Hi!

When I left for work this morning my '01 f-150 (2wd, 4.2L, M5OD) drove very roughly. When starting out, and for 5-10 seconds after every upshift it would shudder and buck, but would smooth out as I got up into the power band. Any time I lugged the engine down a bit it would do the same thing. After about 3 minutes of driving the check engine light came on and started flashing -- for about 30 seconds -- and then went out and stayed out. While driving I dug out the OBD II and plugged it in. It returned two codes: "System too lean (Bank 2)" and "Misfire, Cylinder 3."

After work, on the 40 min. drive home, the truck ran fine, with not one misfire, shudder, or code appearing.

What could all this mean?

The only other info I can think of is that it has been below freezing here for something like 35 days, and just thawed for the first time yesterday. Then it froze again last night, and thawed again today.

Is there some chance that a bunch of condensation had been accumulating, in a frozen state, for the last month and then thawed yesterday, dumping a lot of liquid water into the system, and then some of that re-froze last night, with ensuing mystery consequences?

Many thanks for your insights!

 

The CEL flashing is a signal that raw fuel is getting into the exhasut which can cause cat failure. The two codes tell you exactly where the problem is, cylinder 3, which BTW is on bank 2. The lean code is telling you that the O2 sensor is reading rich, the PCM has tried to lean it out and has reached its limit, thus the lean code. Bottom line, check #3 plug, plug wire (or COP). Repair or replace as needed. Good luck!

 

When you speak of "cat failure", do you mean catalytic converter or catastrophic?

I apologize in advance, but I'm a newbie... If I understand correctly, it puts out a lean code when in fact it is too rich because the cylinder is not firing and the unburned fuel is ejected into the exhaust system?

 

Catalytic convertor failure and you hit the nail on the head.

 

Does a lean code always mean the system has attempted to adjust a too rich mixture to its max limit, or is that only true when it occurs simultaneously with a misfire code?

 

Cyl 3 is on bank 1

3 6
2 5
1 4

front


Probably a plug wire arcing due to the moisture. A tune-up is probably needed.
Check the PCV hose elbows for a vacuum leak.

 

A lean code can be caused by a lot of things, but in general it does mean that the CPU has skewed the mixture to full rich. This can be due to too much unmetered air into the engine intake. It is when the CEL is flashing and the engine is missing terribly along with lean codes, that cat damage can occur. Good luck!