I didn’t mean to imply that the injectors shut off for any length of time, more like a couple seconds or two before readings appeared again. So not really shutting anything down, just closing the tap instantaneously on deceleration. Don’t have any Rivs anymore so can’t really tell now.
The turning shaft pressurizes the tranny fluid, and the computers are certainly capable of keeping the tranny solenoid circuits enabled while still shutting off the injector circuits. And if the circuits are enabled and the alternator is still being spun, whatever the computer decides is what the tranny will do.
I don’t honestly know how common shutting the injectors off is. I too have seen the comment made numerous times, but have never responded because it’s certainly feasible in today’s cars.
I drive a type A school bus that is like a shuttle van. It has a Ford Power Stroke diesel with the 5-speed auto. If I downshift, the bus will pull the engine up without the torque converter being locked up, and it is easy to hear that the fuel shuts off completely a little above idle speed. If the van is pulling the turbine at around 1,500 RPM, it will pull the engine at about 1,100-1,200. If the turbine is around 2,000, it will pull the engine to about 1,900. The slip decreases rapidly as the turbine rises above 1,500. I can’t see why it is so hard to understand that the turbine will pull the impeller along just as the impeller will pull the turbine. That fluid is spinning in there no matter which member is pushing it, and it will push the other member along.
I would have conducted this experiment with my car, but my car is a manual trans.
Here is some video evidence.
If I can find more, I’ll post it.
The whole concept is interesting, because the ECU can shut the injectors down on deceeration and the driver would never even know. Unless, of course, a manufacturer includes it in its promotional materials, which Caddy apparently does.
It would be interesting to find out how common this practice actually is. I tried doing a search and was unsuccessful in finding anything at all.
I don’t get that video. Unless I’m reading it wrong, I see the fuel possibly being cut off because the RPMs are approaching the limiter, not because it’s coasting.
My '83 SAAB Turbo does the same thing with an overboost switch which shuts the fuel pump off when boost pressure reaches a certain stage but that’s in an acceleration mode, not coasting mode.
What people forget is that it’s always been possible to downshift an automatic to take advantage of engine braking. At this point it is not fuel being burned that is keeping the engine turning on any car, whether FI or carbureted. The engine and transmission remain coupled, the transmission (including the pump) keeps turning, and it remains this way until the RPM at the input shaft of the transmission gets low enough that there is no longer enough line pressure to keep the clutches engaged.
A year or so ago there was a long discussion of whether the engine keeps turning and for how long if you turn off the ignition while coasting. Some vehicles fared better than others, but on all automatics, the engine doesn’t just die instantly. I suspect that if the ignition is left on, that vehicles in this discussion would have kept the engine turning a lot longer, even if fuel was totally cut off, because cutting electrical power to an electronically-controlled transmission would tend to affect it differently than a purely mechanical one.
That said, the engine’s computer is certainly intelligent enough (well, programmed well enough) to know what the threshold would be for getting away with cutting fuel and still keeping everything going, and be able to take advantage of this. All it would really need to know would be the vehicle’s speed, gear it’s in, throttle position, and RPM of the engine to make a decision on when to cut and reengage fuel delivery.
I remember the discussion about turning the engine off and coasting and tests out of curiosity on various vehicles around here showed no engine rotation unless it was so brief that it could not be determined.
This leads to the point (assuming for the sake of discussion that the engine remains locked to the wheels) about engine braking being involved. If you’re coasting to save fuel then why would you want any engine braking at all as a free wheeler would be the better option. Of course that would not allow an engine restart either.
I never tried turning the key off with the vehicle moving in an auto trans. I do know that it would drop the tachometer to zero instantly, engine turning or not. The tach only works when the key is turned on.
@ok4450: The point I was trying to make is that the engine will indeed stay coupled to the rest of the powertrain with no fuel flowing, or with the engine being spun by the wheels if you wish to view it that way.
Here is an example, I had a 1971 Ford Pick-up 302 and C-4 auto tranny. It had a dirty fuel tank so I installed a filter right at the gas tank which was behind the seats. ( Would not want to get broadsided with this arrangement )
About every 2 months the filter would clog and acted like I ran out of gas…Was easier to change this one out that the one going into the carb.
Was going about 60 mph when I lost power…a little bucking at first then a dead engine ( no throttle response ) so I knew what it was and a 5 minute fix with a screw driver by changing out the filter.
When this would happen my tach never went to zero, maybe down about 500 rpm to about
2000 RPM. The tranny stayed hooked up until I reached about 15 miles per hour and this was when the engine finally stopped spinning and the dash lights came on. This was a 71 truck so no rear transmission pump, which were removed from most vehicles by 1965 My 59 Thunderbird I had, you could push start this car around 20 mph. Put in “N” turn the key on then drop it into
either D2 or L.
As far as turning off the key in that past experiment, you are killing all the electronics and the ECU
so who knows whats goes on when this is done…
I bet if you had a way on a vehicle on just about any automatic to kill the fuel pump without turning the key off the engine would keep turning until the engine speed dropped down low enough where the front pump could not supply enough pressure, so the tranny neutrals out.
The ECU turning the injectors off is very different from the driver turning the key to OFF. I recommend against doing the latter except in an emergency, where life comes first and the machinery second.