Vtec04

There are a few reasons that I can see to shift from D4 to neutral at idle:
1. The transmission will produce much less heat, possibly extending its life.
2. The engine has much less load on it in neutral since it's not fighting the brakes, so fuel consumption may be reduced.
3. The engine vibrates the car much less in neutral than drive with my solid motor mounts
There are also 2 reasons that I can think of to stay in gear at idle:
A. Staying in gear is safer in case I need to quickly move the car to avoid an accident
B. Shifting from drive to neutral and back may wear out the transmission more.
Not sure what I should do. Can you please help me decide? Thanks.

Chance EG

Don't shift to neutral. Let the torque converter do it's job. Impact on fuel consumption at idle would be incredibly minimal. There would be 0 difference in heat in the trans either way.

Also interesting that you have solid mounts in a car with an auto trans...

Vtec04

Thank you for the reply. I agree that the fuel consumption difference is minimal, but I do disagree with you though that there will be no difference in fluid temp since I did a test and proved that it does. With my Hayden 679 cooler, while idling in neutral, the fluid temp maxed out at 140 degrees F. While idling in gear, however, the fluid temp maxes out at 220, a full 80 degrees higher! So there is definitely a huge difference in trans temp. Also, is it bad to have solid mounts with an auto trans? Thanks again.

Chance EG

Not necessarily bad to have solid mounts with the auto trans, just seems like a very weird combo. Solid mounts generally aren't recommended unless it's a race-only car, in which case you wouldn't be using the auto. If it works for you, then it works though.

I'm amazed if that's the temp difference you're reading at the transmission. I figured all the heat, if any, would get stored in the torque converter (I guess the converter shares fluid with the rest of the AT).

Vtec04

Thanks for the reply. The torque converter does share fluid with the rest of the transmission. The fluid gets pumped through the torque converter. I use the solid mounts because the engine feels more responsive with the solid mounts and handling seems slightly better. Also, why are solid mounts not recommended for street use? The only downside that I see since I switched to solid mounts is the noise and vibration at idle, which doesn't bother me. Thanks again, you have been very helpful.

TomCat39

Shifting to neutral is a safety hazard. If you get into an accident and the car is in neutral there is nothing to prevent the car from rolling into more traffic, into a ditch, off a cliff etc.

In gear when you get slammed off the brake, there is a chance the engine will die and then the transmission prevents the car from rolling much if at all after.

It's not as guaranteed as a manual transmission but still the same principle applies, you are not supposed to put the car in neutral when cruising or sitting idle as a general rule of driving.

Vtec04

Good point about safety, but why would the transmission keep the car from rolling in gear with the engine off? Without the engine running, the transmission won't be in gear anyway because an automatic transmission needs fluid pressure to keep the gear engaged. Just curious.

TomCat39

I see my understanding of the auto is not great. In gear if the engine doesn't die, it's going to roll some unless enough resistance is met to stop it.

And off engine, if being in gear is moot, then it must be fluid dynamics/resistance of the torque converter that resists the rolling of the car. I suspect from what you said, that will be the same regardless if you are in gear or not.

I was always told it that it's legistlation that requires not being in neutral when cruising or idle and no differentiation between manual and auto was given. And was told it came about as a safety issue.

I drive manuals more often than not and so figured the same logic for manuals applies to autos.

So what happens to the car if in gear rolling along and you shut off the engine? Anything?

Vtec04

I tried pushing the car in neutral and in D4 with the engine off, and both took the same amount of effort. D4 is the same as neutral with the engine off.And to answer your question of what happens if you shut off the engine while you are coasting in gear, the car responds the same way that it would if you let off the throttle, which puts the ECU into DFCO- the car slows down from engine braking and the engine keeps turning, which runs the pump in the transmission to create enough pressure to hold the transmission in gear until you get to around 5 MPH, at that point it basically disengages the gear and is in neutral. I think that the safety issue is if you have to quickly accelerate to avoid an accident and can't.

weekendboost

I did for about a month, mainly to see what kind of mpg I would get. I believe I only got about 2 to 3 mpg on average. I'm surprised at the temp difference, I thought It would've been hotter shifting back and forth from neutral.

MAD_MIKE

Accord ATs are more like MTs than ATs.
Only real benefit from shifting into Neutral at a stop would be to reduce the vibrations induced into the cabin with the solid mounts and AT. This is why ATs have the vacuum controlled hydro mounts. When in gear and stopped the mounts allow the engine/trans to 'float' and when the car begins to accelerate the mounts are 'solidified' and act as normal mounts. This is to remove/reduce unwanted vibrations into the cab.