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?

What is the IDLE RPM suppose to be on this vehicle?

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.

It could certainly be done with custom tuning, but that will never pay for itself.

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.

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)

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

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

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

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...

Even if you could implement a simple switch I wouldn't recommend it in an emissions state. IACs are usually a stepper of some sort so a single switch won't work the majority of the time anyways.

The high idle in a manual transmission car is most likely because the manufacturer just took the auto transmission program and flashed it to the manual ECU sans auto trans code.

The reason it is in there for the automatic has to do with transmission safety. Only my Cressida did the high idle when coasting thing and that car couldn't be flat towed. The Tracker would drop to a normal idle at 70 when I put it into neutral and so does the Durango. Both cars are safe to flat tow.

It's likely that if the engine speed isn't high enough in some cars that there isn't enough to keep the bearings pressurized where they need to be. The following is a list of vehicles safe for flat towing. If your car isn't on the list it's likely that you'll have a high idle in neutral.

Cars

Chevrolet
Cavalier Coupe & Sedan
Malibu

Daimler-Chrysler
Neon
PT Cruiser
Sebring/GTC
Stratus Coupe

Ford
Focus
Mustang
ZX2

Hyundai
Accent
Elantra
Sonata
Tiburon

Nissan
350Z
Altima 2.5
Maxima
Sentra

Oldsmobile
Alero GX Coupe

Pontiac
GrandAm SE
Sunfire
Vibe

Saturn
ION Sedan
L-Series

Subaru
Impreza 2.5 Sedan & Wagon
Legacy L & Wagon Versions
Outback Sport
WRX & Wagon Versions

Suzuki
Aerio S/GS Sedan & SX Wagon

Toyota
Camry LE
Celica GT & GT-S
Corolla CE
Echo
Matrix 2WD
MR2

SUVs & Pickups

Acura
MDX

Chevrolet
Avalanche
Blazer 2DR 4WD
Envoy/Trailblazer 4WD
Silverado/Sierra 1500 Series 4WD
Suburban/Yukon XL 4WD
Tahoe/Yukon 4WD
Tracker 4WD

Dodge
Dakota 4WD
Durango 4WD
Ram 4WD

Ford
Escape 2WD & 4WD
Explorer
F-150 4WD
F-250/350 SD 4WD
Ranger 2WD & 4WD

Honda
CR-V
Element
Pilot

Hummer
H2

Hyundai
Santa Fe FWD

Isuzu
Ascender 4WD

Jeep
Liberty 4WD
Grand Cherokee 4WD
Wrangler

Land Rover
Discovery 4.6
Range Rover

Mazda
B-Series 2WD & 4WD
Tribute DX FWD

Nissan
Frontier 2WD & 4WD
Pathfinder SE 4WD
Xterra 4WD

Saturn
VUE

Subaru
Baja
Forester 2.5X
Outback Wagon

Suzuki
Vitara & Grand Vitara 4WD
XL-7 4WD

Toyota
RAV4 2WD & 4WD

If your car isn't on the list, check the owner's manual. If you have a 2wd version of the 4wd listed check the manual as well. Anything about only towing the drive wheels at a low speed and/or a short distance you probably have a high idle in neutral coasting.

Where the engine is in relation to road speed shouldn't affect the synchros. If you use the clutch when going back into gear the synchros are only used to pull the clutch up to road speed before you use the clutch to bring the engine up to speed.

That list is inaccurate, unless it stipulates that the engine must be running. Where did you get it?

I'll double check the manual, but I'm pretty sure that my Silverado/Sierra 1500 Series 4WD doesn't allow flat towing, even with the transfer case in neutral.

I got the list from an RV information site. Won't let you flat tow with the tcase in neutral?

I believe K series GM trucks are flat towable, but not with the electronic transfer case, only with the manual transfer case.

Tomorrow night I'll have to remember to check my manual.

dk jones lied, cow's transmission died

https://www.allofftopic.com/images/smilies/lol.gif I never tried it, and I'd recommend everyone confirm that their particular vehicle, as equipped, is good to go before doing it...

back from the grave...

My little stick shift Metro did the same thing. Found out that the speed sensor was keeping the RPMs high in order to make shifting easier. Once the car came to a complete stop, the idle dropped to factory specs.

I "solved" this by disconnecting the IAC electrical connector. I can now neutral coast at factory idle.

all u need is a console mounted on off switch shizamm!

My Scion xB does the same thing until I stop for about 30 seconds to get the idle to really drop down to 650-640rpm then it will idle low while coasting. It maybe the cat in my case not quite getting up to temp but until I stop for a several seconds the idle is in the 800-900rpm range and burns more gas. Even when it finally does settle down it doen't get all the way down to the lowest burn rate/idle while moving and any power steering use raises the idle a little. It does seem to idle lower on hot days better than on cold days. Forget about nights because the lights increase fuel use. Come to think of it . . . it could also be related to battery charge too.

im gonna try this soon, you dont think it'll set off a service engine light?

hmm someone told me if you dont do this right it can cause some damage.. will look into more..