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Old 12-03-2012, 11:35 AM   #1
p0staldude
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Default P0171 + Really bad fuel economy

I started getting P0171 code (System Too Lean) about 2 months ago. I get one every 2-3 weeks. It always happens when I am stopped at a red light.

Since it started, my fuel consumption has been really bad. Before, it was around 21 MPG but now I do 16 MPG.

Other than that, the car drives fine (no idle problem, no loss of power)

What could be the problem?
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Old 12-03-2012, 11:53 AM   #2
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Need more info. What are your mods?
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Old 12-03-2012, 12:07 PM   #3
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I have an AP but the engine is stock. I had the code with the stock map and with the Cobb Stage 1 91 map
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Old 12-03-2012, 12:22 PM   #4
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Could be a few things. Post MAF leak, bad/dirty MAF sensor, pre-O2 sensor leak, dirty fuel filter. I'd start with eliminating some of those issues and then go from there.
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Old 12-03-2012, 12:48 PM   #5
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Quote:
Originally Posted by swpshgnwgn View Post
Could be a few things. Post MAF leak, bad/dirty MAF sensor, pre-O2 sensor leak, dirty fuel filter. I'd start with eliminating some of those issues and then go from there.
+1, would also check injectors, make sure no leaks are present.
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Old 12-03-2012, 02:24 PM   #6
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Running lean will fast track the car to needing a new motor. Check what was mentioned or start saving money.
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Old 12-04-2012, 01:09 AM   #7
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Thanks for the suggestions!

I cleaned the MAF and checked for obvious leak/tightened the worm clamp.

What is the best way to check for leak? I was thinking about disconnecting the big hose after the MAF and pressurizing it. That part of the intake is not normally presurized, can I damage something? Or is there a better way?

Is there something in particular that I should look at in the Access Port to help pinpoint the problem?
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Old 12-04-2012, 02:00 AM   #8
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Get some brake clean, spray it on your hoses. When you hear the car jump up on rpm's, there is your leak. P0171 and bad fuel mileage is an obvious vacuum leak.
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Old 12-04-2012, 10:11 AM   #9
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P0171 + bad gas mileage and no power loss sounds a heck of a lot like an O2 sensor that has failed lean enough to max out the fuel trim, but not bad enough to throw a code for a failed O2 sensor. Or a vac leak, but normally that would be accompanied with a rough idle. If you had a wideband this would be easier to diagnose.

But follow flynlow's instructions for the leak test. Dont forget to check to turbo inlet tube under the intake manifold for leaking too.
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Old 12-04-2012, 11:54 AM   #10
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Quote:
Originally Posted by Flynlow151 View Post
Get some brake clean, spray it on your hoses. When you hear the car jump up on rpm's, there is your leak. P0171 and bad fuel mileage is an obvious vacuum leak.
Could be bad o2 sensor as well, the upstream o2.

how many miles on the car?
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Old 12-04-2012, 12:00 PM   #11
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71k miles
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Old 12-04-2012, 01:14 PM   #12
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Mine is doing something very similar, just with no code.

I tried to spray on carb/brake cleaner on the hoses... doesn't appear to be a vacuum leak anywhere. With the mileage on yours and age, it's probably not a leak due to age.

Try cleaning the MAF. If it's not those, I'd suspect the front O2 sensor at that point.



**If you have an AP, do a datalog for your AF learning 1 (all values, A, B, C, D and 1). Positive numbers are probably what you are going to see and indicates the ECU is dumping more fuel in. I suspect your numbers may be very near 15% as that is the max the ECU can compensate.
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Old 12-04-2012, 01:20 PM   #13
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Ditto on the front O2 sensor - I'd get that bugger changed. It's very possible it's on it's way out, and hasn't thrown an "O2 sensor" CEL just yet.
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Old 12-04-2012, 03:47 PM   #14
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What are your fuel trims doing via the AP? Have you checked for boost leaks yet?
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Old 12-04-2012, 08:51 PM   #15
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My fuel comsumption is back to normal (~19 MPG). It looks like cleaning the MAF and tightening all the worm clamps helped.

I tested for vac leak with brake cleaner. I sprayed all the intake joints twice without noticing any change in the RPM. I then sprayed a little bit of brake cleaner in the airbox to see by how much it would raise the RPM but nothing happened. I am really not sure why, should any brake cleaner work?

I noticed a pretty loud "air" sound when I depress the throttle quickly. It happens for a fraction of a seconds before the RPM starts to rise. I made a video of the sound. I am not sure if it's normal



Here is the AF Learning data from the AP:

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Old 12-04-2012, 09:01 PM   #16
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I also noticed an unpluggued wire behind the airbox. I could not find where it is supposed to go


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Old 12-05-2012, 09:58 AM   #17
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Your C and D fuel trims are a negative value beyond the recommended 8% max. I would check for boost leaks using a better method. The Brake Cleaner method is not very good.
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Old 12-05-2012, 11:05 AM   #18
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Quote:
Originally Posted by Tripintaz View Post
Your C and D fuel trims are a negative value beyond the recommended 8% max. I would check for boost leaks using a better method. The Brake Cleaner method is not very good.
Positive numbers indicate fuel is being added, aka a lean condition, aka leaks or a poor MAF.

Negative amounts indicate fuel is being taken away... which would mean what mechanically?
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Old 12-05-2012, 02:23 PM   #19
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Quote:
Originally Posted by cmiovino View Post
Positive numbers indicate fuel is being added, aka a lean condition, aka leaks or a poor MAF.

Negative amounts indicate fuel is being taken away... which would mean what mechanically?
I am not sure, but I would also like to know.

I ordered a new front o2 sensor, hopefully it's going to help
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Old 12-05-2012, 02:37 PM   #20
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Quote:
Originally Posted by p0staldude View Post
I am not sure, but I would also like to know.

I ordered a new front o2 sensor, hopefully it's going to help
How much was the front 02? I know the rears are around $200.
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Old 12-05-2012, 02:44 PM   #21
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It was 200$ from the dealer but I found one for 159$ on eBay
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Old 12-05-2012, 03:05 PM   #22
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Quote:
Originally Posted by cmiovino View Post
Positive numbers indicate fuel is being added, aka a lean condition, aka leaks or a poor MAF.

Negative amounts indicate fuel is being taken away... which would mean what mechanically?

Think about this for a moment.... If the MAF is measuring ~20gps but there is an after MAF leak then the trims would go rich because the intake manifold is in vacuum and its sucking in ambient air. The O2's see the fuel mixture as being lean since extra air is getting into the combustion chamber that was not measured by the MAF.

Now lets go into the higher MAF gps values that you would see while in boost. So lets say you start to get into boost and the MAF sees 100gps of air. Since you are in boost and the intake manifold is now pressurized above atmosphere it is not sucking in any extra air, its actually blowing air out of where the leak is. So the MAF reads 100gps, but only 80gps of that metered air makes it into the combustion chamber. The O2 sensor now thinks the car is running rich and starts to pull out fuel.

So in this example the ECU thinks you're lean when in vacuum and rich when getting into boost.

OP needs to check more thoroughly for a boost leak.
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Old 12-05-2012, 03:17 PM   #23
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Quote:
Originally Posted by Tripintaz View Post
Think about this for a moment.... If the MAF is measuring ~20gps but there is an after MAF leak then the trims would go rich because the intake manifold is in vacuum and its sucking in ambient air. The O2's see the fuel mixture as being lean since extra air is getting into the combustion chamber that was not measured by the MAF.

Now lets go into the higher MAF gps values that you would see while in boost. So lets say you start to get into boost and the MAF sees 100gps of air. Since you are in boost and the intake manifold is now pressurized above atmosphere it is not sucking in any extra air, its actually blowing air out of where the leak is. So the MAF reads 100gps, but only 80gps of that metered air makes it into the combustion chamber. The O2 sensor now thinks the car is running rich and starts to pull out fuel.

So in this example the ECU thinks you're lean when in vacuum and rich when getting into boost.

OP needs to check more thoroughly for a boost leak.
In your previous post you suggested using a better method than the brake cleaner to check for leaks. How would you do it?

Thanks!
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Old 12-05-2012, 04:28 PM   #24
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Quote:
Originally Posted by p0staldude View Post
In your previous post you suggested using a better method than the brake cleaner to check for leaks. How would you do it?

Thanks!
There are numerous threads with this information on this forum. The search feature will serve you well here. Basically pressurizing the system is the way to go. But I have read where some have used a cigar and blown smoke into the intake thru a vaccum line(obviously after sealing off everything). You're going to have to do some more reading on this.
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Old 12-05-2012, 04:42 PM   #25
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Start with that damn intercooler to tb connector. Notorious for leaking. Or check your intercooler end tanks to see if they started to separate.
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