Guys I am on a quest to solve this (172) lean condition but I am about out of ideas. My truck is a 2nd owner bone stock 1996 F250 4WD, Auto, 5.8 liter with 133k miles. The truck is in very nice shape and has been well taken care of by the previous owner. Let me start by saying.

The truck starts perfect, idles perfect , accelerates perfect, shifts perfect. You would not know anything was even wrong with the truck but throws a code 172 CEL.

Now what's interesting is: It only throws this code once you are out on the highway at cruising speed. If you are putting around town all day, I have never had the CEL come on. This is the only code that it has ever registered.

I have owned the truck about 5 months and this is what I have done to the truck:

1) Fuel pressure regulator
2) Front fuel pump (crossflow problem fixed and great fuel pressure now)
3) Fuel Filter
4) 02 sensor (truck only has one)
5) Pan gasket
6) Radiator and thermostat (small leak in radiator)
7) Plenum gasket
8) Repaired broken bolt in exhaust manifold
9) Repaired slight leak in EGR tube

As far as I can tell the truck now has no exhaust or vacuums leaks it sounds and runs great but after everything I have done I have not effected the (172) lean code at all. I can just about tell you where on the highway it is going to light up it is so predictable and frustrating.

The one thing I have not done is clean the fuel injectors. Any thoughts or additional questions would be greatly appreciated.

Thanks,
Mike

 

Have you checked vapor return? Could be as simple as running down the hwy it's pulling a vacuum on the supply because of not venting correctly? I changed to locking caps at one time and created a similar problem

 

I haven't checked that but I do have dual tanks and the code will appear from either tank.

One thing I may mention that probably has nothing to do with anything is:

The temp gauge continually, slowly oscillates between the "M" and the "O" while running at operating temp. It will climb then cool, climb then cool, etc. It has always done this before and after the radiator and thermostat. Maybe this is common but I think it tends to run a little too cool.

Just thought I would mention it.

 

What O2 sensor did you get?
Code 172 is a direct O2 sensor failure.... might try a better O2 sensor.

 

The code could be a faulty O2 sensor or due to not enough fuel getting to the motor at highway speeds. When the computer demands more fuel to the motor and can't get it, a lean code will be thrown. If it's not your pump then it's got to be the sensor. An exhaust leak will make the O2 sensor detect a lean condition but the exhaust leak shouldn't do that for all driving conditions.

While it is difficult, you may be able to test the O2 sensor under working conditions. One of the three wires is ground, one is incoming voltage and the last is the value that the computer uses. If you probe the ground wire and the wire that gives the reading to the computer, you should be reading around 0 or 0.1v while motor is not running (lean) and while the motor is running you should see 0.4-0.5v (stoichiometric) and anything above 0.6v will be rich.

 

According to my Ford Service manual Code 172 can be displayed in the Continuous Memory display, historical, or during the Key On Engine Running tests. When are you seeing this code displayed?

I disagree that is an indication of a faulty O2 sensor, in most cases it's a symptom.

 

Thanks for getting back to me guys.

I am seeing this code after the CEL has come on then I check it with the jumper wire and with the engine off. I think that code is stored until I unplug the battery for an extended period. Then of course the next time the CEL comes on it is registered and stored again. Hope this helps?

I kind of agree, while it's possible I received a faulty 02 sensor not likely that both are bad and emit the same code under the exact same driving scenarios.

While my fuel pressure is 32lbs at idle and spikes to near 40 under throttle (appears strong and sufficient) Is it possible the actual injectors are plugged enough to not supply the cylinder with the needed amount of fuel and cause a lean condition....Like RIKIL eluded to?

I don't notice any stumbling or misfiring but I do notice a little pinging upon acceleration at a certain rpms range. I have not checked the timing. I don't know if that could in some way cause a lean code. Like I said, I get no other codes, ever.

keep the thoughts coming guys.

Mike

 

It isn't the injectors. All of them would have to be running lean, they aren't.

It's the O2 sensor/s not switching when it needs to.

 

They way I read the OP's opening statement Code 172 was happening with the original O2 sensor and with the replacement. The odds of two bad O2 sensors are slim, but not impossible.

 

That is correct. I replaced the original Ford 02 sensor with a Bosch 02 sensor and they appear to act exactly the same. I am not going to rule out a coincidental bad 02 sensor but it seems unlikely.

I thought I would test and inspect the wiring to the sensor this weekend as well.

Any ideas, keep them coming.

Mike

 

Could you explain in more detail your statement in bold? Does the pressure "spike" then drop back to ~32 under full throttle?

 

Sure.

When I first got the truck the front tank pump supplied about 28lbs at idle and would actually decrease pressure when you throttled the engine.

After replacing the pump it now holds a steady 32lbs at idle and gradually increases as you throttle the engine. It does not spike or fluctuate. Sorry for the confusion.

 

Dirty MAF Sensor?
I know I have seen them dirty and get the A/F ratio out of whack making a lean code pop up on occasion...

Simple to clean with the spray they sell at Vato-Zone.

 

He said he only has one O2 sensor, so he probably has Speed Density like me and no MAF to clean.

I have a similar problem with mine, but I only get the code 172 during a KOER engine running self test, NEVER during normal operation.

 

Flecker, you were right.....sort of. It was the 02 sensor. I'm an idiot. Let me explain:

I replaced the 02 sensor in an attempt to fix the lean code....and it acted the same way as the initial 02 sensor did.....so I assumed it was not the culprit.

Unless if you don't plug it in all the way. Then it acts exactly as the failed sensor did.

The plug sits atop the transmission and requires that you sort-of bear-hug the tranny and feel your way. I guess I just didn't plug it together far enough the first time. It nearly fell apart when I checked it. I knew right then what had happened. I firmly plugged it together until I heard the snap of the connector latch and have had no problems since.


Thanks for all the input guys, I really appreciate it.

Mike