the computer doesn't read the fault in 55 cycles or 5? I've been reading some threads and it seem that i should take the engine cover off and look for some unmetered air situation..I guess i can start there. The car runs fine I just dont like driving it with that damn light on
About a month and a half ago I took the X to the dealer since I was lost for an explanation for a rattling inside the cab (they found & fixed it, by the way). While one of the tech was out searching for the source the SES light came on. I told them to reset it and I put well over 500 miles on the X5 with no light. Then it came on--so my question is: does it always take this long for the light to come back on? If there was a big problem I would assume the light would come right back on. My biggest concern is damage to the catalytic converters with tweeked air to fuel ratios. Ive read some threads that have mentioned that this code is not a real serious code and is likely due to a sensor going bad that has at least one other sensor to back it up. I'm leaning toward this idea since my pcv vent hoses were changed and that area of the engine was fully inspected in late August. I'm not really sure on this information I came across in some threads but I believe it was mentioned in more than one thread.

Thanks guys

[Icer006]

If it's a lean bank, it could be an aging O2 sensor or cat going bad, no?

A leak won't cause a lean bank unless it's a fuel leak.

I think the small evap leak is P0456

Originally Posted by weasel56

The gas cap would not cause a lean mixture fault, but a vacuum leak or any other form of unmetered air would. If the fault was not detected for so many drive cycles (55 I think?) the light will turn off. And it has to see the fault for at least a couple drive cycles first before turning the light on.

May be time to search down a leak. I forgot, you got a 3.0i, or a v8?

[Weasel]

Icer006, an air leak DOES cause lean mixture faults. What happens is the DME reads the mass air flow sensor to know exactly how much air is coming in the engine, and tailors the fuel injector output (pulsewidth) to that... then reads the o2 sensors too see the quality of the combustion cycles, ie not enough fuel or too much fuel. If it sees too much unused o2 in the exhaust it knows not enough fuel was burnt hence lean mixture. What happens with a leak (unmetered air to be specific) is that air makes it into the intake system without being metered by the MAF sensor, so the DME doesn't know it's there and doesn't add fuel for it. The DME can only compensate for soo much of a leak by adding fuel (very minuscule really) without raising the engine speed as a result. So it throws a lean mixture fault.

And the DME would usually throw a fault for "o2 sensor bank _ sensor 1 aging, slow to respond" when the sensor gets old and slow as that is one of the things it monitors.

And as for an aging cat, BMW's will put a fault code for "cat converter efficiency, bank _" and tell you if it is loosing efficiency, and a clogging cat would cause lack of power/stalling issues. But not lean mixture.

And a small fuel leak is more of a safety issue than a running problems issue.

GmX5
Some times with higher mileage M62 engines its just time to reseal the intake manifold. I usually replace the manifold gaskets, front end plate gasket and throttle body gasket, rear end plate w/gasket. And while it's off you'd be stupid not to change the coolant valley pan. But that's a decently big job... probably at least a full day as a DIY.

[Icer006]

^Gotcha! Thanks!

[Weasel]

No prob bob!

[GmX5]

I popped the engine cover off and looked around air intake everthing looks normal. One thing I can think of is the gasket that connects the MAF and air tube to engine (dont kno part name). Once when I had it off it was a bit warped and didnt fit quite the way I wanted it to. This seems like it would produce a small magin of air, if any. There might not even be a leak there. I dont know where else to look. Do you think there is any chance of damaging the cats cause I've been driving it around lately? The light hasnt come on. Again, why was the interval for the light turning on again so long?

[Weasel]

It shouldn't damage the cats. And for the light interval, it has to see it either 2 drive cycles in a row, or 2 times within 3 drive cycles. And it probably has to idle for a while to see it, as it doesn't usually see the small leaks above idle.