Accidental Transmission discovery (bump started my automatic) - Page 2 - Fuel Economy, Hypermiling, EcoModding News and Forum

apgrok1 · 03-07-2008, 02:31 PM

I agree with elhigh. As everyone has experienced before, during deceleration, engine rpm's will not return to idle unless you throw it in neutral. This means that the trans is driving the engine, which is a common occurance.

apgrok1 · 03-07-2008, 02:38 PM

A quick calculation shows that stator rpm in top gear at 30 mph is 1090rpm. If the stator is running faster that the engine by 25%, then engine rpm is 812, which is probably lower than idle. Also, the slower the TC speed, the less efficient it will be. There is probably an exponential drop off in which the TC housing would finally quit turning.

apgrok1 · 03-07-2008, 02:47 PM

More quick calculations based on 75% engine to trans ratio;
50mph = 1814rpm trans, 1360 rpm engine
40mph = 1451rpm trans, 1088 rpm engine
These engine rpms should be adequate to provide full prssure to the clutches as it is the engine/TC housing that turns the pump, not the input shaft.

apgrok1 · 03-07-2008, 02:50 PM

One more reply. The correct way to do this is to set up a kill switch. That way I could leave the key on and still watch engine rpm's. As long as I would restart it before engine rpm's went below 1000rpm, I should be fine.

RH77 · 03-07-2008, 04:47 PM

Quote:

Originally Posted by apgrok1

They still inject a small amount to keep the engine running, even on deceleration. My proof? If they did shut off, the effect would be exactly the same as turning the key off in gear.

My shop manual states:
________________________
Fuel Cut-off Control

*During deceleration with the throttle valve closed, current to the fuel injectors is cut off to improve fuel economy at speeds over following rpm:

B18B1 engine: 970 rpm (Canada model: 1050 rpm) My Engine -- U.S. Version
B18C1 engine: 970 rpm (Canada model: 1050 rpm)
B18C5 engine: 1300 rpm (Canada model: 1400 rpm)

___________________
2 years ago, I rigged up an injection deactivation device. On decel above 950 rpm or so, there was no difference in engine operation.

RH77

EDIT: Adding...

So, then the engine is still operational providing power from the alternator, and all other ECU-controlled functions. The resistance between the compression of the engine and the turning transmission may be a point for wear if IGEOC (in-gear engine-off coast)...

Whoops · 03-07-2008, 05:05 PM

Quote:

Originally Posted by apgrok1

I probably won't make this a habit, but I might try this for 50 miles or so to find if FE increases a bunch. However, I beg to differ on everyone's claim that the injectors shut off when you let off the pedal. They still inject a small amount to keep the engine running, even on deceleration. My proof? If they did shut off, the effect would be exactly the same as turning the key off in gear. Your trans would be driving the engine.

Believe what you wish. I'm basing my comment on the fact that on my 89 Honda, by monitoring the Oxygen Sensor Voltage, it is clear that the injectors completely shut off, if I am in gear, my foot is off of the pedal and my rpm is above 1200. You are correct, it is just exactly the same as having the engine off, with the key, while being in gear.

You might have a point, in an Automatic. On my wife's Chrysler, I think the injectors do inject some fuel, largely to give people the impression that the car is able to coast farther than it really is.

My primary point was that if you want to risk killing your automatic transmission, knock yourself out. It will have an effect on it, and you will have to replace it sooner, rather than later. I personally don't like replacing or rebuilding transmissions any sooner than I can possibly get away with. I shared the information I did with you, so you'd have a better idea of the risks associated with it.

dremd · 03-07-2008, 08:49 PM

AE340 does the same, it will even keep to motor turning if you downshift (either manually or electrically). however once you put it in neutral with the motor running, it is all over.

diesel_john · 03-07-2008, 09:23 PM

99 windstar 3.8L auto
i pop the trans into neutral and coast engine on. it coasts a lot farther this way.
Question how can i get the engine to idle, it still tracks the vehicle speed even in neutral.
If i put a momentary interrupt switch in the VSS circuit do you think the engine will idle? this bugs the daylights out of me, i know it uses a lot less gas at idle. and i know it won't hurt the trans. coasting in neutral as long as i let it idle.

i believe the ECU is adding air with the Idle Air Control (IAC) which increases fuel because its MAF system.

s2man · 03-08-2008, 10:32 AM

John, my Cavalier keeps the engine turning around 1100rpm when I NC. But the Scanguage reports idle fuel consumption rate, 0.4gph, regardless of rpm. As long as I'm burning fuel at idle rates, I'm not concerned what the rpm is.

I don't think interrupting the VSS will get your engine to idle. I think the ECU is sending idle-amounts of gas, and our engines are just getting a little extra push from the not-completely-decoupled slush boxes.

RH77 · 03-08-2008, 02:41 PM

Quote:

Originally Posted by dremd

AE340 does the same, it will even keep to motor turning if you downshift (either manually or electrically). however once you put it in neutral with the motor running, it is all over.

I think it's VSS and Vacuum. I've wondered the same...

I'll dig out the shop manual and see if I can figure out what mine does...

RH77