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post #1 of (permalink) Old 11-01-2007, 12:38 PM Thread Starter
 
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check engine code p0171

Hey guys,
My 1998 jeep wrangler 2.5l has a check engine light throwing a P0171 lead code. The exhaust has a rich smell and the new spark plugs are carboned fouled. I have checked for vacuum leaks and cleaned the MAP. The exhaust pipe has a small leak just before the CAT. Any Ideas?
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post #2 of (permalink) Old 11-01-2007, 02:53 PM
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Did you try searching?

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post #3 of (permalink) Old 11-01-2007, 03:30 PM Thread Starter
 
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Yes, I have tried searching. Most descriptions are for a vacuum leak or dirty MAP. The carbon in the exhaust and on the spark plugs indicate a misfire, pushing unused oxygen and fuel into the exhaust. Since oxygen sensors only read oxygen levels, the sensors are reading the oxygen from the misfired cylinders. All of the sparkplugs are carbon fouled, so a bad injector is not the cause. A bad coil could be the problem but I have never heard of a coil going bad on a small unmodified engine. The jeep is still drivable but it runs rough sometimes and then runs fine. I think a bad coil would be bad all the time. The plug wires, cap and rotor have been replaced.
What I need is some sort of diagnostic trick to determine the cause.
Thanks for taking my call.



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post #4 of (permalink) Old 11-01-2007, 04:05 PM
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I guess it would help if I would read the full text....

Yea, you're right....maybe.

The computer is throwing a lean code because it thinks the mixture is too lean....and as a result is pushing more fuel in (rich mixture).

If you had a OBDII scanner you could look at the signal from the O2 sensor. You could try measuring the output of the sensor with a voltmeter....you will need to look up what the correct signal is....but if the O2 sensor is reading low, it's going to make the computer think you are lean and the system will throw more fuel at it until it hits the high end.

You might want to also check the temp sending unit. It could be telling the computer you are colder than you really are...

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post #5 of (permalink) Old 11-01-2007, 05:03 PM Thread Starter
 
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Thanks for the advice. My jeep has a modification to the the air intake that brings in cold air from outside the engine compartment. That could be causing the air temp sensor to read cold. Also I did notice an exhaust leak at the end of the CAT right before the 02 sensor. Both of these could cause the computer to change the air/fuel mixture. The cold air could cause the computer to think the engine is cold and make the mix rich (like a choke). Or the sensor behind the CAT could be reading the leak as outside oxygen level and sending a screwup signal to the computer.
The cold air could be checked by putting back the original air intake setup.
I will do that first, then fix the leak with some metal sealer.
Thanks again I will keep you posted.
As usual this forum has provided me with valuable information and allowed me to bounce ideas around.
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post #6 of (permalink) Old 11-01-2007, 09:59 PM
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an exhaust leak can and will make the O2 sensor read lean

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post #7 of (permalink) Old 11-02-2007, 08:44 AM
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While it's running - try directing a blast of compressed air at the exhaust leak. You should hear an immediate RPM change. The extra air on the O2 sensor should change the mixture even more.

Remember, the exhaust is not just pressure - the pulses also create a vacuum that "sucks in."
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post #8 of (permalink) Old 11-04-2007, 04:40 AM
 
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these guys are sure right. an exaust leak before the sensor will draw more air past the sensor. this in turn will tell the computer to add more fuel. causeing a too rich situation. then fouling your plugs and making the whole engine run bad. put a new pipe on it, clear the codes and drive it.
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post #9 of (permalink) Old 11-04-2007, 01:31 PM
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Isn't it interesting how computers throw out simplicity.
It used to be lean meant lean - but in this case the engine ran rich!
Yes - the O2 SENSOR saw it as lean. But we aren't O2 sensors.

A bad spark plug can do the same, or similar thing. Even a vacuum leak that affects just one cylinder can do it too.

For example - think about an injector that is not closing properly - a mechincal problen with the injector - dirt, carbon etc.).
A little drip makes that cylinder run rich, the O2 sensor doesn't see enough O2 from that cylinder, so it leans the entire system. Now the rest of the cylinders suffer a little. Adaptive memory will change to where it thinks that's normal.

But then as the drip gets worse and that cylinder starts misfiring because it's flooding out, the extra unburned O2 from that cylinder causes the O2 sensor to think the system is way too LEAN, so it richens it up and sets a lean code.
One thing affects other things!

We have to learn to think backwards sometimes. It make it interesting, and very challenging at times. And sometimes downright frustrating!
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post #10 of (permalink) Old 11-05-2007, 02:21 PM Thread Starter
 
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So guys here is the skinny on the problem so far. I found an exhaust leak near the forward 02 sensor and the CAT. Fix both of them using a metal sealer. Also found a small vacuum leak in the hose leading to the ventilation vents. Fixed that. Then I cleared the code and fired up the jeep.
Check engine light came on again with code p0171 lean. The exhaust is still running rich.
Is there any way to check the other monitors, (02, map, coolant & air temp) to see if they are working properly? Has anyone ever used the software Autotap? I read it is useful in reading all monitors and their levels at any time.

I've been searching the web for any information and found that code p0171 is a real hard nut to crack. Apparently it is one of those codes that indicated a mulitude of possibilities. Fuel pump, weak coil, bad plugs, bad 02, MAP, fuel injectors, air & coolant temp, exhaust leak, vacuum leak, bad spark plug wires, and some other things I have already forgotten.
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