In my '67, when you rev the engine fast, and take your foot off fast, the engine drops down fast back to idle. Yet, do that in my 5.0, and it will go up fast, and then stick and hold at about 1,500RPM on the way down, and then gently ease back down to 850RPM idle... Why is that? I'm curious...
Brad
Entering into the unknown, V'ger seeks information used to maintain his Vintage Burgundy 1965 Ford Mustang 2+2 w/289 ci 4v oem A Code V8, C4 Trans,
16x8" Vintage 40 wheels, with BF Goodrich gForce T/A 225/50ZR16 tires, American Racing "Mustang" Centercaps, and a whole lot of other stuff; )On Sat, 09 Oct 2004 05:48:47 +0000, Brad and Mia rearranged some electrons to form:
No computer in the '67 controlling the idle speed.
Holding the RPM above idle during decell is an EPA item I've been told and dates to the computer controlled carbs. The reason is to keep some air flowing to burn fuel more leanly that is being pulled through the engine because of the increased vacuum.It is speed related and drops to idle somewhere around 5 MPH.
bradtx
They do that on purpose. When you suddenly close the throttle, there's still some unburned fuel in the system, which causes a sudden burst of smog out the tail pipe. By closing the throttle just a little more slowly (as you describe), that fuel gets burned properly and emissions are controlled better.
The computer holds it up like that. Its suppose to help emissions.
MadDAWG
Actually it's the IAC that holds it, for emissions purposes.
The '67 likely runs much richer than the 5.0, and the richer the mixture, the quicker it will return to idle.
More importantly, the PCM code in your later-model 5.0 uses the IAC to gently ease it back to idle to reduce emissions. When the throttle snaps shut, a very rich mixture is created (fuel still spraying but air is nearly cut off), which increases tailpipe pollutants.
In later years on systems using mass air systems, a dirty MAF will make this problem worse. The dirtier the MAF element is, the leaner the mixture will be, causing pinging/detonation problems as well as a slower return to idle.
-JD
Are you ready for the real "skinny"????
From the factory, your '67 would have come with an anti-stall dashpot. This was mounted on the carb near the throttle linkage and (when properly adjusted) would allow the rpm to drop abruptly to the 1500 rpm range and then slowly allow the throttle to close to idle from there. As the name indicates, it main purpose in life was to keep the motor from stalling in that brief period when manifold vacuum would surge higher than the usual idle reading.
Your 5.0 (obviously computer controlled) has this dashpot effect programmed into the PCM strategy. Yes, it helps reduce emissions but it's main purpose in life is to "keep the fire lit".
And just what controls the IAC? :)
MadDAWG
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