Reducing RPM's in Neutral - Fuelly Forums

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Old 09-25-2009, 05:08 PM   #1
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Reducing RPM's in Neutral

when I coast in neutral (manual here) at about 30mph my rpm's stay at 1300 about, but when stopped they go down to 900 rpm, I realize The car stays in this range to prevent wear to the synchros in my transmission when re-engaging into gears, but I was thinking about making a switch to reduce to the rpms to 900 somehow while in coast, any one have any ideas how this could be acheived, I considered hacking the speed sensors somehow, but this wont work to increase my mileage on a electronic odometer at the least.. far to complicated an approach.. any other good suggestions? mechanical hand throttle?
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Old 09-25-2009, 09:01 PM   #2
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What is the IDLE RPM suppose to be on this vehicle?
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Old 09-25-2009, 09:19 PM   #3
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This has been mentioned before. My truck does this. It will idle in Neutral stopped @ about 600 RPM, but when moving it will idle in Neutral at about 1,000 RPM. To my knowledge there is no way to override this behavior.
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Old 09-26-2009, 03:34 AM   #4
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It could certainly be done with custom tuning, but that will never pay for itself.
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Old 09-26-2009, 09:57 AM   #5
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It may be related to the CAT temperature but I also see higher fuel consumption while coasting and less when I stop for about 15 seconds the idle and fuel rate goes a little lower 0.15 moving 0.12 gph stopped - not much can be done from what I can see - just the slightest movement of the power steering increases the fuel rate too. They pretty much have it dialed in to burn fuel as much as possible - even when in gear going downhill turning on the AC will add fuel to the burn even if not needed unless the RPM is way up in the 2k or greater.
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Old 09-26-2009, 11:34 AM   #6
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I believe ECUs look at the vehicle speed sensor to see if the vehicle is moving, and if it is, maintain high idle. I'm unsure of the reason for this.

An 80s through mid 90s Chryco ECU will fault the VSS as follows...

- Throttle has been closed for 7 seconds
- Engine speed 768 rpm above target idle speed
- Speed sensor not producing a signal

In other words, during engine braking or coasting in gear it will fault the sensor as bad if there's no signal after 7 seconds.

If other ECUs have similar parameters, then rigging a relay off the neutral safety switch to cut out the VSS while in neutral should not theoretically throw a code..... though other problems may of course be possible... like if your speedo and odometer are using that sensor (Highly likely post 1990)
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Old 09-27-2009, 08:49 AM   #7
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its to keep the synchros in your transmission from wearing out when re-engaging in gear, the closer your rpms are to the speed it will be in gear, the less wear when reengaging, there's a technique to downshift where you rev the rpms to the speed the lower gear will be at when engaging the clutch, it's called rev-matching

my point is, if you understand rev-matching, you can theoretically use a switch, or something, to lower rpms, then undo the switch before you go back into gear
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Old 09-27-2009, 09:01 AM   #8
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im just not into engine off coasting it just seems like my car is more into continuously running as opposed to being turned on and off
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Old 09-28-2009, 07:18 AM   #9
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Many auto-transmission ECUs are programmed this same way too. Kinda annoying, frankly.

In order to make a switch, you'd have to disable the speed signal to the ECM (which would probably also disable your odometer and hence be illegal). However this could make your ABS system go haywire (since they are all interrelated). Alternately, you could have the ECU reprogrammed.

-BC
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Old 09-28-2009, 08:20 AM   #10
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What about a switch that overrides the computer's control of the IAC or throttle (whichever it's using on any given car)?

It would probably throw a code for failure of the IAC/electronic throttle actuator, though...
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