My 97 Explorer has just around 100,000 miles (if I remember right) and is now getting error codes P0171 (System too lean) and P0304 (Cylinder 4 misfiring). A few months ago I changed all the spark plugs (except 1 that I couldn't get out), the plug wires, and the fuel filter. At that time I was getting a misfire as well... A few months before that I replaced an O2 sensor...

Most of the forums I've read said that these error codes mean I've got a dirty MAF sensor or leak in the intake somewhere after the sensor. I cleaned the MAF sensor with the CRC cleaner designed for such a task, but the vehicle seems to be running the same. I read one site that said if you unplug the MAF sensor than the vehicle kicks into bypass mode and runs a little rich, but smooth. I tried this trick today, but still no difference.

This, I would think, would outrule the possibilty of there being a leak in the intake, since it really doesn't even know how much air it's getting... Any advice is welcome!!

Thanks in advance,

Ben Reese


P.S. For others with the same issues: The MAF sensor comes off with a secure/anti-theft/anti-tamper torx driver. I got mine from O'Reily... There is 2 small bolts and one is covered with a plastic cap, but I just used pliers to extract that bolt...

will check at work what this could be..im a front end tech for a ford dealer so ill ask one of our tune up guys what they think it may be..

will try to post tomrrow if i can

Was the plug you could not get out the back one on the passenger side? If it is, that is cylinder number 4. You need to change the plug you could not remove first. Then worry about the P0171. You may be able to erase the code and see how long it takes to come back before it worries you much. Curing the P0304 may cure both of them.

Was the plug you could not get out the back one on the passenger side? If it is, that is cylinder number 4. You need to change the plug you could not remove first. Then worry about the P0171. You may be able to erase the code and see how long it takes to come back before it worries you much. Curing the P0304 may cure both of them.

Yeah, I'm pretty sure that was the plug I couldn't get out. It had what looked like a heat shield around it so I couldn't get my socket on it.
I figured the misfire was a symptom of the lean mixture.