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Discussion Starter #1
My engine has thrown 2 P0171 codes in the last few days... so after extensive searches here it seems the thing to watch is my short term fuel trim... my understanding is that it should be +/- 3% when at idle. My STFT was starting above +3% and creeping to >+5% at idle... that didn't seem right. It just so happened that I was low on fuel, so I went to a different gas station and fueled up. Now the STFT at idle is +/- 1%, and never goes above +3% after stabilizing at idle.

The service manual lists "Water in fuel" as a potential cause of P0171...

Anyone have any ideas? No engine/intake/exhaust mods and otherwise the car is running fine.
 

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You mean to see if it's dirty? No, but I thought a dirty air filter would throw a P0172 (System Too Rich)...
Sorry, fat fingers. Seems like I read in the past where that code could represent either condition, but that's from feeble memory. If not, you might try cleaning the MAF.
 

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Discussion Starter #5
Sorry, fat fingers. Seems like I read in the past where that code could represent either condition, but that's from feeble memory. If not, you might try cleaning the MAF.
Sounds like that should be the next thing to check if another P0171 gets thrown, thanks.
 

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My engine has thrown 2 P0171 codes in the last few days... so after extensive searches here it seems the thing to watch is my short term fuel trim... my understanding is that it should be +/- 3% when at idle. My STFT was starting above +3% and creeping to >+5% at idle... that didn't seem right. It just so happened that I was low on fuel, so I went to a different gas station and fueled up. Now the STFT at idle is +/- 1%, and never goes above +3% after stabilizing at idle.

The service manual lists "Water in fuel" as a potential cause of P0171...

Anyone have any ideas? No engine/intake/exhaust mods and otherwise the car is running fine.
I get P0171 anytime I run a tank of Shell (93-oct).
 

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Did the gas stations in your area recently switch to "summer blend" gasoline?
 

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Discussion Starter #10
I get P0171 anytime I run a tank of Shell (93-oct).
Coincidently, the fuel tank that threw the codes was Shell (but Cali-blend 91 oct). Do you reset the code manually, or let the ECU reset the MIL itself?

Did the gas stations in your area recently switch to "summer blend" gasoline?
That's a good question, I'm not sure how I'd find that out for sure. The article you referenced said that in California they start selling "summer blend" in May, so that's possible.
 

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THe CEL light pops up for me more often when they do the switch between winter and summer blends. Before i got tuned wheh i was stock, i usually just reset the ECU and then the ECU "learns" the new gasoline.
 

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Big Bear Grizzly » News

California motorists get what they pay for. The state requires a special blend of cleaner burning gasoline and Californians pay for the difference each time they fill up their cars.

“Gasoline is a cocktail,” said Susanne Garfield, California Energy Commission spokesperson. California’s mix needs to burn cleaner to meet state air quality standards. The result is an extra 8 to 9 cents per gallon paid by the consumer at the pump, she said.

In 1996, the state’s 14 refineries were required to remove butane from gasoline during the summer months, said Joe Sparano, president of the Western States Petroleum Association. Butane improves gasoline’s octane rating and is clean burning. It also makes gasoline less dense and more prone to evaporation. Evaporating gasoline pollutes the air.

During the summer, butane is replaced by ethanol, which is very expensive, Sparano said. Summer gasoline flows through the gas pumps from April 1 to Oct. 31. The butane is put back during the winter to make starting the engine on cold days easier, he said.

“Of course you want cleaner burning gas,” said Sharon Hendricks, about summer blend gasoline. Hendricks lives and works in Big Bear. She doesn’t have a long commute to work, but said gas prices make everyone’s wallet lighter whether you drive or not. Higher grocery bills and the downturn in the economy are all related to gas prices, Hendricks said.


Before the refineries can produce the summer blend, they must clean all the equipment to remove winter blend residue, Sparano said. “It is illegal to sell the winter blend during the summer,” he said.

The turnover takes four to six weeks and the down time at the refineries causes the price spikes common in the spring, Sparano said.

Garfield disputes Sparano’s claim that the refineries close to convert to summer gasoline. She said she has never heard of any refinery shutting down. The refiners have to deliver the product to the gas stations by April 1, but they can’t guarantee that there isn’t winter blend in the retailer’s tanks after April 1, she said.

Patrick Kelly, fuel associate with the American Petroleum Institute, said the conversion from winter to summer gas varies from place to place. “Typically refiners don’t shut down the whole refinery,” he said. Different segments of the refinery are taken off line and retrofitted for summer gas. The process takes less than a month, he said.

A representative from BP America, who spoke on the condition of anonymity, said the company does routine maintenance to its refineries, but never completely shuts them down. BP America is the largest refiner of crude oil in California, according to the California Energy Commission.

The conversion from summer to winter gas is much easier, Sparano said. It isn’t illegal to sell summer gasoline after Oct. 31, and refineries don’t need to halt production. “We can gradually work back in the winter blend,” he said.

California’s special blend is also hard to come by. The large refineries along the Gulf Coast, which produce and deliver most of the nation’s gasoline by pipeline, seldom produce California’s cocktail, Garfield said.

However, when demand spikes, like it does during the summer months, refineries along the Gulf Coast get requests for California blend gasoline, Sparano said. If the refineries are already configured to make the blend, it still takes 14 to 17 days for the gasoline to arrive by ship, he said.

California’s expensive gasoline cocktail is worth the price, Garfield said. The cleaner-burning fuel has reduced smog levels in the state even though the number of cars has increased. “We have 25 million cars registered in California. Imagine if we didn’t have cleaner gasoline,” she said.

Hendricks has seen the price to fill up her Nissan Altima shoot up from $30 to $50 in the last year. Her face sagged as she filled her tank May 2 on her way home from work. “I would use dirty gas if it was cheaper,” she said.
 

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Discussion Starter #14 (Edited)
It could be bad gas, typically most cars sit at around +3% on the STFT, %5 isn't abnormal, over 17% is.
Thanks Charlie...

The stored frame data had +16.7% STFT, +7% LTFT....

1100 RPM, 23 gps on the MAF, 8 MPH... so probably just cracking the throttle from a stop...

I haven't cleared the last P0171... but it's no longer pending (obviously). if the ECU senses another P0171 condition can I tell? Will it throw a second code of the same type? Or would I have to reset the existing code first?
 

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The ecu internally stores two sets of codes, one readable with all working OBD II reader, one not so much, it then counts for a preset number of times/starts that the condition exists, if it does it'll throw the code and the light comes on, theres also the 'just happened' codes, and the happened on a previous run codes, pending vs stored.

Usually if there is something amiss it registers the code internally within 10 seconds, monitoring the STFT to see if its >17% will give you an idea.
 

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Discussion Starter #16
The ecu internally stores two sets of codes, one readable with all working OBD II reader, one not so much, it then counts for a preset number of times/starts that the condition exists, if it does it'll throw the code and the light comes on, theres also the 'just happened' codes, and the happened on a previous run codes, pending vs stored.

Usually if there is something amiss it registers the code internally within 10 seconds, monitoring the STFT to see if its >17% will give you an idea.
Thanks again. My OBD II reader has a graphing function that shows the last few hundred seconds of data, plus high and low readings during the session. I'm going to try driving it around with the reader monitoring the STFT...

The service manual says that if the +17% STFT condition is met on two consecutive "trips", the code is thrown. It can't be thrown at idle since it doesn't meet the enable criteria. I'm not sure what the ECU considers a "trip" though... an engine on/off cycle, or maybe a minimum number of seconds of run time, temperature cycle, etc.
 

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Discussion Starter #18
Thanks again. My OBD II reader has a graphing function that shows the last few hundred seconds of data, plus high and low readings during the session. I'm going to try driving it around with the reader monitoring the STFT...

The service manual says that if the +17% STFT condition is met on two consecutive "trips", the code is thrown. It can't be thrown at idle since it doesn't meet the enable criteria. I'm not sure what the ECU considers a "trip" though... an engine on/off cycle, or maybe a minimum number of seconds of run time, temperature cycle, etc.
OK... the results of a short but varied drive show that the STFT varies from -15% to +27%... but the flash RAM in the reader doesn't hold enough data for me to see how long it as at +27% and what the concurrent values of the other parameters were (i.e. the MAF went from 3 gps to 200 gps... RPM's topped out at 7800, but I can't correlate that to the peak in STFT). Tomorrow I'll get a (spare) USB drive so I can store more data.
 

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Discussion Starter #19
The latest update: called my dealer's service manager to get his opinion... he told me to bring the car in if the MIL didn't reset after 5-6 trips. It didn't, so I did. Diagnostics indicated car was running fine, so they cleared the code, it didn't throw another, so they gave me the car back. So far, so good. So, it's quite possible that it was the gas...
 

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My 2007 Exige S is throwing the code like there is no tomorrow. I finally have a OBD-II scanner with live data. The car idle sounds fine without any abnormal movement but the STFT at idle is well into the double digits and over +20.

Would the ECU gurus please look at this thread and give suggestions.

Thanks a lot.
 
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