Cat and o2 power loss
 

To fix a catalytic deficiency code on my '06 5.4 F150 I was told I needed new cats. The mechanic replaced both cats with Walker direct fit. I was told they would operate just like Ford cats at a lower cost. The deficiency code went away after the replacement, but I now have a o2 lean code on the passenger sensor, before the cat. This was replaced with a bosch o2 and the code remained. The truck runs fine, but I feel a loss in power, especially when towing my travel trailer. Any ideas? I'm tired of throwing money into parts and want to get to the bottom of it. Also, cops and plugs were replaced 20,000 mi ago.

may try an O2 extender if you have room, get it out of the flow a little. May be the new cats are flowing a bunch more air than the old ones. Or not.

Thanks for the recommendation. I've been long debating a programmer to regain some lost towing power. The truck runs great it just seams like with every key part replacement I lose some power. Sometimes I fee like it's an uphill battle. Does anyone have any experience using a programmer in this situation?

vacuum or exhaust leak?
 

@gdavis17
You could have a vacuum leak or small exhaust leak on the passenger side (although you did not mention any "tick" or other noise symptomatic of an exhaust leak). However, either one will result in positive fuel trim to compensate for the un-metered air getting to the front O2 sensor - and cause a "too lean" code. It would be helpful to know what live readings are for short and long term fuel trim is bank 2. Generally an exhaust leak will effect one bank where a vacuum leak will usually (but NOT necessarily always) effect both banks. But positive fuel trim could have resulted in excessive fuel metering and the destruction (by over heating - melting) of the catalyst that caused your "catalyst efficiency low" code.

The O2 sensors are such a vital part of the ECU's positive feedback (closed loop) concept, I would not tamper with an extender. Not sure how @tsigwing's O2 extender moving the sensor out of the direct exhaust air stream would actually change the stoichiometric ratio of the combustion gases. It would instead dampen the sensors responsiveness to changes in injector pulse width - which is a vital part of the ECU's constant real time catalyst efficiency tests! I would be more in favor of replacing all of the O2 sensors (preventatively) if the vehicle has > 100K miles or something has already resulted in the destruction of the cats.

As for a programmer regaining some lost power - one would have to know what parameter changes (that the ECU isn't already trying to do) would actually accomplish that. I for one don't know what it would be that wouldn't effect something else negatively - even if it be only fuel economy.


Do you have a scanner that will give you live data readings?

If the code is from the front o2 sensor using an extender will not solve this!

I would feed it another O2 sensor and reset the ECU. If it continues then I would pull plugs and see which cylinder is running leaner than the others...

Cheap tests first
 

There SHOULD NOT be any cylinder actually running "lean" when the ECU is reporting lean on bank 2! - That's because the ECU will increase the injector pulse width (fuel trim) to its MAXIMUM before reporting "lean on bank 2". [ Even with 1 injector totally plugged up to the point of missing (& gdavis17 says it runs fine), the ECU will increase fuel trim (+) enough to normalize lambda at the upstream O2 sensor. Then 1 plug will be white and the other three will be black. Then you _might_ have a lean indication.] This DOES assume the "new Bosch O2 sensor" is working properly. Which assumption I am willing to make until an inexpensive tests is done rather than condemning the O2 sensor. I'm a strong advocate of "challenging" a sensor in some way to see if I get the expected result.

I would suggest two (inexpensive) things as follows: 1.) spray starting fluid (ether) or carb cleaner around / under each of the intake manifold runners on bank 2 - with engine at idle [and while monitoring O2 sensor signals with a scanner if possible]. Of course - not while you're smoking a cigarette or without a fire extinguisher nearby. If there is a vacuum leak - idle WILL CHANGE, and the O2 sensor signals will GO CRAZY!
2.) If test number one is negative, I would use a stethoscope (or piece of 3/4" radiator hose) and listen for an exhaust leak ALL around the bank 2 exhaust manifold down to the CAT (including around the "new" O2 sensor - to make sure it was tightened properly. If there is a leak ----- FIX it.


After this - I'll be scratching my head some more -------

Thanks for all the great advice. After reading all this I'm wondering if it's something easy. I noticed that the replacement cats came in two pieces that sleeved inside one another in the middle. When I got home I noticed the mechanic didn't weld or clamp the connection. The connection was just dry fit. I called and asked him and he said the exhaust will plug it up and it's all they always do. Is this true or can it be the culprit?

I am in no way an expert on cats. But my gut feeling is I can't imagine anything in the exhaust system being "dry fit" without leaking. (Why do they put gaskets on the Exhaust Manifolds, and weld header pipes? I looked around online for some info on Walker cats and did not see anything bragging about two part feature, so I do not know.


What I do know - O2 sensors (both fore and aft) are extremely sensitive to minor changes in oxygen concentrations. A very small hole in the exhaust system will upset its readings. Seems to me that a two part (un welded) cat would be very susceptible to allowing fresh air in.


The variable valve timing system on a 5.4L 3v engine spends a majority of its time "retarding" valve timing to some extent. Very frequently as much as 60 crankshaft degrees. Part of the reason for this is to regurgitate exhaust gases (exhaust valve closes late - after tdc on exhaust stroke) sucking some exhaust gases back into the cylinders (like an EGR valve does). It also seems that could suck fresh air into the upstream O2 sensor on a two part cat if the joined parts are not sealed.


JUST A THEORY. But I believe I would be listening all around that joint with my poor man's stethoscope (three foot piece of 3/4 heater hose).

Yeah I don't believe the mechanic on his dry fit snake oil.