Is the revs going down fast all the way to 900-1100 revs after letting off the gas a bad thing?

[ArcticsLeftGucci]

Hey guys i own a er34 25gt Auto N/a and had a friend tell me that the revs go down fast right after you let off the gas is weird and sure enough when i hopped on his lancer i saw how his rev meter after he lets go of the gas goes down much much slower. Firstly does anyone know why it goes down much faster and second is it a good or bad thing?

Thanks guys

[Duncan]

I've got to be honest, I don't really understand what you are describing...if the car drives smoothly and does not stall when you let off the throttle I would not be concerned

[ArcticsLeftGucci]

Yeah thats a fair call, theres no issue i can see from first glance concerning this. What i was trying to describe was that when i let go of the gas (Car is still rolling), the revs go back to neutral revs very quickly compared to my friends lancer

[GTSBoy]

Back in the grim old days of carburetors, and particularly in the emissions era of carbies before EFI came along to save the day, a lot of carbies had a fairly significant dashpot on the throttle linkage to slow down the closure of the throttle. I used to hate that, particularly on an old Navara that we used to have on the farm.

Nowadays, the same thing is making a comeback in EFI cars, particularly newer ones with e-throttles. The manufacturers are doing it as an emissions control thing. I can't remember exactly what it is that a rapid close causes, probably NOx. Given that the Lancer is probably massively newer than your dirty old datto, and your dirty old datto certainly has no such dashpotting function, it's likely to be that.

[joshuaho96]

When you snap the throttle closed the primary concern is hydrocarbon shoot-through on the catalyst, if you suddenly snap the throttle closed you will pull high vacuum which will cause port injected engines to flash a bunch of fuel on the intake walls into vapor and cause a rich spike that is hard to control for, even with x-tau transient control it's normal to have a bit of a rich spike. Even with GDI engines it still seems to be challenging to control fuel in transients so the easy fix is to just smooth out the decay with drive by wire.

As for why an automatic from 20 years ago will have revs fall to near idle the instant you let off the accelerator vs a modern automatic or a manual transmission, the reason is that almost every older automatic I've driven unlocks the torque converter when you let off the gas and they don't lock the torque converter until you're at something like 64 kph anyways, it was mostly done to improve efficiency of the overdrive gear. Modern automatic transmissions in an effort to improve fuel efficiency will lock the torque converter as soon as 9 mph these days and depending upon what the ECU/TCU thinks you're doing will either keep the torque converter locked to keep the engine in DFCO to help slow down the car or it might unlock to reduce the amount of KE loss in coasting. Manual transmissions never had a torque converter so they don't have torque converter slip.