Any Harm To Coasting?

phillyjoec · June 28, 2008

I've often wondered if it does any harm to the transmission to put my car or truck in neutral when going downhill.

I tried it as a means to get better gas mileage and it works. My Sierra 1500 went from 13 MPG tp 15 MPG!!

Any thoughts?

Zembonez · June 28, 2008

I've often wondered if it does any harm to the transmission to put my car or truck in neutral when going downhill.
I tried it as a means to get better gas mileage and it works. My Sierra 1500 went from 13 MPG tp 15 MPG!!

Any thoughts?

The only "possible" issue I can see is switching from neutral into drive while you are at speed. The trans is designed to be put into drive when stopped. This might not be a problem when it happens every once in a while but on a constant basis it may cause trouble.

98silvz71 · June 28, 2008

I think that we had a topic like this and the GM tech said that the engine almost shuts the fuel off while you coast so the difference you are seeing could be other attributes. Different size hills, wind at your back, not so aggressive on the throttle, etc. If you could do it over 10 tanks and yield the same results I could see it being worth it. But the money you save on gas won't make up for the money you'll spend fixing the transmission when it craps out on ya.

cammiller · June 28, 2008

I had said it in another post, and since then when I have been on test drives, I have been watching the scan tool. When going down hill, off the throttle, the truck goes into Decel mode. Basically, after 2-3 seconds, the injectors turn off and there is no fuel going to the engine. If you put in in neutral, the engine will idle using fuel, not to mention the possibility of causing transmission issues. Bottom line, neutral going down hill will use more fuel than just coasting.

BigJayDogg3 · June 28, 2008

I had said it in another post, and since then when I have been on test drives, I have been watching the scan tool. When going down hill, off the throttle, the truck goes into Decel mode. Basically, after 2-3 seconds, the injectors turn off and there is no fuel going to the engine. If you put in in neutral, the engine will idle using fuel, not to mention the possibility of causing transmission issues. Bottom line, neutral going down hill will use more fuel than just coasting.

Isn't there also the fact that you use the engine to slow down, and putting it in neutral makes you rely on your brakes, which in turn melt?

VMax2007 · June 28, 2008

I had said it in another post, and since then when I have been on test drives, I have been watching the scan tool. When going down hill, off the throttle, the truck goes into Decel mode. Basically, after 2-3 seconds, the injectors turn off and there is no fuel going to the engine. If you put in in neutral, the engine will idle using fuel, not to mention the possibility of causing transmission issues. Bottom line, neutral going down hill will use more fuel than just coasting.

Question: If the injectors shut off ALL fuel to the engine when coasting, then how come the engine doesn't shut off? It must keep firing to keep running. Also, when coasting, my DIC shows anywhere from 20 MPG up to 99MPG, depending on how fast I am coasting. If the engine wasn't using any fuel, the DIC should show 99MPG (assuming that is the max readout) all of the time (after 2-3 secs). Right?

cammiller · June 29, 2008

The torque convertor is locked up which keeps the engine turning. As for mileage in the DIC, I would not put much faith in it. As I mentioned, I have watched the O2 sensors while coasting down the 2 mile hill at work and they all read 0mv, that means no combustion.

BJD3, yes, in gear and coasting will provide some engine braking.