High Negative Long Term Fuel Trims - 00 Ranger 3.0L

[fast240z]

I have a 2000 3.0L FFV that is having high negative long term fuel trims at idle, and I'm not sure what to troubleshoot next. The trims stabilize around -8 LTFT/+2 STFT when driving, but will spike to -15 after pulling off the freeway and idling.

I bought the truck 5 years ago and discovered it was misfiring badly due to low compression from bad valve seats (common 3.0 issue). A rebuilt set of heads fixed the issue and the truck ran fine. Last year, the heater hose coupling failed and I lost all coolant and warped the heads. I resurfaced the heads and truck has been running fine once again. Recently it started running a little rough at idle and I'm trying to track down the root cause.

I've performed the following troubleshooting items:

1. New O2 sensors - both upstream and downstream. Originals were very slow to update and were biased rich.
2. New idle air control valve
3. Fuel pressure test from fuel rail - 59 psi KOER, 50 psi KOEO. Pressure would hold with key on but would immediately leak down after turning key off. Fuel trims/O2 showed very rich on both banks, and replacement injectors fixed leakdown.
4. New injectors - old ones failed leakdown, cleaning didn't fix issue.
5. Smoke test on intake - smoke leaked from EGR valve, replaced EGR. No other leaks found.
6. New Intake air temp sensor - original was stuck hot
7. New coil pack, plugs, and motorcraft wires - original plugs were toast and old coil pack had cracks in it
8. Replacement MAF from Autozone - original looked slightly burnt and BARO value was slow to update

The only other issue I can think of is maybe plugged exhaust. The misfire and leaking injectors make me suspect the catalytic converters are plugged, but the truck accelerates to redline fine.


Driving at cruising speed -

After exiting freeway and sitting at idle:

WOT in 4th gear-

 

[adsm08]

Do you have an FF_inf (Flex Fuel Inferred) or similar PID available showing the calculated alcohol percentage in the fuel?

If that number is skewed high it will be running rich and thus have very negative long term fuel trims.

 

[fast240z]

I have FF Hz, FF% and FF Status. FF Hz stayed stuck at 5400 and FF% stayed stuck at 100% unless I blipped the throttle. Sensor shows no fault. I run 87 octane currently.

I guess I need to dig into the FF system.

 

[adsm08]

Looks that way.

 

[fast240z]

I pulled another flex fuel sensor from a Ford Taurus today and the replacement unit now reads 5700 constantly vs the 5400 constant that my original unit did. Are both of these bad??

I cleaned the connectors for the sensor and the main body connector but no changes happened.

 

[bills4065]

Test drive the truck at WOT and watch your 2 upstream oxygen sensors. If they drop lean at WOT , there is a fuel delivery problem. If your oxygen sensors stay rich, above 800 mv. at WOT, then fuel delivery is good, you most likely have a plugged exhaust.

 

[fast240z]

Test drive the truck at WOT and watch your 2 upstream oxygen sensors. If they drop lean at WOT , there is a fuel delivery problem. If your oxygen sensors stay rich, above 800 mv. at WOT, then fuel delivery is good, you most likely have a plugged exhaust.

The upstream sensors go to .88 mv at WOT.

 

[RonD]

Rich and lean fuel trims are open time for injectors

Computer KNOWS its running a 3 LITER engine, so already knows exactly how much air it can pull in at any RPM and throttle position, its just math
And it KNOWs air fuel mix for gasoline is 14.7:1, this is a WEIGHT ratio
14.7 pounds of air to 1 pound of gasoline
14.7 grams of air to 1 gram of gasoline

And it KNOWS the size of fuel injectors(pounds per hour flow) and the fuel pressure, 55-65psi for 1998 and up Rangers

So computer calculates 0 STFT based on that, these are actually air/fuel mix Tables in memory

What computer doesn't know is the WEIGHT of the AIR
Thats what the MAF and Air Temp sensors are for

The WEIGHT of air changes quite bit by elevation above seal level, and temperature, "hot air rises", why?, because its lighter than cooler air

Computer then adjusts its calculation based on air weight and temp, and thats the new 0 STFT, but wait its not done

LTFT(long term fuel trim)
This is an average of STFT over last few weeks/months, say its -5
So computer adds that to its calculation, and THAT IS the 0 STFT

So if you see -6% STFT and LTFT is -5 then its actually -11%

So why would STFT go to - numbers
"+" numbers are usually vacuum leaks(false low air weight reported) or low fuel pressure
So "-" numbers could be high fuel pressure or "false" HIGH air weight being reported

 

[fast240z]

So "-" numbers could be high fuel pressure or "false" HIGH air weight being reported

I checked the BARO against the local aiport that's 5 miles away and was within 1inHg which is accurate. The IAT was reporting incorrect numbers and was replaced with a new unit that now reports temp accurately. I don't think it's false air weight at least from the MAF side of things.

Fuel pressure checked out at 59 psi when running and revving engine. I didn't try driving it on the freeway with the gauge connected, as the negative trims occur at idle.

I still suspect the flex fuel sensor might be the culprit. I'm reading that typical sensors read 50-150hz and two different sensors have showed 5000+hz. I cleaned the wiring and verified that disconnecting the Flex Fuel sensor throws a CEL, so the wiring appears to be good.

 

[RonD]

You can get fuel sensor replicator
And if you look in the wrecking yards you may even find one on a 1999/2000 3.0l flex fuel Ranger/Mazda, they were a popular conversion

The fuel sensors were $600-$750 to replace, so a company made a device that replicated the "gas only" feed back to computer, it was plugged into the 3 wire connector instead of fuel sensor
This removed the ability to run higher than 15% ethanol but was way less expensive, I think they ran $225 when they first came out

This is what they look like: https://www.ebay.com/itm/Flex-Fuel-Composition-Sensor-Replicator-1999-2000-Ford-Ranger-/280869660232

In 2001 Ford changed the software in the Flex computers and injector size, so it could compensate for the richer ethanol mix needed vs having a fuel sensor
And no, you can't run a 2001 computer, lol, there were other changes as well

 

[bills4065]

I would also check for a plugged exhaust. Should be less than 2 psi at 3000 RPM.

 

[fast240z]

I tested the exhaust backpressure from the DPFE port and showed no backpressure at idle or when holding steady at 2500 RPM. If I snap the throttle then the backpressure shoots up to 3 psi or so. Is this a symptom of plugged exhaust or normal operation?

I also have started to believe that the Flex Fuel sensor is operating properly but the PID is missing a decimal in FORScan. This article shows that most composition sensors operate between 50-150hz and my sensors were reading 5400 and 5700, which if we infer that this is 54.00 and 57.00 is within spec for fuel with a slight ethanol content. https://www.hpacademy.com/blog/how-to-find-out-what-percentage-of-ethanol-is-in-your-fuel/

 

[Mechrick]

If you have 3 psi of back pressure that's excessive. Are you sure of those numbers? Back pressure would restrict airflow through the engine, but the MAF should compensate. Fuel delivery should be in line with what the MAF is reading.
Do you have a baro sensor? Most newer Fords use math from the MAF reading at WOT to infer a baro reading. A plugged exhaust would trick the EEC into thinking the truck was running at high altitude and make it run lean, not rich.
Rich running is always easier to diagnose than lean. The list of possibilities is *much* shorter.

High fuel pressure
weeping fuel pressure regulator (or pulsation damper) -unplug vacuum hose and look for fuel
MAF sensor reading high voltage (unlikely)
Skewed ethanol sensor
Excess fuel in engine oil (enters through PCV valve)
Excess fuel in carbon canister (enters through purge valve)
Skewed O2 sensors (rare, but coolant contamination can cause this)
Incorrect parts (don't laugh, I can't count the number of times incorrect injectors or MAF sensor has caused this. Beware of cold air kits disturbing air flow through the MAF also)

 

[fast240z]

If you have 3 psi of back pressure that's excessive. Are you sure of those numbers? Back pressure would restrict airflow through the engine, but the MAF should compensate. Fuel delivery should be in line with what the MAF is reading.
Do you have a baro sensor? Most newer Fords use math from the MAF reading at WOT to infer a baro reading. A plugged exhaust would trick the EEC into thinking the truck was running at high altitude and make it run lean, not rich.
Rich running is always easier to diagnose than lean. The list of possibilities is *much* shorter.

High fuel pressure
weeping fuel pressure regulator (or pulsation damper) -unplug vacuum hose and look for fuel
MAF sensor reading high voltage (unlikely)
Skewed ethanol sensor
Excess fuel in engine oil (enters through PCV valve)
Excess fuel in carbon canister (enters through purge valve)
Skewed O2 sensors (rare, but coolant contamination can cause this)
Incorrect parts (don't laugh, I can't count the number of times incorrect injectors or MAF sensor has caused this. Beware of cold air kits disturbing air flow through the MAF also)

I verified the BARO sensor is working and reporting proper inHg and is pulling 15lb at WOT which seems accurate for a 3.0L according to some calculators I checked online. You can see the BARO PID in my original post.

Pulse damper is new and no fuel leaking from hose.
MAF is reading fine, tried a new one from autozone and a replacement from the junkyard with no changes.
Engine oil was replaced this week, no changes to trims
Pulled a new ethanol sensor from junkyard, read similar values to the original composition sensor. Seems fine.
PCV valve is new and engine oil is new. Old engine oil didn't stink of fuel.
O2 sensors are new and not skewing rich
No cold air kit or any mods, this truck is all stock

I haven't checked for excess fuel in the carbon canister yet. I did try to perform an EVAP test using Ford IDS and it failed due to "excessive vacuum", yet the canister pressure PID didn't increase during the test. I don't see anything visibly wrong with any hoses but haven't had a chance to look at the charcoal canister yet.

 

[RonD]

If all the sensors are reading correctly and the fuel pressure/injectors are within computers parameters then only conclusion for a fuel trim error will be computer internal error

If you fix a fuel trim error it can take a week or two for LTFT to come back to +/- 5%