Hello, ive been hunting a problem in which i cannot find the answer to. Hoping to find some experience here.

Truck is a 1996 ranger 2.3L 5 speed 200k miles nice truck really. The problem is ill start the truck and it fires fine cold idle is high but while watching the obd scanner my fuel trims start going to -20 and the truck starts running like crap. Thats within 2 minutes if the truck running. While the trims are -20 it barely wants to move. Then eventually itll come back to around 0 and truck runs fine. My question is what causes the -20 fuel trims?

some history on the truck.

ive changed in the last year
pcv valve (aftermarket)
egr (aftermarket)
dpfe (aftermarket)
Idle air control (aftermarket)
coil packs wires and plugs (oem)
smoked checked all the vacuum lines
replaced 02 sensors
cleaned the MAF

checked my fuel pressure is at 30psi and at 40psi when i unhook the fuel pressure regulator.

pulled the plug on IAC while running rpms did drop.

i was thinking the the truck changes the fuel trims only after the truck is warm and taking good readings of the 02 sensors. But itll cold start fine run for a second then the fuel trim just starts to drop off down to -20 ish and itll run like crap.

any help or advice would be much appreciated, ive tried reading in several other post which not a ton of luck.

 

Welcome to the forum

Nice looking Ranger

You are correct the one upstream O2 sensor should not be used until engine is warmed up a bit more, but maybe it is and that's the problem

Fuel trim is the calculated Open Time for each injector
Prior to the O2 sensor coming on line the computer uses tables in memory to calculate Open Time based on RPM, throttle sensor, MAF sensor and air temp sensor
O2 sensor needs to be heated above 600degF to work

Short Term Fuel Trim(STFT) is calculated by the computer and that is represented as 0%
+ STFT means computer is opening injectors longer that it calculated, +5% would be 5% longer that calculated
- STFT means computer is closing injectors sooner that it calculated, -5% would be 5% sooner than calculated

-20% or +20% is extreme and sets a code
But unless Long Term Fuel Trims(LTFT) are at -20% then fuel trims really shouldn't change much UNTIL the O2 sensor starts to work

LTFT is used with engine cold or warm
LTFTs purpose is so the computer doesn't have to Learn the engine system on each start up
A new engine may have 38psi fuel pressure, clean MAF sensor and no vacuum leaks, and LTFT of 0%
A 10 year old engine may have 30psi, dirty MAF and small vacuum leaks, and LTFT of +/- 5%

When 10 year old engine is started cold computer looks up the air/fuel mix tables in memory and then adds or subtracts 5% based on LTFT as the new 0%, and it does that even after O2 sensor comes on line
Say vacuum leak is fixed or MAF cleaned or a new fuel pump or filter added, then computer would see say a change in STFT so its running +/- 3% steadily, it would then change LTFT to +/- 2%, to reflect the change

Back to -20%,if this is LTFT then that's a problem in the computers memory, not sure you can clear that, LTFT is usually not re-settable by most OBD2 readers
If this was a problem before new O2 sensor then computer issue, if not then swap O2 sensors around and see if it changes

Look at O2 sensor voltage as engine warms up
0.1v is high oxygen(lean)
0.9v is low oxygen(rich)

Computer tries to keep O2 voltage at 0.45v, by varying the open time of injectors, so voltage will jump around between 0.2 and 0.6v

-20% means O2 is showing close to 0.9v pretty much all the time and computer has reduced open time to lower the O2 voltage
And it its not a true O2 higher voltage from Rich mix then engine will run like crap because there is not enough fuel coming in

 

wow thank you so much for all that information, that really helps me understand all that is involved and how it works to help figure out whats going on. We just got some snow so im unable to work on her right now but id like to add that the LTFT never gets to those extreme numbers its always the STFT. And currently the truck is warmed up and idling good but the fuel system says OL_Fault and the STFT is locked at 0% and its running good and LTFT is at 1.6%

now when i shut the truck off and clears that Ol_fault ill restart the truck fuel system says CL im just letting it idle and the STFT slowly dives to -20% while the LTFT is at 1.6% it idles rough until the fuel system goes into fault and when that happens the STFT sits at 0% and it idles good again. Now thats all with just idling.

when i go to drive after a few minutes the fuel sytems status will go back to CL im assuming that means clear. And truck is running good the LTFT never seems to get over 6% but when driving it’s typically around 2% but if i come off the gas or stop and let the idle drop to below 1000rpms the STFT will immediately drop to -20 % and run like crap until i start driving and or it goes into fault.

my O2 sensor up stream while warm bouces back and fourth from .1v to .6v

down streams seems to sit around .025v

with that being said i do want to ask, when i smoke tested the vacuum system i notice i got some smoke out of the vent cap on the IAC now that im not sure is common but i did replace that with a aftermarket one a year ago. Not sure if that is a problem. If so id like to source a good IAC assembly ive been having troubles finding a oem part number or motorcraft replacement.

This photo is at idle when the truck is running smooth

 

This is also at idle truck is now not falling into the -20 stft . It happens more on cold starts and first 30mins of driving. but years ago i did mess with the anti diesel screw adjustment thinking it was a idle screw and my tps is at 15.3 % is there a way to correct this back to factory? Could it be an issue ? Does my MAF numbers look correct for a 2.3 idling at 850ish rpms

 

Also at idle while truck has been ran for over an hour

 

O2 sensors are ignored at idle and WOT(wide open throttle)
Idle is TPS(TP) 19% or lower
WOT is TPS 90% or higher
With key on engine off push down gas pedal all the way and watch TP %, should go to 90%+
If not Google: Ford Ranger Throttle cable mod

TP at 15% is lower than expected, 17% to 19% with foot off the gas pedal is expected, so it may be a replacement TPS that needs to be adjusted physically
But if you get 90%+ with gas pedal down all the way then all is well
If you don't have 90%+ then computer never sees WOT so you are missing out on full power
Computer is all about MPG in fuel calculations, best efficiency/highest MPG
Until it sees WOT, computer then uses tables in memory for best power, MPG be damned, lol, so its good to have WOT when you need it

STFT -5% to +10% is fairly normal

O2S11 is upstream O2 and at idle 0.5v is normal, computer has to run engine slightly rich at idle to prevent over heating, but its running fuel mix from tables in memory and not using O2s
Upstream O2 jumping around 0.1v to 0.6v is correct, 0.4v is what it tries to average

O2S12 is the downstream O2(after Cat converter) so Cat is not working or O2 isn't, should show 0.75 to 0.85v, little or no oxygen left in exhaust after Cat has used it up

MAF sensor looks OK, but change it to grams/second, 42lb/min is about 3.2gr/sec, at 850rpm that seems OK

 

wow thank you for that mound of great information. I was thinking the downstream 02 sensor was to be less than the up stream so ill have to figure that out. Could that pose an issue with my fuel trims? Im guessing my cat needs replaces as ive changed the o2 sensor already. But also i want to add that the STFT never drops to that negative-20% untill im off the throttle and rpms drop down to 1,200 or so or less. If i keep my foot on the throttle at say 1,500 rpms the STFT stays around 0% + or - and the truck runs smooth. Its as soon as i go for a shift or stop that it then drops down to the -20% and runs rough. Start driving and it comes back up and or goes into fault and sticks at 0%.

after just driving ive noticed that sometime ill get heavy into the trottle and i seen the STFT go up to +32% !! What the heck is going on with this thing.

im still at a loss on to where to go from here

ive checked for vacuum leaks and just seen some smoke come from the vent cap on IAC not sure if thats normal.

MAF seems to be showing correctly

IAC shows correct

TPS truck off goes to 93% pedal all the way down

new plug and wires oem

clean air filter

35 psi fuel pressure and 40 psi when i unplug regulator

possible bad cat or downstreem o2 seems im getting only .025v after warm

upstream cat shows correct numbers

new egr valve and dpfe

im about to pull my hair out, surely its not good on the engine when it runs lean at -20% stft

LTFT seems to be ok from what u guys are telling me

anyways i know ive said alot and asked alot just want to thank you guys again for taking the time to reply with such great information.

 

When you take you foot off the gas pedal and coast in gear, the computer should shut off the injectors, its one of the fuel saving benefits of Fuel Injection
It will then turn on injectors when RPMs are under 1,500 when coasting
In a manual trans if you press clutch pedal down coasting computer will keep injectors on for shifting reasons

Decelerating and seeing -20 shouldn't be an issue, same as +20 when accelerating

I would check the TP % engine off, 18 or 19% foot off the pedal, 90%+ pedal down all the way

 

ok are you said i should adjust the screw until i get 18-19% on the tps? Or is there a way to adjust just the tps ?

 

Is this from a cold start ? or has the engine been running for awhile?

I'm just asking because of the ECT being @ 183 is a warmed up engine.

 

yes the truck has been running

 

Im going to put a new catalytic converter on it. Truck has roughly 220k miles and exhaust is all original.

 

The screw can be used but needs to be done the right way
Warm up engine, let it idle
Unplug the 2 wire connector on IAC Valve
Idle should drop, if engine stalls turn screw clockwise 1 full turn, restart engine and see if it idles now
If it idles check TP %
Turning the screw clockwise increases %

You want the engine to idle but BELOW computers Target idle which is set using the IAC Valve
Yes, if you can get TP to 18/19% without idling being too high that would be good
Plug back in IAC Valve and idle should go up to computer Target RPM
REV the engine a few times to make sure idle is not too high

Downstream O2 voltage should be jumping around like upstream O2 voltage if Cat is bad

 

Thanks for the info! This is the truck warm at idle. The O2 downstream really stay around .080v ish hit the gas itll jump up to .8v ish and then drop back to a steady .080 ish so that shows possible bad cat?



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I was reading the o2 down stream should be less than the o2 upstream so my voltage of downstream at .080 ish is good correct?

its staying pretty steady around .040 to .080

edit*** ok when im driving the o2 downstream is jumping steadily between .7v and .1v at idle it just sits at .080v

 

O2s only see Oxygen
0.1v high oxygen
0.9v low oxygen

The Cat converter burns up most of the Oxygen in the exhaust, as it uses the oxygen to burn up pollutants, so after Cat O2 should be 0.75v - 0.85v, low oxygen
O2S12 = 0.070v is high oxygen, less than 0.1v, so very high Oxygen

Upstream O2, O2S11 = 0.500v which is OK, 0.45v is spot on

Up at the top of the screen in orange is 0.820 which means Cat is good if that's downstream O2 voltage
So your reader is very confusing to me, lol

 

ok thanks, yeah thats the max levels up on top, when driving itll jump up to that and down to .080ish back and fourth. At idle warm it sits around that .080 ish range