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Hi, I use Auto Enginuity for my scan tool. Today, I did a test drive in a 1996 Mercury Grand Marquis. Connecting my OBD2
dongle to my computer through Wi-Fi, I had several data pids selected during the drive. I noticed the IAT and the ECT both had a brief blip of dropping many degrees lower than average during the drive. I also noticed a brief blip of the absolute throttle position dropping lower than the normally fully closed position during the drive. (With a clean throttle body, this cars TB plate rests at 20% open when it's closed. So seeing a brief blip where it was reading 5% didn't seem possible). I'm also assuming the coolant and the air could not drop several degrees lower in one second and then return to normal in the next. I redid the test drive and got similar results the second time. Then, I wondered if my Wi-Fi connection might have been an issue so for the third test drive I connected my OBD2 dongle with a USB cable to my computer. I did not see any of the dropouts this time. Anyone ever seen anything like that before? I'm not sure if maybe there is some power or ground issue somewhere intermittently in the vehicle, or maybe if it is my scan tool connection that is the culprit. The car came to me with a couple of misfires in history, but not misfiring currently. I cleared the codes and then went on a test drive to see if anything came back and no codes reappeared. Fuel trims looked good. The only thing that caught my attention was those momentary drops.
Last edit: 2 years 10 months ago by timothy.spencer. Reason: Spelling error
Sounds like either a scan tool glitch or a problem with the 5V reference. What I'd do is hook two scope channels to the TPS 5V and sensor ground wires and repeat the test drive to see if there are any spikes or dropouts happening at around the same time as the scan data dropouts. On a '96 any scope glitches will probably happen a second or two before the scan glitches because of the slow data rate. USB scope would be best so you can get everything on the same screen and record it.
I agree with Matt T, the scope will tell you if there's really a drop out or if there's a glitch in the scanner software.
Also, just an FYI, it's notoriously difficult to get that era Ford product to produce an accurate misfire code.
You could just about remove a plug wire and go about your day with that thing running on seven cylinders and there's a good chance that when it does finally set a code, it might be for the wrong cylinder.
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