I've got a box stock 4.2 in an 03 scab. About 3 months ago I finally changed the Plenum gaskets and Isolator bolts, it is so much smoother now. But the MPG's went down a little and that shocked me. It has come back up just a smidge but not like it should be.

Now for the question. I hooked it to a borrowed scanner to check on the O2's and now I'm confused. I'm not getting any codes or pending codes and it just passed Calif emissions with flying colors. But the scanner said thet the B1S2 is rich (that is where I think the milage went). So here is what I've got:

short term fuel trims are
B1S1= +2.2 to -2.3
B2S1= +4.0 to -3.4

Long term f/t is
B1=3.0%
B2=5.0%

B1S2 stays on rich
B2S2 switches rich/lean like it should.

Now there is 160k on the original O2's. Am I to assume that the front O2 on bank 2 is finally junk or do I need to look at something else? (I know I'm about 60 to 70 thousand miles past "normal life" ):o

Also seems bas-ackwards for the B1 to be rich. Can some one "SPLAIN" it to me, and type slow b/c I'm a slow learner!

The only thing I have done to the truck since the gaskets is oil changes and clay bar-don't think either one would make a diff here. ha ha

Thanks in advance
Jim

 

It wouldn't hurt to replace both front (S1) O2's. Use only Bosch or Motorcraft.

The rear O2's (S2) don't affect the way it runs one bit - all they do is monitor cat efficiency and trip a CEL if they detect a problem. If you are running rich due to malfunctioning front O2's, you could start to clog the cats.

 

With the numbers posted, are the front O2's with in range?

Also Bank 1 has the lower numbers at sensor 1 but sensor 2 stays on rich?
Would that be the Cat starting to go or O2?

 

I don't know how to interpret the numbers, but with 160k on the original O2's and the loss in gas mileage, I would replace the front O2's on general principle and go from there.

 

okay, i saw you said B1S2 B2S2 you are talking about the rear O2 sensors correct??

because actually the rear O2 sensors are only there to check cat efficiency, and SHOULD NOT be switching.. O2 sensors read oxygen you should have more oxygen after the cats than before so they will read a little high, but they should not switch, if your rear O2 sensors are switching that indicates that the cat is going bad. because you should be pretty steady on the rear O2.

and on the fuel trims, your LTFT reads +3% and +5%. if that is right, that means the vehicle was running LEAN, and the computer is trying to ADD fuel, due to adding percent. now that will lower fuel economy due to extra fuel being added. um i would say that there is most likely a small vacuum leak, you are getting more air than fuel, so my thought would be vacuum leak. i would check for one, small hoses cracked, gaskets cracked, just small things like that would cause one..

i would borrow that scan tool again, and watch the PIDs for front and rear O2 sensors the fronts should be switching, the rears should read what most people would say is rich and pretty steady, the more the rear switch, the worst the cat is doing its job of cleaning the exhaust.

also just pay attention to the LTFT, the STFT really is not a good diag tool..

 

Underdog96U
Member
Underdog96U has no trucks in their garage Join Date: Dec 2009
Posts: 44

Yes rear O2's

It was lean with codes before I did the Plenum gaskets and bolts. The battery was un-hooked all that day, could it be that the LTFT is stored and has not reset itself? I would not think that it would stay in the memory and the re-learn process would adjust to the new non leaking plenum, or do I need to do something to make it adjust?

How about the EVAP system, is that a possibility?

Bank 1 sensor 2 on the scanner says "rich"
Bank 2 sensor 2 on the scanner switches from "rich to lean"

So first order would to search for vacuum leak, correct?
What is PID, I hate to show my ignorance but it is outta the bag now!

I thought that the fuel trims were developed by the info from the O2 sensors, at least in part. That is why I posted them.

Thanks a lot Underdog that gives me some direction to start looking.
Hope it aint the cats!
Jim

 

well your LTFT have been reset when you unplugged your batter due to it being in the ECM Keep alive memory, it will stay stored in memory until the power has been cut for long enough for things like the radio and clock to be reset (those are also stored in the KAM) how long has it been since you have reset the ecm by unplugging the battery.

due to the battery being unplugged the current LTFT stats would be correct.

while i don't know the technical definition for PID, i can tell you that it is the stat that is read when you are looking at a scan tool, so the LTFT would be a pid for instance.

if they are switching just a little, i would not worry about them, the added fuel would most likely not be caused by them..

you said the are no pending codes, or codes present, again the question comes up how long has it been since the battery was unplugged.

as for you saying the vehicle passed a CALI emissions test, i would say again, don't really worry about the cats. with a 160K on the cats thats is still good. as long as you pass inspect i really wouldn't worry about them.


with that many miles i would like to ask, what if any vacuum hoses have been replaced. has the PCV (positive crankcase ventilation valve) ever been replaced?

to be honest please try to give me as much info as you can, and i can try to diag your problem.. such as how many miles since battery, what hoses have been replaced, PCV,, um on your vehicle the only real things on the EVAP system would be your EGR, DPFE, O2 sensors, and cats (if i am not mistaken,

now this might seem a little out there, but if you can get the scan tool look at your DPFE stats since that is what ford uses to see if the egr valve is properly working, i only say this, because you EGR valve could be stuck open slightly causing egr to flow back into your intake, causing a slightly lean reading on your air fuel due to the way egr works. (if you would like to know if you don't i can explain later,) with a slightly open EGR you will see a lower MPG, and LTFT will add some more fuel to bring the oxygen back to proper levels for the computer to like (i would not really worry about this, and DO NOT just assume its the egr and replace.)

how does the car idle??

Steven

 

OK Steven are you ready for this?

The truck had a P0171 and 174 (lean bank 1&2) for quite awhile and I tried ALL the cheap stuff to fix it. Checked and re-checked the vac lines and replaced the one by the starter relay (the one that goes to the resivour). Also replaced the one that goes from the VMV to the plenum, everything else looked good.
Replaced the rubber at both ends of the stove pipe for the PCV. Also replaced the PCV (from Ford) it's the heated one. None of that stopped the 171/174 from comming back.
Bit the bullet about 3 or 4 months ago and did upper and lower Plenum gaskets and Isolator bolts (from Ford). Cleaned the crap out of the plenum and checked the EGR ports while it was open. I was suprised at how clean the EGR ports were for the miles on the truck. Any way it looked good inside, also while it was open I checked the bushings and gromets on the IMRC cause I didn't want to have to dig into it later. All was good.
All this was about 4000 miles ago. Truck was smooth as silk after the plenum job and has not thrown a code of any kind since, just the milage is down. Running the same load and same driving pattern as before the plenum job and I'm off about 2 1/2 to 4 MPG.
As for the battery disconnect it was off for maybe a day and a half. When I hooked it back I don't think I had to re-set the radio. How long will the KAM hold?
The idle is smooth and rock solid at 800 RPM. Starts good either warm or cold, but it does feel sluggish now. That is why I went on the hunt about the O2's. Could I have cooked them by running lean with the 171 & 174 codes? I just figured they were like me (old and lazy) ha ha!

As for just throwing parts at it, not to worry that aint my style. That is why I ask here first, I like to understand what is going on.

Anyhoo you have been a ton of help so far and I hope the above info will shed some light on the cure.

 

Replace the front O2's or take it to Ford for a full WDS (diagnostic). Cost would be about the same. With 160k, those front O2's don't owe you a cent.

 

Well bit the bullet today and replaced both front O2 sensors. Seems to have a little better power at mid range. So I topped it off with gas and we'll see how that does.

Next up new plugs next weekend. If that doesn't help then I guess it's time to start chasing the EGR stuff.

 

Okay, first let me say that I have an 04 F-150 Heritage 4.2L Auto 4X4 standard cab with 158K miles on it. The truck has a CAI, Electric Fan, SIDO Magna Flow Exhaust. The Intake Plemum isolator bolts and gaskets were replaced at 120K as were the Front O2 sensors. I have an SCT X3 programmer running on 87 OCT fuel program. I have also run a custom program for 89 oct fuel in the past.

Now for the issue, I have noticed a distinct drop in fuel mileage in the last 2-3 months of about 3-4 MPG. Iused to run 18 in the summer and 16-17 in the winter assuming I didn't have to go into 4X4. Recently the gas mileage has dropped to 13-15. No CEL and seems to idle okay and run fine just the mileage drop. I did replace the plugs, Yes with Motorcraft, but no change in mileage. The MAF has been cleaned with the MAF Cleaning Spray, the filter is clean and the driving habit haven't changed.

I did get a Blue Driver Blue Tooth Scan Tool and came up with the following readings

LTFT Bank 1 and LTFT Bank 2 run about15% / 14% when at idle but when accelerating or at highway speeds the LTFT Bank 1 runs at 10% - 14% while the LTFT Bank 2 runs at 2 - 4%.

The Bank 1 O2 Sensor is switching but for the most part is staying in the 0.4V to 0.7V range. The Bank 2 O2 sensor is switching and running in the 0.1V to 0.7 volt range.

Running on the road at 55-60 MPH, which all the data is from the same time frame, the RPMs are running 1550 to 1900, throttle position is varying from 20% to 42% and mass air flow rate is running in 1.6 to 8 lb/min range.

So the question is why am I seeing such a wide spread between Bank 1 and Bank 2 when running above idle conditions? O2 sensor, Injectors, graviational pull??? If anyone would like the actual data I can supply that too.

Thanks, Michael

 

Michael, good to see someone using the search.

If you think it's the O2 sensor going out on you can switch them side to side and see how the readings are. If the reading follows the O2 sensor then it would be the sensor. If not then you may have a leak.

Have the sensors ever been changed, if not then I would think it may be time.

But it could be gravitational pull, is it better or worse during a full moon? JK

 

Yes the O2 Sensors Bank 1 Sensor 1 and Bank 2 Sensor 1 were both changed at about 115K during the illusive search for the cure for lean condition. Of course when that and checking for vacuum leaks didn't resolve the condition the Isolater Bolts and plenum gaskets were swapped on a cold winter day!! I can swap the O2 sensors but, if I'm taking them out I might just go ahead and swap them..I don't know...I hate just throwing parts at it. I really need to get this resolved before April, we are heading to florida to pick up our boat and bring it back to Missouri, and it's a 23 ft cuddy so I wanted this puppy in top form. I need to get a picture and post it once the feaking snow stops!! I am ready for summer!!!

Oh yeah, I did replace the vacuum line from the heated PCV valve to the passenger valve cover about the same time I replaced the O2 Sensors.

 

A little more information on my readings on my truck;

LTFT bank 1 14% at idle
LTFT Bank 2 13.5% at idle

LTFT Bank 1 10% at 55 mph
LTFT Bank 2 0% at 55 mph

Dynamic Throttle position at idle 20.78%

Both Bank 2 and Bank 2 O2 Sensor 1 are switching + and -.

I'm confused.

Michael