2002 EuroVan CEL P0171 Too Lean - But runs fine

[jjvincent]

Another good one you can look at as that error is quite common for many VAG vehicles:
http://wiki.ross-tech.com/wiki/index.php/Fuel_Trim_Info

Watch the video at the end too.

[jjvincent]

Also, any oldtimer mechanics know (especially if you ever worked on any early Bosch fuel injection system, that carb cleaner sprayed in areas where you think there's a leak, is a cheap effective diagnostic tool for finding a leak (as in the video).

[Abscate]

A decent OBD scanner like the Autex 500 series or up,will monitor fuel trims. Graphing and plotting functions are cool but not really necessary, you only need two points, idle and 2500 rpm

Quick version

Engine runs lean. That means too little fuel or Too much air.

Air comes from two places, metered air ( measured by the MAF) and unmetered air ( not seen by MAF)

At idle , a leak is large compared to idle air, so SHORT TERM FUEL TRIM is large ..20% or more.

At 2500 rpm the throttle plate passes gobs of air and the leak is now small relative to the throttle metered air , so STFT falls to 5% or less.

Voila!
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.ssS!

[Masta Shake]

Had a leak where the PCV hose attaches to the intake. Replaced the intake and the codes have gone away. I feel like the van is still running pretty rich though. Took it to another exhaust guy and he checked for leaks. Nothing obvious but he suggested that if I thought the upstream O2 sensor was more than 80k miles old to replace it. Based on his observations he agreed its running rich. Replacing the O2 sensor this weekend. Plugs are new and the engine has 13k miles on it since I bought it from VW as a rebuilt. No other obvious vacuum leaks per the smoke test at a shop and my starter fluid testing.

MAF sensor is next.
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As a good friend told me when I got the Syncro: "Welcome to the an intense love/hate relationship".

[menko]

Reviving this topic as I am facing a similar problem. I replaced the brake booster vacuum line last year, but haven't been able to locate where the air is getting in.

based on this fuel trim at idle, I'm far beyond .20

Are there some likely suspects I should start looking at? I'm not super familiar with where to look for these vacuum lines on a modern car, but I can spray carb cleaner if I know where to look I tried listening with a stethoscope to see if I could find any, but didn't discover anything obvious.

thanks!

[Abscate]

Your trim is good if its truly 0.78% and not 0.78 (22%)
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.ssS!

[menko]

Haha, didn't catch that part I'll reset it and then see if it comes back. Thanks!