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Discussion Starter #1
My 97 T-Bird has never coasted well when off the throttle - compared to our old 95 T-Bird and 96 Cougar. It does come in handy when approaching slower traffic and red lights, but can't be great for gas mileage. Or does it even matter since these cars don't shut off the injectors while off the throttle anyway?

I have checked the brakes - there is nothing dragging.

I'm bringing this up because after having the battery cable off over the weekend, the car has really been nose-diving when you let off the throttle - it's pretty extreme. It doesn't do it every time - most likely when you suddenly let off the throttle.

Is that controlled by the IAC?

Thanks,
Al
 

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What do the rpms do when you let off the gas? What speeds/gears does this happen at? Does the car have a tune or j-mod or anything done to the trans?
 

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It all depends actually. In the stock program the injectors do in fact shut off under certain coasting conditions at highway speeds. The amount of engine braking you feel is controlled by the dashpot function, which is the IAC valve. Normally it's barely perceptible - nothing like the feeling of light brake pressure. If you're nose-diving.... something is out of whack for sure!

Do you have any data monitoring tools to see what some engine parameters look like as it's doing its thing?
 

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Discussion Starter #4
The car is 100% stock. It happens in OD. Not 100% sure what the RPM does, but I think it drops for a split second, almost has to. Will verify.

I have a reader with the torque app I can plug in.

This is the first time the battery cable has been off since I installed the Big 3 mod two years ago. Maybe it's time for a new IAC.

I've had issues on my Toyota where a PCM reset leaves the IAC on the wrong "step" and you end up with an idle that's either too low or too high. Sometimes it required another PCM reset.

Al
 

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According to the Powertrain Control / Emission Systems Manual for my 95, if the battery has been disconnected for a while then the PCM has to relearn its adaptive idle air control strategy and adaptive fuel control strategy. But the nosediving sounds extreme. Is it downshifting? Something in the logic between OD and the torque converter?
 

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The Parts Guy
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Sounds like adaptive is relearning the misfire signature since you cleared the KAM by disconnecting the battery. That "nose diving" should go away after a few drives.

If you really want it to coast with your foot off the accelerator, go ahead and turn off coasting fuel shut off in the tune.
 

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Discussion Starter #8
But the nosediving sounds extreme. Is it downshifting? Something in the logic between OD and the torque converter?
No, not downshifting. It only does it for a second or two, then it goes back to it's typical rate of slowing down while coasting.

It had never done this in the past when I've had the battery cable off.

Al
 

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The Parts Guy
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The symptom you're describing is textbook relearn of the misfire signature, via CFSO. It's even called out in the tune file that it's often shut off in the tune due to complaints from customers during relearn.
 

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Discussion Starter #10
But the CFSO saves gas by shutting off the injectors, correct?

It seems to be getting better. The RPM's definitely drop from 1400 to 1200, but it's not as harsh. This is at 50 MPH. Will try at hwy speeds today.

Thanks,
Al
 

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Yes it does save a little fuel.

Once the EEC is done relearning the misfire signature, CFSO will be nearly unnoticeable.
 

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Discussion Starter #12
It's totally gone now, and back to how it was. Still doesn't "coast" per se'.

There are good things about the way the car "decelerates" instead of coasting. I just took a drive across the beltline, and high traffic street. Speeds varied from 65-75 down to 45 for both exit ramps with traffic. Not once did I touch the brake pedal. Well, not until the red lights.

I'm good at estimating speeds of other drivers though. Some drivers hit the brakes every 10 seconds for no good reason, other than they're 4 ft from the car in front of them and get a sudden paranoid feeling that should have kept them from getting that close in the first place.

Al
 
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