Aloha, I know this has been discussed ad nauseam but I think I have a unique issue. Here goes,

So, I've got a 2004 fx4 lvlII with a 4.0 sohc, manual trans. It's got a K&n intake tube and throttle body spacer installed from previous owner. The cone filter that was on it was done for, It has been replaced with an aem dry synthetic cone filter. I have installed a new IAC, a new MAF, and have ruled out vacuum leaks and exhaust leaks as possible issues.

So here's the problem. I have a solid stft, it goes from between -2 and +3 under all load conditions, and usually it's close to 0. Great, I love it. However, my ltft is -13 bank 1 and -12 bank 2, also steady and under all load conditions. I have new o2 sensors being shipped now, which was my last guess as to what might be causing this. Anyone have suggestions? Is it possible the air filter/intake/spacer is causing the issue? I'm still getting 15-16 mpg city, which seems decent for a truck with 170k miles on it.

 

When did this start ? I read many post on this forum about the aftermarket cold air intakes were junk didn't listen.Ordered any way kept setting lean code. Replaced stock air intake all is good.But it was nice and shinny.

 

Yeah that's what I was thinking too. I've been monitoring the fuel trim for about 6 months now. The short term started at around +‐5-7 and I've worked it down to near zero with new plugs, new iac and new maf plus the new filter element. It's always had the negative low teens long term, at least for the 6 months I've been watching it. In that time I've relearned the pcm fuel trims like half a dozen times. It starts up fine and idles at around 750 after about 10-15 seconds at 1300 on a cold start. I've replaced the exhaust gasket post cat and tightened down the manifold to exhaust flanges since I had to pull the exhaust when the tranny was out. I'm stumped, no pun intended :)

 

LTFT(long term fuel trim) are used when O2 sensors are not available, i.e. cold starts before O2s warm up above 650degF, they can't work below that temp

LTFT can't be cleared by most readers/scanners, I think Ford Dealer shop can clear them

LTFT is learned by averaging upstream O2s and downstream O2s, this gives the computer an idea of engine condition so it doesn't have to re-learn STFT on every start up

LTFT is used in the base fuel trim calculations
So after warm up if STFT is +3, and LTFT is -12, then actual STFT is -9
If STFT stays at +3 then that will charge LTFT over time, it would change to -9, putting STFT back closer to 0

O2 sensor create their own voltage by contact with Oxygen in the exhaust
.1 volt is high oxygen, Lean
.9volt is low oxygen, Rich

The upstream O2s should switch around very fast as computer adjusts open time on the injectors, sweet spot is .3-.5volts, the computer does run each bank Rich once in awhile to give the Cats unburned fuel to keep them hot
Downtream O2s should be steady at .7-.8volts, low oxygen because Cat Converters use up most of the oxygen to burn up exhaust emissions

So yes new O2 sensors may solve your problem, they do run out of chemicals, like batteries do, after 125-150k miles

But as said it may take a few weeks of driving for the LTFT to start coming back closer to 0

STFT or LTFT won't set codes until -20 to +20 are reached and stay that way for a bit

 

Thanks Ron, that's cleared up some stuff for me. I didn't realize the ltft was used after warm up. So it looks like my actual trims are around -11 bank 1 and -10 bank 2. I only replaced the clogged up nasty old k&n filter a week ago when I did the maf and iac, so I'll keep an eye on it while I wait for the o2 sensors to show up. If it starts to trend back toward 0 I'd be very pleased.

The scan tool I'm using is a bluetooth odb2 dongle with real time readouts and the forscan app for android. Not the most accurate setup. It let's me clear codes from each module(pcm, can, ect.) Individually but I have no way of telling whether it clears the ltft. When I install the o2 sensors I'll clear it again and see what the ltft reads immediately afterward.

I have noticed that aside from the odd -1 or -2 now and again the stft has been relatively steady at +2 to +4 since I did the maf, iac,filter last week. I'm guessing that is what improved and that after a few weeks of driving it will end up with 0 stft and around -8 ltft. Still not great but getting better.

So what kinds of issues would be associated with running slightly rich for extended periods? Is it just a mpg thing or am I gonna be fouling catalytic converters? Or something else I haven't even thought of yet?

 

Its not running Rich or Lean, the upstream O2s tell you its running as it should, -5 to +5 is normal range, and thats real time from upstream O2s

The LTFT does bias that BUT................fuel trims are actually OPEN TIMES for a fuel injector
Say computer calculates open time for a fuel injector as 100ms(milliseconds), this is 0 STFT, at that moment
If after its burned O2 showed .2v, lean, then computer would change STFT to +2, thats +2%, so open time is now 102ms, +4 STFT would be 104ms, -2 STFT would be 98ms

So as the computer varies open times you see that as STFT, BUT.......the upstream O2s are always giving feed back so engine is never actually running lean or rich
Even if STFT was -15 or +15 engine would not be running Rich or Lean, these numbers are open times for the injectors

If tail pipe is blowing grey smoke you ARE running Rich
If engine is pinging/knocking then you ARE running Lean
Other than that the STFT are just the open time numbers to balance upstream O2 sensor voltage at .4/.5


What does your scanner show for downstream O2?
After warm up, should be steady, .7v/.8v

 

Downstream o2 sensors are steady at around .7 once its warmed up. Sometimes the downstream sensors drop to .1 or .2 but it's not very often, maybe once or twice a minute while driving. Upstream bounce around between .2 and .8 but it only reads every 500ms so I'm sure there is some data I'm not seeing. Sure would be nice to log it and have a detailed graph but I dont have the equipment to start doing that yet. I'm replacing them because the truck has 170k on it and they are the original sensors. Does the occasional .1 readings for downstream sensors point toward catalytic/sensor issues or is that normal? They definitely dont stay at .7 to .8 100% of the time but they never drop for longer than 500ms because the next reading is always back to .7 I didnt even notice they were doing it until I had my wife driving so I could watch the scan tool. Thanks for walking me through this, I'm starting to understand the fuel trim function a bit more clearly.

 

Yes, it may drop to .1 now and then, its a way the computer tests if its working, but should be way slower switching than the upstream O2s, as you described

 

Thanks Ron, I've got a good idea of my next steps. I'll update this once I've run the new o2 sensors for a while and see if there is any change.

 

Okay, so a little MAPP love and some coaxing with a nifty new 22mm wrench and out came the four o2 sensors. Wrench was a Milwaukee "max bite", gave me loads of grip and I had all four original sensors out in like 20 minutes, in a 2004 truck with 170k miles on it. Ive used o2 sockets in the past, and cut my own with an angle grinder, plus beat my fair share out with everything from air chisels to vice grips. I've never had such an easy time with o2 sensors, just thought I should share.

Anyway, the new sensors are in and I got about 50 miles on them so far. Ltft is slowly creeping toward zero, went from -11 at 2krpm with load to -9 same conditions. Stft is still close to 0 + or -2 depending on load.

I have read about resetting the KAM, and in my forscan program there is an option to reset all learned values in the pcm. Would either of these options be a good idea, or should I just keep driving it and let it learn it's way back out of whatever the old sensors had taught it to do?

 

Let it re-learn, you will learn more, as will the PCM, lol
You can reset it any time, lets see what happens
And I am not sure LTFT can be reset with Forscan or any off the shelf OBD unit, except a Ford unit

Yes some O2s can be hard to get out, especially on where they are located after vehicle is assembled

I was having a Lean on passenger bank issue and decided to change my O2s
When I got down under the vehicle to change it, it was almost all the way unscrewed, lol, that was my Lean code, but I changed them anyway, it was time

 

Hey Ron, I've got some new information regarding my fuel issues. It seems that my truck is missing a fuel pressure damper. One fuel rail is capped with a plate, and one has a Schrader valve. It's a 2004 sohc, everything I've found says the driver side fuel rail gets a damper. I don't see the vacuum hose that would be connected to it either. I'm working on tracking a damper down, so far I've only found one for sale and its $260. I'm guessing the P.O. didn't find one and just capped it? What kind of issues would the pulse waves cause? I've traced the whole fuel line under the hood and there is no pressure regulator or damper to be found.

 

https://www.rockauto.com/en/catalog/...re+damper,6120
https://www.rockauto.com/en/moreinfo...=2234&jsn=2234

 

Yes, lack of a damper would cause fuel trim issues

In 1997 and earlier Rangers there was a fuel pressure regulator(FPR) with a rubber diaphragm on the fuel rail, that served 2 purposes, pressure regulator and damper, these ran 35psi pressure

1998 and up Rangers switched to returnless fuel system, FPR was moved to inside the gas tank, so a pressure damper(rubber diaphragm) was added to the end of the fuel rail, these ran 55psi pressure

Each time an injector opens and then closes it creates a pressure wave inside the rail, if these waves are allow to bounce around and COMBINE with other waves from other injectors, then pressure in the rail can swing up and down in waves, maybe 30-60psi swings
So when an injector opens it may only have 30psi pushing fuel in, and the next injector opens and it has 60psi, so the amount of fuel flowing in changes.
The damper absorbs pressure waves and prevents them from combining and creating big swings inside the rail
This wouldn't show up on a pressure test, or would only show a little needle bounce
The injectors are only open for milliseconds, so as a wave passes by the amount of fuel that flows in changes depending on if its in the trough or crest of the pressure wave

 

So I'm planning to replace the missing damper, but there is a small issue. The driver side fuel rail back end is crowded with wiring harnesses and the coolant hoses for the pcv and there isn't any room for the damper. The passenger side has the Schrader valve and loads of room behind it. Will the damper work the same if installed on the passenger side fuel rail?

 

Try this part number: Ford 1l2e-9f775-ab

No, it really needs to go where Ford put it
You may need to bite the bullet and pull off some stuff to install it

Be sure to pull the fuel pump fuse or relay and start engine to drain pressure in the system
After its installed put fuse or relay back in and turn key on and off 3 or 4 times to check for leaks before putting it back together

 

Thanks, yeah it shouldn't be too difficult. Probably need to pull the distributor and free up the injector wiring harness is all. I guess I'll replace that pcv while I'm back there too. I'm planning to pull the intake and the valve covers here in a month or two to replace lifters, arms and gaskets. Is this damper something I should replace ASAP or can it wait another couple hundred miles? I can only imagine how long it's been running without one, probably 10k miles or more so far.

 

Its fine to keep running the way it is

 

Well, I ended up replacing a load of stuff. I had to do the timing so it just seemed like a convenient time. The laundry list includes:
Valve job and heads cleaned
new lash adjusters and roller followers
new gaskets and seals on everything
new and tested IAC, MAF, TPS, EGR, CKP, CMP
new fuel pulse damper
new pcv valve
new plugs
new distributor
new and tested O2's
cleaned and tested injectors
triple checked vacuum connections and manifold gaskets

I've put about 200 miles on it since I finished the work. P1000 cleared and its learning trims. The ltft has been dropping on both banks.
bank 1 is -10
bank 2 is -17

While running stft is always around 0, sometimes jumping to + or -3 at the max and then returning quickly to 0.
O2 sensors show cats working, downstreams are both at .7-.8v steady when I'm on the throttle, and drop back to around .3v when coasting.

I'm gonna test fuel pressure to rule it out, waiting on test kit. I don't think it's the cause though.

I have a few guesses. The first one is exhaust leak. I've checked all the exhaust joints between the engine and the o2 sensors with a stethoscope under multiple throttle positions and can't hear blow-by or suction, but that's not a confirmation.
The second one is bad cats. Could they cause negative trims?
Third guess is idle. The engine had a bracket for an auto transmission cooler line on it. I'm thinking it was swapped into this truck from an auto. The idle is at around 725 rpm when warmed up. Isn't the idle on a manual supposed to be around 600? I feel like the computer and the IAC are the only things that should determine idle, but I dont know for sure.


I will update with fuel pressure and if anyone has any ideas I would love to test more stuff. I'm just running out of ideas/reaching the limits of my understanding.

 

The Computer would set a code if downstream O2s were seeing a "bad" Cat converter, very sensitive to that as its an Emissions system

+/- 5 STFT is normal operation and always higher at idle to prevent overheating, idles need Rich Mix
If LTFT is coming closer to 0 every few days then its going to equalize itself after all the repairs, so give it a few weeks or a month

Manual trans idles have been set higher and higher to meet emissions standards over the years, yes, my 1994 idles at 625, but later years 725, the higher the RPMs the leaner the idle mix can be, but its still a Rich Mix
RPMs also hold high when shifting, this is to meet emissions as well

Newer vehicles have to shut off the engine when stopped to meet new "idle emissions", lol

 

Thanks Ron, it's nice to know the cats seem to be working properly. Also takes idle issues off the table since it seems I'm right within spec.
The ltfts have been steadily moving negative since the work was done. Bank 1 averages another -1 every 40 miles, bank 2 is closer to -1 every 25 miles. When I started driving it after the rebuild the ltft started out near zero.

 

If its going farther to the negative then yes check fuel pressure 55-60psi is spec, if fuel pressure regulator in the tank is stuck closed it can be 75+psi

At -15/-20 or so it will set a code

Exhaust leaks usually cause Lean trims, + side
Same for MAF issues

I assume the vehicle is running well, no misfiring or lack of power now and then?

Long shot would be Barometric pressure error, its a number thats easy to check with reader, and local airport will have current barometric pressure to compare it to
Baro is height above sea level, higher elevations have less oxygen so computer adds less fuel, lower elevation more fuel
If computer thinks you are a "submarine", lol, and at -2,000ft it may be calculating too much fuel needed and so correction would be - STFT and then - LTFT over time

 

Yeah the truck runs as well as it ever has, no missing, no power loss. Just the usual creaks and groans from my skyjacker leaf springs when I take off from a stop. Wish I could stop that but I've replaced bushings and it's still noisy.
I'll check barometric pressure and fuel pressure as well once I get the gauge. I honestly wouldn't mind if the fuel pump needs replacing, itll give me an excuse to pull the bed and do some rust inspection and remediation.

 

Alright, so I did some driving just to get a better idea of the trims at different times.
First off, barometric pressure is dead on, so that's all good.

Warm start, after the rpm drops to 725.
stft1 0.62
stft2 0.45
ltft1 -10.5
ltft2 -16.2

Driving downhill, 2nd gear with no throttle, rpm 1600, 22 mph
Stft1 1.44
stft2 2.45
ltft1 -11.72
Ltft2 -13.28

Driving flat, 5th gear half throttle, 2300 rpm, 65 mph
stft1 0.91
stft2 0.87
ltft1 -8.99
Ltft2 -11.54

While cruising I had my wife watching the trims. Short trims stayed around -+1 and long trims were around -9 bank 1 and -12 bank 2
As soon as I pulled in to the garage and let it idle, Ltft bank 2 jumped to -16 again.

Ltft2 changes from around -12 while driving or under load, to -16 as soon as its idling but the stft is relatively steady under both conditions. When I was doing my downhill drive in 2nd with no throttle the stft1 jumped up to +10 for a second then back down about once a minute or so. Stft2 was steady, aside from one +10 spike that coincided with a spike on stft1. I have no idea what this means, or if it even means anything. Just grabbing whatever data I can.

 

Okay, so fuel pressure checks out. I'm now embarking on the evap system testing journey. Any help would be greatly appreciated. I was planning to start on the engine side stuff since it's easy to reach.