I have a 2002 AWD V6 Ford Escape. My Escape has 210,009 miles on it and was running good up until about a day ago, actually it still generally runs good but it’s throwing two error codes. They are:

PO171 diagnosed as System too Lean Bank 1, and PO174, System too Lean Bank 2.

To double check the lean running diagnosis I examined a spark plug and found it to be bone white. FWIW, I replaced the plugs about 681 miles ago with the exact same Motorcraft AWSF-32F spark plugs. The old worn plugs I replaced were light brown just as you’d expect from a good running engine, now they’re white confirming the lean running engine codes. BTW, since I had to take off the intake manifold to replace the rear 3 plugs I also installed new manifold gaskets to avoid any intake manifold leaks and up until just yesterday the cars run fine (except for the annoying cold engine rough idle I’ve noticed). The engine doesn’t stall but you can hear the roughness at idle and the RPM’s drop slightly.

I live in SE Arizona and earlier in the day yesterday, just before the check engine light came on, I’d taken a 12 mile drive on a very rough surfaced gravel road up into the nearby Huachuca Mountains. The drive was 6 miles up with a 2,000’ elevation gain followed by 6 miles down on the same rough road.

My first thought was the 12 mile rough drive had stirred up gas tank sediment which then partially clogged the gas filter. Gas filters are cheap so I replaced the filter but it didn’t solve the problem.

I inspected all the vacuum hoses and found no obvious leaks or disconnects.

When I replaced the worn spark plugs a few weeks ago it was because gas mile had dropped from averaging 18mpg down to 15.5mpg. After I replaced the worn plugs (but before this trouble started) my mileage was back to 18mpg.

Lately, the car’s been running a little rough at start-up but it soon settles to normal running when the engine warms up.

About the only thing I can think of is that it needs new O2 sensors, but since I hate simply throwing parts at a problem, I’d really like to hear some other people’s thoughts on the matter.

Steve

 

Vacuum leaks can be there and we not hear them....shoot some smoke in there and chances are you will find it, but, 200k miles is about 100k past the replacement schedule for the O2 sensors so I think you have a couple of things to check out.

 

Oxygen sensors will NOT cause this symptom, they're simply reporting what they're "seeing". FWIW, I only replace them when they exhibit a specific behavior that indicates failure. You can replace yours pre-emptively, if you want to. It's skin off your knuckles (good luck on the bank 1 upstream sensor!), not mine.

My money is on a vacuum leak also. Since you just took everything apart, I'd further wager that something isn't tightened or connected properly. Having worked on the 2.5 & 3.0 L Duratec engines for 20 years now, I've had plenty of opportunity to do things the wrong way first.

Since I don't have a smoker, my preferred method is to use my scan tool equivalent (either Torque Pro or Forscan) to watch the short term fuel trims while selective spraying a flammable (I usually use brake parts cleaner since I have it on hand most of the time, Ford recommends an unlit propane torch). The STFT reacts almost instantly when that stuff is sucked in allowing you to see where the unmetered air is coming from.

BTW, "lean" codes do NOT mean the engine is actually running lean. Usually it's just the opposite. Due to unmetered air (usually) entering, the O2 sensors are reporting a lean condition in the exhaust so the PCM adjusts the fuel injector pulse widths to ADD more fuel in an attempt to reach stoic. When the combined short and long term trims limit out (maximum enrichment) but the O2 sensors still report a lean exhaust condition, the PCM sets the appropriate DTCs.

 

Thanks projectSH089!

I appreciate your detailed response. Just one thing, since I’m not a professional mechanic I don’t understand a few of the acronym’s that you’ve used. I know what a PCM is of course and I think STFT is your scan tool (about the only thing that makes sense!). But, what are DTC’s?

I certainly don’t want to replace the O2 sensors if I can help it. I haven’t inspected the engine for the rear sensors exact location but I’m certain the sadists laughingly figured out somewhere painful to put it.

Thank you again for your help.

Steve

 

STFT = Short Term Fuel Trim (Ford uses "SHRTFT"
DTC = Diagnostic Test Code aka "fault codes"

I've attached a couple of TSBs that may be of use.

 

Bah, Mine went 299k and never once changed an O2 sensor.

 

I finally ran some tests on my Escape. I first hooked up the scanner and started the engine. Earlier I’d driven the car a few miles so it was warmed up and running okay. I’d also removed the air filter so I could do the test that projectSH089 recommended, only I used my butane torch. When I started the engine with the scanner hooked up it ran okay. However, the STFT – Bank 1 read 0.0, Bank 2 gave a reading of something like 3.2 (being Arizona it was sunny and the scanner was very hard to read). When I opened up the butane torch valve the engine immediately picked up RPM’s and ran better. I read the scanner and the STFT – Bank 1 remained at 0.0 while the STFT – Bank 2 bounced all over with a high reading of 19.7, once again it was hard to read the scanner in the sun. Unfortunately, I have no garage.

After I confirmed that it actually was running lean, I looked for vacuum leaks the only way available. I used the tried and true method of disconnecting vacuum hoses and either blowing into them or sucking on them (if they appeared to be non-fuel related such as the power brake vacuum hose).

I didn’t find anything that appeared to be an issue, though there was one large hose that went into the intake manifold close to the firewall that when I blew into it it seemed wide open. However, I tried running the engine with my finger over the manifold fitting and it ran okay, when I removed my finger the engine stalled. I’m guessing it was maybe a vacuum line for the fuel tank.

I hate to say it but I think I need to remove the intake manifold and check the 6 intake gaskets that I installed over 681 miles ago when I replaced the spark plugs, only this time when I put it back together I’ll spread a very small amount of silicone (probably using a Q-tip) into the gasket slots so that the gaskets are lightly “glued” into place.

I want to thank the folks here who have been helping with this lean running issue I’m having. It’ll probably be a few hours before I begin replacing the manifold gaskets, if anyone sees anything I’m doing wrong or that I should check please feel free to let me know. I’ll recheck this website before I begin work.

Steve

 

First off I want to again thank those who've helped me with this lean running problem I've been having. Apparently the problem was caused by badly rotted vacuum lines but my work somehow seems to have caused a single cylinder mis-fire problem that I've written about in a separate thread.

Steve