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Old 11-17-2010, 01:48 AM   #1
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Default DTC P0171 Theory

I have a 97 Buick Park (not SC) with 172k and have been chasing the 171. I have read the numerous threads and have the service manual, so I am not going to ask what to check.

My question has to do with the conditions for setting the DTC. The manual notes the typical range for SHORT and LONG fuel trim is +/- 10%.

From the circuit description: "The PCM'* maximum authority to control long term fuel trim allows a range between -23% and +25%...short term fuel trim...between -11% and +25%."

Under the many conditions listed for setting the DTC, long and short fuel trim is at or near maximum authority of +25%. In addition, MAF is between 9 and 180 gm/sec.

The way I read the list, all the conditions have to be met. When my DTC sets (happens at idle for me), short trim is around 10%, long trim 16%, and MAF around 4.

The DTC should not set, should it?? Maybe the P0171 is just a fluke not to be worried about??
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Old 11-17-2010, 07:22 AM   #2
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P0171 is not a fluke and is something to take seriously. P0171 is the pcm telling you it is exceeding the allowable positive fuel adder in it'* programming. At X point it keeps adding fuel, but has been programmed to inform the driver through the SES light.

The typical causes of P0171 in the FSM are not what I have found them to be and I have become quite familiar with this code.

The typical causes I have found:
Corroded battery cables. Pull the bolt out at the battery and slide the rubber boot off the cable. Does corrosion (usually greenish in color) go down into the insulation of the cable? Is the ground cable to the engine clean and tight? If you have corrosion, I would suggest new cables.

EGR to LIM tube. For some reason this tinfoil tube has the ability to crack right at the LIM where it is held by it'* retainer and bolt. Usually impossible to confirm with carb/tb spray I suggest removing it and pulling back the retainer while holding the tube up to light and looking around it'* edge. If you see daylight, you found a large vacuum leak.

Other vacuum hoses, if they are old, cracked and leaking, it'* time to replace them.
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Old 11-17-2010, 11:51 AM   #3
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Quote:
Originally Posted by BillBoost37 View Post
P0171 is not a fluke and is something to take seriously. P0171 is the pcm telling you it is exceeding the allowable positive fuel adder in it'* programming. At X point it keeps adding fuel, but has been programmed to inform the driver through the SES light.

The typical causes of P0171 in the FSM are not what I have found them to be and I have become quite familiar with this code.

The typical causes I have found:
Corroded battery cables. Pull the bolt out at the battery and slide the rubber boot off the cable. Does corrosion (usually greenish in color) go down into the insulation of the cable? Is the ground cable to the engine clean and tight? If you have corrosion, I would suggest new cables.

EGR to LIM tube. For some reason this tinfoil tube has the ability to crack right at the LIM where it is held by it'* retainer and bolt. Usually impossible to confirm with carb/tb spray I suggest removing it and pulling back the retainer while holding the tube up to light and looking around it'* edge. If you see daylight, you found a large vacuum leak.

Other vacuum hoses, if they are old, cracked and leaking, it'* time to replace them.
What is LIM tube?
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Old 11-17-2010, 11:56 AM   #4
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That'* the tube that goes from the EGR valve to the lower intake manifold under the throttle body.
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Old 11-17-2010, 04:27 PM   #5
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Thanks Bill will keep looking.


Quote:
Originally Posted by BillBoost37 View Post
P0171 is the pcm telling you it is exceeding the allowable positive fuel adder in it'* programming.
Still wonder why the scanner results for short and long term fuel trim do not appear to indicate the maximum authority of the pcm is being reached (+25% according to the manual)?
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Old 11-17-2010, 07:56 PM   #6
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Thats the good ol lean code.

Basically, if your fuel pressure isnt low or have clogged injectors, you prolly have a vacuum leak. A quick cleaning of the MAF would be a simple and easy first diag step. Dirty MAFs report low airflow, means less fuel, but the O2 will show lean.

As has been stated, the EGR tube likes to crack which is a common problem.

Best bet, start the car and let it idle. Pull up your fuel trims on your favorite scan tool and view the live data. Working one spot to the next (anywhere that there could possibly be a small or large vacuum leak) spray carb and choke cleaner. If you spray in one area and the fuel trims change a fair amount, you found the source of the vacuum leak.

Another method that I love to use, but you have to be very very careful with!!!! is to use propane and a propane accessory!!! A simple small cylinder and the torch head works. Use that, point it at an area that could be leaking, open the valve to let the propane out right in the area and observe the fuel trims.
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Old 11-17-2010, 08:27 PM   #7
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Thanks BXX...more good advice.

I still wonder if the PCM is throwing the code too early. Nice to know of a leak or failure early though.

Also makes one aware that the computer can compensate for some variance, but when things get too far out of tolerance, a code will be indicated.
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