I've had this happen intermittently on my 2003 Prius for a few years now: When I turn the ignition to Start instead of revving directly up to idle it seems to pulse to high RPM and back down. It's about one pulse per second and either one or two pulses before the engine stays running. So it seems like the car is having trouble starting the engine, but from the sound of it I'm pretty sure it's revving under gasoline power and not just MG1 spinning it up. I haven't noticed a pattern to the conditions when it does this (can happen on warm and cold starts). I estimate the pulsing happens about one out of every four starts. As far as I know there's never been any code stored related to this, but it seems odd. Does this happen to anyone else?
I've noticed odd rev-up (albeit with a 3rd gen) reliably, after a 12 volt battery disconnect. For the next two-three starts. My speculation is it's "relearning" something, maybe throttle response. Could battery be ailing?
I think the battery is going on 4 years old and I don't drive often so that's quite possible. I'll do some investigation, and if it does the same thing consistently after disconnecting the battery that'll be an indication to replace it.
My hunch is to check the engine air filter, and clean the MAF sensor (although IIRC a dirty MAF sensor will set a code). BTW, “ normal” idle speed on my ‘02 is 1280 rpm.
Being intermittent and ongoing behavior, if it's a 12 volt supply problem, also possible it's a loose connection somewhere.
The engine air filter was a bit dirty but probably okay. I may go ahead and clean the MAF sensor just for good measure. Haven't had a chance to check whether disconnecting the battery triggers this, but I had it happen the other day after the car sat unused for a while. It actually happened at the beginning of each of two 30-minute trips with a few hours in between, and in both cases not right away. The revs happened maybe 10-20 seconds after starting while the car was in motion.
Let it run its course. Most of the time this happens right on startup, so I don't start out until the engine is steady on. When it's happened in motion I haven't really reacted to it: kept the accelerator at the same position throughout. I think the car was keeping the overall power output steady so I only noticed it by ear. It never seems to be more than 3 revs so it's over pretty quickly.
Just a random thought, seeing you're in WI...the VAV on your cat may be getting rusty and starting to stick. That and the rusted fuel filler tube are common in snow states. Others would better explain why that VAV may cause your problem, but IIRC, it can stick open or stick closed, and maybe that messes with the fuel mixture for a few seconds until it either unsticks or the exhaust temps get to normal. Maybe, outside temps matter, if it's stuck open in cold weather, or won't open in hot, but Idk.
