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Lean codes but negative fuel trim

 
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Old 03-12-2019, 07:39 PM
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Default Lean codes but negative fuel trim

I am completely confused by the symptoms I have. Rather than theorize about it, and in so doing possibly pollute people's thinking, I want to just lay out the facts and see if anyone has an idea about it . Any suggestions much appreciated.

- Occasionally I will get a restricted performance message, after which Codes P171 And P174 are found . This seems to happen never under load ... but rather usually while coasting or braking.
- Monitoring fuel trim shows negative(!) numbers, as much as negative 17. This is seen primarily around idle; more normal trims, mostly negative low single digits, appear at higher RPM .

Has anyone ever seen a condition like this? Any ideas about it?

TIA,
 
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Old 03-13-2019, 06:47 AM
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Those two codes are sometimes triggered by failing VVT solenoid O-ring seals. They sit there and bake in the engine bay, get hard and brittle, then begin to allow oil to seep out and air to seep in. In my wife's 2006 XK8, they only last two to three years before they need to be replaced again....
 
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Old 03-13-2019, 07:03 AM
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When the codes flag a lot of data is stored - known as freeze frame data - so read it and you'll see what was happening at the time the codes flagged.
 
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Old 03-13-2019, 07:28 AM
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Thanks guys,

I've cleared the codes and will watch for the next event. I've never tried to retrieve freeze frame data so I'll have to learn about that.

The codes, and when they trigger, seem to me like classic symptoms of an induction air leak. I had started to look at the usual suspects for that. But the fuel trim data says the computer is trying to correct an over-rich condition. Curious, ain't it?
 

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Old 03-13-2019, 10:03 AM
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Keep in mind the fuel trims are really a table of many values, indexed by rpm and load. It is possible some trims are positive somewhere in the table, while being negative somewhere else in the same table (say, different rpm), at the same time. For an '03, the air flow meters are dirt cheap (same model used in MANY cars), so I would start there. The catch is to make sure the o-ring that seals it up is also functional (replaced or somehow seals). Once you have reliable sensor, best is to do a smoke test to identify all possible leaks. There are so many opportunities for leaks, it is hard to go around and repair them all.

Best of luck, keep us posted.
 
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Old 03-13-2019, 10:29 AM
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The MAFS can also cause this. Try some MAFS cleaner.
 
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Old 03-13-2019, 01:33 PM
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I have cleaned the MAF. New one on order.
Also looked over the prime air leak suspects, but seems like there must be something more than air leak going on; it would not account for negative fuel trim.
 
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Old 03-18-2019, 06:50 AM
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Installed new MAF. Both this one and the one it replaced are Denso units, and the old one was in place only about 3 years, 20k miles.

But ... they read differently. My interpretation is that the new one senses less air going by; i.e. it seems that it is calling for less fuel, so high negative fuel trims are gone. But now we're working with LTFT values built up using the old MAF, mixed with readings from the new MAF, so hard to reckon what the new STFT values mean. I'll drive it some and see where all the FT values go.

Still don't see how old MAF could have led to P0171 / 0174, but it may be I have a second problem that has not reappeared yet.
 
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