Hey y'all,
So recently I've been trying to hunt down a strange surge that happens sometimes in my dad's '94 Corvette. This only seemed to happen when stopped with your foot on the brake in Drive, so initially I suspected a vacuum leak or brake issue. Thankfully I managed to get it to do it while I was logging, so I was able to see what was causing it. What's apparently happening is the PCM is detecting a non-existent momentary spike in engine RPM, which causes it to wildly alter the IAC position, followed by the PCM very quickly realizing its mistake and wildly altering it back to where it was, causing an RPM surge. I was only speedlogging at the time, though, so I only had the high res pulse initially to see.
The EEHack log I've attached contains both high and low res pulse data from the Main datastream. It's from an hour-long drive along a freeway, followed by then coming to a stop when arriving into the city and hitting traffic. There does not appear to be any fluctuation in either high or low res pulses the entire time on the freeway. But the moment we slow down, the signals start getting funky. There were also a few momentary signal dropouts in the data, but I'm not sure they're related. This car has always had a "noisy" serial bus, and it could also be as simple as the cable being loose on my laptop and momentarily disconnecting. Or it could be related. I never did discover why this car's ALDL seems to cause so many packet failures.
I replaced the opti harness as a cheap hail mary but that made no change to the issue. Haven't yet inspected the entire body side harness for the opti. That it only happens at low RPM makes me wonder if it's something with a ground or voltage issue, but I'm not sure. Before I start tearing into stuff, figured I'd ask around, see if anyone here has experienced an issue similar to this one.
Thanks!
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