Stanley Wright

Having trouble with check engine light. The light came on so i went to the part store and check the code. The code was P10171. So hoping this was a fluke I disconnect the battery. A couple of days later the light came on again. Most websites said to clean MAF sensor and disconnect the battery so I did and no light for a week and then tonight the light came back on!

I have a 99 Cobra with 110,XXX miles on it. Any ideas before I take it to the shop.

JC316

Either a vacuum leak, weak fuel pump, or a bad 02 sensor.

Stanley Wright

Hey thanks is there a way to determine which out of the fuel pump and O2 sensor? Also, I took the entire intake duct off and found dirt and dust all over. The throttle body was covered with a little grime on the outside and thick black grime inside so I used throttle body cleaner and cleaned it up. Disconnected the battery and cleaned the MAF and Temp sensor reconnected everything hoping that would solve the problem. Took it out for a ride and no light but it usually take a couple of days. So should I wait and see or go ahead with replacing parts?

spaseghost

Wait and see. You cant fix what you don't know isnt broken. Check your PCV as well, seems to be pretty common. You could always make/buy some 02 sims...

cliffyk

Here is what the shop manual says about P0171, with possible causes and diagnostic tips. Work the list of causes from the top down, however as it is a bank 1 (passenger side) DTC focus on things that would only affect one bank. These would be predominately the front O² sensor and exhaust leaks.

One of the first tests should be to use a wideband O² system to determine if the engine is really running lean, or is it's just a bad O² sensor or exhaust leak making the PCM believe it is lean. A dead/dying O² sensor will produce no or a low voltage output, this PCM interprets this as the AFR being lean (when it really isn't) and it then attempts to enrich the mix. When it reached the limit of its ability to enrichen the mix (+25% more fuel) it will throw the "system lean" DTC.

Exhaust leaks before or near the O² sensor will also create a false lean indication.

The exhaust flow is not one homogeneous stream, but rather a series of pulses created by each exhaust event. These pulses have a head, body, and tail; each with different characteristics, their pressure relative to the outside world being the most significant.

• The head is at a pressure higher than ambient, when it passes the "leak" the gas flows out into the world; however the chemical composition of the gas in the pipe remains unchanged;
  
• The body is at or close to ambient pressure. When it passes the leak there is no flow in or out of the pipe as the exhaust gas and ambient pressures are nearly the same--again the chemical composition of the gas in the pipe remains unchanged;
  
• The tail is at less than ambient pressure (nearly a complete vacuum), and when it passes by the leak it sucks in ambient air--changing the exhaust gas chemical composition and making it appear "leaner" to the O2 sensor;

This is why a leak makes the PCM believe the mix is lean, a big or close enough leak can cause the sensor response to slow or even appear "stuck".

wifes pony

Excellent information!

spaseghost

Amazing, I fade from my old forum and reference cliffyk in another post and *BAM*, here he is. Truly a savant like he says.

Stanley Wright

Wow this is very helpful and I can't thank you all enough. I will keep you up to date with this issue of mine.

Stanley Wright

I have a small filter in place of the PCV and I cleaned it when i cleaned the intake duct. So is the filter on it a problem?

Stanley Wright

Update time! The last owner put a breather in place of the PCV. So I replaced it with the original PCV line that runs to the CAI. I then reset the r check engine light and it hasn't come back on since. Thanks for your help