Short & Long Term Fuel Trim

I have a 1997 Explorer Sport 4.0L 72,000 miles that has a intermittent missfire. A code was set for "cylinder 4 missfire" . I investigated with a scan tool and noticed that both Short Term Fuel Trim and Long Term Fuel Trim were displaying a static -100%. Additionally the vehicle speed displayed 0 at test drive. The scan tool was connected to my 2000 Mustang and it operated properly showing varing values of Short and Lonf Term Fuel Trim and vehicle speed.

Any ideas why the fuel trim table appears to be without useful data? Is the PCM shot or should I start to look at the MAF/O2 sensor.

tyqchur said:

I have a 1997 Explorer Sport 4.0L 72,000 miles that has a intermittent missfire. A code was set for "cylinder 4 missfire" . I investigated with a scan tool and noticed that both Short Term Fuel Trim and Long Term Fuel Trim were displaying a static -100%. Additionally the vehicle speed displayed 0 at test drive. The scan tool was connected to my 2000 Mustang and it operated properly showing varing values of Short and Lonf Term Fuel Trim and vehicle speed.

Any ideas why the fuel trim table appears to be without useful data? Is the PCM shot or should I start to look at the MAF/O2 sensor.

Click to expand...

When the scanner starts seeing the PCM data stream showing 100% fuel trim on both short and long terms, it is a sign that something else is amiss in your PCM circuit. The fuel trims are computer compensated for other variable factors on your truck, such as a dirty air filter, bad O2 sensors, plugged cats, bad wires, etc. What happens first is the PCM will compensate fuel flow to try to hit the proper air/fuel ratio via the oxygen sensor input with the short term trim value. When that hits (and stays) at approximately 50% the long term trim will begin adjusting (learning) to bring the short term trim back to center. (Think of the trims as a pair of scales, from 100 - 0 - 100, with 0 being centered, and perfect.) As the short term trip continues to compensate for whatever is amiss in the engine control system, the long term trip will continue to move to the end of the scale. When neither it, nor the short term trim can compensate, the engine will start to run rough (or rougher as the case may be).

I'd start in my diagnosis of this problem in a couple of ways...

1. Figure out what is causing the mis-fire on #4. It can only be a couple of things -- bad injector (or plugged fuel rail), bad plug or wire (or coil), or an intake leak -- presuming that all else is good mechanically. (Some times a mis-fire on one cylinder will also prompt codes on the next cylinders to fire, as the mis-fire code is set via crankshaft counts, and one cyl. affects the next couple in order with decreasing counts in the next couple.)

2. Check the air cleaner and MAF. Clean both.

3. Check for oxy sensor function. Make sure that they are responding when you blip the throttle and are not out of specs. (Note well that an out of spec oxy sensor is not necessarily a bad sensor! It may be reading perfectly, but out of specs from some other issue. Don't believe the Zone when they read your codes and automatically sell you a sensor!) The back oxy sensors should be flatlined once the cats are at operating temps. The front ones will continue to respond to engine load and fuel compensation.

4. Check for intake leaks. A couple ways to do that... Use a propane feed (a small volume of propane is shot around the intake and vacuum line ports while watching the scanner and/or lab scope -- look for rpm rise, and/or cylinder fluctuation) or use starting fluid judiciously (PLEASE don't just hose down the engine compartment! That stuff can and will burn your truck to the ground!) A small shot in the expected trouble areas can pinpoint problems when the engine revs up from an intake or vacuum line drawing the fluid into the combustion chamber.

5. Check plugs, wires, etc., also.

This should allow you to pinpoint the problem/s.

at 100% you should have a code for rich or lean adaptive limits reached on the fuel system.... something is awry here. You only get the one code? How do the O2 sensors seem to be working? Are they switching?

Through the expierence I've seen with Computers and their vehicles - very RARELY is the PCM the cause of the problem. Sometimes it isn't putting out the right signal because it isn't recieving the right input.

-Drew

Thanks for the reply. The truck is running fine except for the intermittent missfire. The O2 sensors seem to be working and the PCM only sets only one code. The short and long term fuel trim tables show a value of -100% and are static, they don't change, also the vehicle speed on the scan remains at 0 even while driving.
Two questions: Does 97 explorer support the LT,FT Tables and if so is the "Bad Data" a sensor issue or a PCM issue.

Thanks for the reply. I'm getting all the inputs, ECT,IAT,MAF etc. The PCM does seem to be storing any data in the Keep Alive Memory. All the values for the fuel trim tables are a static -100%. Does the Keep Alive Memory reside on a seperat power circut from the rest of the PCM?????

Thanks for the reply. I'm going to attempt to isolate the misfire this weekend and I appreciate your advice. Was wondering if the misfire might be casued by the hosed up fuel trim tables.

tyqchur said:

Thanks for the reply. I'm going to attempt to isolate the misfire this weekend and I appreciate your advice. Was wondering if the misfire might be casued by the hosed up fuel trim tables.

Click to expand...

Probably not if it is only on one cylinder.

If it showed a misfire on all cylinders, then perhaps, but on one only, I'd go looking for another problem.

I have a little more info. Connected a scan tool to view the O2 data. All four of the O2 sensors read 0.00 mV and all fuel trims are at 100 percent. Additionall, vehicle speed still displays as 0 while driving by the scan tool.

I'm still confused on this one. I am doubting that the actual outputs is zero V from the 02 sensors... I imagine they are working just fine... the Q is why can't you read them.

I agree with glfrederick... look elsewhere for your misfire.

ps. I have a personal (private) list of knowledgeable members who, when they speak, I listen. glfrederick is on that list)

Well the misfire is resolved. After eliminating fuel pressure, vacuume leaks, egr, burn valves and a bad coil pack, as a last resort before pulling the fuel injectors, I pulled, the 750 miles ago new Motrocraft AGSF22FM spark plugs, and found the electrode insulator cracked on the #4 cylinder. Replaced the plug and the 4.0L runs better then it ever did.

Still having a problem with the scann tool displaying fuel trim and 02 data. Noticed one thing; if I force a code, the Freeze Data feature displays the Fuel Trim, 02 and Vehicle Speed data at the time of the fault.

I've got an e-mail into Actron tech support. The scanner is an Actron CP9145

Thanks everyone for all your help!

This is Actron's replay:

"We are aware of this problem. This is a common issue when using a global OBD II scanner on some '97 Ford light-duty vehicles. It is a software issue in the vehicle (The same results occur on the FORD NGS tool if set to scan in global mode), and cannot be rectified. The tool would need enhanced scanning capabilities, which this tool does not. You would need something along the lines of a OTC Nemisys (wwwotctools.com 1-800-533-6127).
Thanks,
Vic Ellinger
Actron Tech Support
1-800-ACTRON-7 "

There ya have it. Thanks for sharing that info!

97 explorer v8 5.0 help

i have a 97 explorer that has a rough idle, and giving me codes 430,340. i replaced the cam sensor stub and cap, made no difference. if i unplug the sensor and start it up it makes no difference still runs the same. has anyone else had anything like this.

In your case, I might begin to suspect the ECM itself. Sounds like it is no longer controlling your engine variables. Have you hooked something more informative than a code reader to your computer? A scanner would be great -- tell you what is actually going on in there -- and you could see if the cam sensor is reading or not. A code reader will never tell you anything except that a particular item is out of specs for the built-in data tables in the ECM.