As for the carb cleaner trick, if it is a MAJOR vacuum leak you would detect it... for a small leak, you would not as the DME does compensate quite fast!

What I do when tracking down a vacuum leak is either use a smoke tester or read the upstream o2 sensor signal voltages then spray the cleaner. When you hit the leaks with the cleaner the o2 sensor signal voltages stick at the high voltage.

I'm with you 01, will test it out this Friday...on a side note, I wish I never installed that K&N filter, I keep reading how the oils can damage MAF sensors...

Ahh, go to the parts store and get a can of MAF cleaner... odds are that's not the main issue, but it will help and should be done by anyone with an oiled filter.

Thanks for your response weasel56. Is there a DIY guide (I did a quick thread search to no avail) to read the upstream o2 sensor signal voltages? I think that this will save me both time and $$ and be useful to others as this seems to be a very common problem...

I read the values through the diagnostic equipment at the dealer (GT1). That is the easiest way to read the values it through data communications in a diagnostic computer (some indy shops have euro software for the snap-on modis) Otherwise you would have to backprobe the signal wire at the o2 sensor plug and read it with a multimeter, which can be a pain.

Otherwise a smoke tester is good because it pumps nitrogen smoke (like a party fogger) into the intake system and you just look for the smoke coming from the leaks.

Funny - I just started to get these codes as well. 2002 3.0 with 202,000+ miles.

Last week I had a brief stumble in the morning, followed by the SES light on the drive to work. Other than the stumble the car drove just fine, with one significant difference - much BETTER gas mileage! I usually get around 19, I suddenly was getting 23+. When I was able to read the codes I got:

P0171 (System too lean, Bank 1)
P0174 (System too lean, Bank 2)
P0300 (Random/multiple misfire detected)
P1347 (Misfire cylinder #3 with fuel cutoff)
P1348 (Misfire cylinder #4 during start)
P1350 (Misfire cylinder #5 during start)

I noted them, cleared them & left the reader connected. The next day the SES returned with all the above codes and these:

P0302 (Cylinder #2 misfire)
P1083 (Fuel control mixture lean, Bank 1)
P1085 (Fuel control mixture lean, Bank 2)
P1342 (Misfire cylinder #1 during start)

I took it to my indie repair shop (good guys) and they replaced the pre-cat O2 sensors and the intake boot which had a crack in it. I could discern no difference in behavior of the car before or after the repair, but right after I picked it up my fuel consumption was back where I thought it should be. Then the next day I got the SES again with just P0171 and P0174, along with the reduced fuel consumption, albeit only up to about 21 MPG. I reset the codes again, and although the light has not come on again I can see P0171 and P0174 are pending.

What's interesting is that this began to happen immediately after I filled up at a station I don't usually go to, but I was low and running an unplanned errand. It was a Miristar and the pump said the fuel could contain up to 10% Ethanol - has anyone else experienced any such connection?

Still, the car runs great - fires right up without hesitation, not lacking power, smooth, and if not for the lights/codes and the increased mileage I'd have no idea anything was going on!

I too have the codes P0171 & P0174 and failed my states emission test to renew my license plates. I just received the Air Mass Meter from Partsgeek.com ($314.95) cheapest I could find. Replaced it and used my ACTRON to read the codes. Same codes popping up. I went ahead and erased the codes and will drive for a week and see if they return. I have a 2001 4.4 with 87K miles. Never replaced the plugs, should I? I check for leaks around the air filter housing and the boot that attaches the Air Mass Meter and found no leaks. Any more helpful suggestions?

What is connected to the front bottom of the Air filter cover that has a hose connected to it?

I had the same codes about a year ago. Erased them with my OBD reader and everything was fine until 3 days ago when the Check Engine Light came on. To my surprise the OBD reader found the same codes (I think there were 11). Cleaned them and so far no Engine Light. Hopefully I'll be spared for another year.

well, I wasn't that lucky this time.
adnoid....are the pre-cat O2 sensors the same as regular O2 sensors? I don't see any other type of sensor on realOEM other than the regular ones.

As far as placement goes, pre-cat monitors the air/fuel mixture for running purposes, post-cat monitors the performance/efficiency of the catalyst as far as cleaning the exhaust.

You can see in the link it calls one "regulating" and one "monitoring".

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