Issue: 97 4.0L OHV, auto

No DTC's, PCM in closed loop, LTFT and STFT all registering -100%. Engine runs smooth. Exhaust tip tan colour.
First start since Nov 2010 (winter stored outside) in less than 1 revolution.

Question,
Why no DTC set?
Why -100% on fuel trims? ... Philip

 

What tool are you using to make the measurements? Have you ever received "good" measurements on this vehicle with the same tool?

-Rod

 

Hi Rod,

I have never in the 45yrs of working on cars have I run into a situation like this.

I have used both my laptop (used for yrs) and a hand held scanner Autel MaxiScan® MS509.(first time on this Vehicle)...

If I cannot get any ideas I will run a 5 gas on her, but I'm sure it will be withing spec.

Tomorrow will scan with my smartphone/BT.. Philip

 

I guess the next thing to do would be to watch the function of the sensor 1 oxygen sensors to see if they are giving voltages that would suggest a consistently rich condition and monitor the injector pulse width. Negative fuel trim numbers would suggest the PCM is REALLY shortening the injector pulse width to compensate for a rich condition.

And, yes, I'd agree that once the fuel trims reach 15-25%, I would expect a code to be triggered.

-Rod

 

I should have posted that O2 voltage flat lined at 0 (zero)... Philip

 

I would then start by checking in to why the O2 voltages are flatlined. You may have a short circuit there contributing to your fuel trim values. Are the O2 heaters drawing current?

-Rod

 

Morning Rod,

Haven't gone out yet but yes the heaters are functioning.

At intial cold start, PCM reports (OL), withing <1min that changes to (CL), this confirms the heaters operational...

It should be notes that all my Monitors are set as complete incl O2 and Heaters.. Philip

 

Can confirm no shorts/opens on O2 circuits prior to connector.

Diagnosis, internal shorts in O2's, will change out some time next week when I have younger more supple help, LOL... Philip

 

Good grief people, stop speculating on this and give it a rest.

Allow me to explain what is happening.

Some of the early OBD-II systems, namely some of the Ford Rangers, Aerostars, Explorers, and F series were not fully OBD-II compliant. Some of the PIDs (the live values the scan tool can read) were still proprietary Ford stuff and were not in compliance with the OBD-II standards that had been established. The values that are most often difficult to read are going to be your fuel trims (both long term and short term), your O2 sensors (both banks, both upstream and down), and depending on the model, some other values may be effected.

Your LTFT% is not actually -100%, otherwise you would have a code. This is a default value that basically means the scan tool cannot retrieve that value.

The solution is to use a scan tool that understands which Ford models are not compliant, and how they are different. If the scan tool knows they are different, then it can access the data using the Ford specific OBD-II protocols. Unfortunately such scan tools are not readily available to the public.

I know this from experience as well. My 1997 Aerostar 4.0L behaves exactly the same way, STFT% and LTFT% are both at -100%, and the O2 sensors both read 0v. This is normal and does not mean the vehicle has a problem, just that the scan tool doesn't know how to access the data. If you have any codes in memory, the freeze frame data will contain the LTFT% and STFT% at the time of the code.

When you plug in a more complex powerful scan tool (Snap-On has good ones), it asks for information about the vehicle, and once that is entered, it reads the actual values without a problem.

 

Khan,

You also must step back take a deep breath and admit you do not know it all... get your facts straight before you throw out misinformed opinions..

Thank you for your dissertation, however... Now explain why I have been able to read these values and graph them for the previous 10yrs. (>360K).... and after a 6 month storage period find I cannot access the information...

I am also using a Ford Extended PID set.. Regards, Philip

 

So your same scan tool worked to retrieve these same values on this same vehicle before?

If that is the case, then:
your ECU has frozen up and likely needs a hard reset. To complete a hard reset you need to disconnect the positive and negative terminals from the battery, then connect the positive and negative terminals to each other (not connected to the battery). This allows the entire electrical system to completely drain, including draining the residual charge from the PCM memory. This will restore the PCM to a factory new condition.

or

The PCM needs a computer reflash. You can give the vin to a dealer to find out if there are any software updates for your vehicle.

or

Your scan tool has frozen up and is not running the Ford PIDs.


How can I get my facts straight when you did not state that you previously used this same scanner with the vehicle. When things are not said, we are left to assume things.

You are going out and testing the components the hard way, only to confirm that what I said is true, your scan tool for whatever reason is not communication with the PCM properly. BTW, the Ford Extended PIDs do not cover the exception vehicle. The Extended PIDs give coverage to CANBUS and Ford specific codes, but the early OBD-II exceptions are just that, exceptions. You need a scan tool that knows what kind of vehicle they are working on, otherwise it will use the generic Ford Extended PIDs, you need the Vehicle Specific Ford PIDs.

 

Vehicle stored for 6 months, battery not in vehicle for winter, basic....
or
Work OK on sister truck 10' away.......

You jumped right in, giving an opinion without asking ANY QUESTIONS... NEVER ***-U-ME!
AGAIN, SCANNER WORKS ON SISTER SHIP!

 

Khan, review posts #2 and #3 above.

-Rod