'91 corolla - idle won't drop till car stops rolling

[dlb]

Since I've had my '91 FWD corolla, it's had this weird thing where if I coast in neutral, the rpm hover at about 1700-1800 until the car physically stops rolling. Only then will the idle drop the rest of the way to 800 rpm. It's not a big deal, it's not like I spend a lot of time cruising down hills in neutral, but I do wonder what's up and would like to sort it out if something is amiss.

Another funny detail about this is that my previous 1991 corolla (a 4wd) did the same thing. I can't imagine this being an intentionally designed detail though, could it?

[Petros]

I think the ECM is confused, you should never be coasting in neutral with the engine running, bad things can happen. the ECM sees speed, but no throttle position, so it gives it extra gasoline. do this does not save gasoline, the ECM cuts off the fuel supply when you are coasting with it in gear, it lowers emissions and improves economy.

I have seen people do that on long down hills, thinking it improved economy, but it does not. if you bump it into reverse accidentally you can trash the trans. just leave it in Drive.

[dlb]

Hmmm, good thinking, that's certainly a possibility. I don't know if the OBD system in these cars is that sophisticated though, as the speedometer runs off a cable.

[Ace]

Maybe the ECM was anticipating that if the transmission is re-engaged with the vehicle rolling at some speed, there is less shock to the drive train with the engine RPM elevated somewhat, if the driver doesn't try to match RPM appropriately. Speculation.