Mods: I know I put this in C4 tech, but this is where the collective brains of knowledge can be found so please don't move this...

Back to the topic: When a motor is spinning at any high RPM that is not at idle, with no throttle being applied, and put into neutral (or the clutch is applied), shouldn’t the RPM quickly drop back to idle? Shouldn’t the response be immediate?
Example: Car is in neutral and you stab the throttle… the RPM will shoot up, but once you take your foot off the pedal the idle should come right back down or if you are cruising at speed and slip the car into neutral or apply the clutch

Is it a problem if the RPM hangs or is very slow to return to idle?
What could the problem be if any?
Am I just paranoid?
TIA for any advice.

FYI: This has been an ongoing thing on my wife’s 99 Cougar. The Cougar forums are just a bunch of kids who think their Domestic import is really cool with a fagtard muffler and ironing board wing… this is why I am asking this here.
We bought my wife’s car in late 01 with 25k miles. It has always done this to some extent, but it is getting increasingly worse now with close to 90k on the clock. I am just trying to figure out if there is going to be some substantial investment needed on her car… because it will significantly impact the amount of mods I can do on our vette over the winter.

 

Yes the engine should return to idle pretty quickly. Start by verifying that the throttle is being allowed to close. Could be something as stupid as the gas peddle getting caught on the carpet. Freyed cable, weak or broken return spring, ect. If all that's fine, my next guess, and that's all it is, would be the idle air control is heading south.

 

No, actually, the days of instant return to idle have been long gone since the advent of computers and fuel injection. It has to do with fuel management, emissions and economy (cafe standards) as well as driveability. Matter of fact, in almost any car today, if you push in the clutch while driving at any speed the engine will not go down to idle rpm until the car is stopped.

 

As far as I know, the design is to prevent stalling. If you were around in the early '70's than you might remember how bad vehicles idled (if they idled at all) as the OEMs leaned out the fuel mix and added emissions controls without the benefit of a computer to control anything. And you might also remember that none of them really worked with the Chrysler K cars probably the worse, and the Chevy Citations (and others) not far behind. This led to a general mistrust of anything relating to emissions controls, some of which is still prevalent today, though since the mid '80's it's all really pretty good and has led to engines that probably run better and last longer than if they were still burning leaded and purring along with a fuel mix that tended be a bit more than rich.

 

Not to argue with you SunCr, but if you go back to the 60s cars before emissions stuff, most cars idled well even after dropping the accelerator pedal. Today's purpose built cars which use the older tech still return to idle rpm quickly, some stumbling, but not much. Not unless there are wild cams and other significant mods. You're right, tho, since the emissions stuff, much of it is for driveability issues as well as emissions and fuel economy (such as the 1-to-4 skip shift CAGS).

 

It's the ECM sending a higher "desired idle" to the IAC.

If you look at a scanner as you drive along you will see that the target idle is higher than when just sitting at idle. I never did a scan to see how it behaves when I crack the throttle and let off immediately.

 

air filter clean?


the engine will slow at its own pace, thou the little motor shoul slower quicker than bigger ones. what will keep the engine running (at any rpm) is spark, fuel, and air. since its the flow of air that controls spark & fuel, look for ways for air to get in the motor, cracking old vacuum lines are the normal culprit.
I'd start at your map senor vacuum lines. sometimes they'll even fill with engine soot.

 

LT4CompYell is correct, most all newer cars do not return to base idle until the vehicle speed is ZERO (Fords are noticably slow to idle back down, my wife's '99 GT does the same thing). An electronically fuel injected motor would stumble and blow black smoke if the idle was forced down as soon as the throttle blades were closed.

 

All of my hanging Fords get a throttle air bypass "restrictor plate." This makes the car much more fun to drive. Don't go too small on the hole or the computer won't be able to adjust for it. Check mileage before and after (several tanks) to be sure. Use the gasket as a pattern. A 3/8" hole works well on the 4.6L. You might try a 5/16" hole on the smaller engine. Leave the entry hole full size.