Contour Enthusiasts Group Archives Forums General Troubleshooting Who knows their stuff about Long Term and Short Term Fuel Trim on OBD-II?

My fuel trims are still running close to +50%. They seem to be coming down after my intake seal job but they're still up there. Otehr OBD-II people (not necessarily Contour people) are telling me fuel trims over 25% are OUTRAGEOUS so what's the deal here?

id say you have a vacum leak

I work in the department that contains the OBD-II feature designers and calibrators and I write tests that the assembly plants use to test a variety of engine controls. While I've done relatively little with fuel trim directly (other than explain to plant repairmen what the likely causes of lean and rich errors are for their vehicles), I've seen that most vehicles stop trying to adapt at +/- 25-30% errors. Personally, I a bit suspicious of how the scan tool is displaying what it is reading from the PCM. What scan tool are you using and does it have an option to display data in raw versus engineering units?

I don't have it with me today but I'll get back to you. It gives me RPM, IAT and ECT temps, timing advance, TPS %, "Load", MAF lbs/hr, fuel system status (open closed, system 1, system 2, don't even know what that means), O2 percentages (don't know how to interpret them) and voltages for both sensors on both banks, long-term trim by bank and short-term trim by bank, expressed as percentages.

DOH! I had a brain fart. I typically disable long-term fuel correction during my testing, so the +/- 25-30% range of authority that I've observed is strictly within the short-term fuel trim. If you add the long-term fuel trim range of authority, I can see how the vehicle can be + 50%. Still, that is a pretty serious error. I'd suspect a serious vacuum leak OR maybe even an exhaust leak (O2 that didn't burn coming through the engine looks the same as O2 that can in after the engine).

Does disconnecting the computer for several hours effectively "clear" the computer, or is it still adapting from a pre-existing condition? I obviously had a vacuum leak, the injector-manifold seals were toast, but they're all new now, with new upper and lower gaskets, a new PCV valve, vacuum lines inspected for problems, etc. No more P0171 lean codes and it purrs like a kitten now but just for giggles I threw the scanner back on and saw these high trims. Lower than before, and possibly coming down (that's why I ask about clearing the computer) but still high. I don't hear any evidence of an exhaust leak and have no reason to believe one exists but I suppose it's possible.

The official line is 'no power for 5 minutes clears all adaptive memory.' However, I think that this is the case where the question went through 10 people and each person doubled the previous person's answer. The specs I've seen indicate that KAM should be clear in 2-6 seconds, depending on the PCM, but I've heard many people swear that it takes considerably longer than that.

Does the EEC-IV in the '95 Contours output the long term/short term fuel trim parameters when connected to the proper diagnostic equipment?

I've never been able to get a straight answer on this. I have an autoxray scantool, which shows some parameters (such as engine temp, IAT voltage, TPS voltage, etc) but the long term/short term fuel trim are missing (as are one of the O2 sensors--it only shows one O2 sensor, and it's a V6).

Brian

The scanner I'm using is an Actron with a plug-in application cartridge, nothing fancy.

Regretfully, it has been so long since I've worked on EEC-IV that I don't recall which parameters it supported (Arthur Anderson is storing my EEC-IV documentation). I'm rather confident that the PCM supported the short-term fuel trim. If the vehicle has two upstream HEGOS (OBD-I/EEC-IV did not have downstream HEGOS), then it will support fuel trims for each bank. I know that back then, our tests did not even attempt to read long-term fuel trim, so I don't know whether or not the PCM strategy used/supported it.