Volkswagen Passat Forum banner
1 - 20 of 32 Posts

·
Registered
Joined
·
318 Posts
Discussion Starter · #1 ·
:crazy: I was thinking about something the other day, does our 1.8t engine actually shut the fuel off to the engine when coasting down a hill in gear? I don't really get concerned too much with gas mileage but I did have the initial idea that putting the car in neutral while going down a hill would actually SAVE gas mileage. I now think that if you leave the car in gear (with your foot off the gas) and coast, the fuel gets shut off to the engine, SAVING gas even more.

I noticed this while observing the instantaneous MPG reading on the trip computer. Does anyone have any insight on this? Is it true that the car shuts off the fuel supply while coasting? :roll:
 

·
Registered
Joined
·
2,156 Posts
Leaving your manual transmission Passat in gear and with the throttle at idle will shut off the injectors. That's why your MFA will show blank lines on the instantaneous mileage when you do this. The MFA is based on injector duty cycle.

The automatic transmission cars do not shut off the injectors at coast down.
 

·
Registered
Joined
·
318 Posts
Discussion Starter · #4 ·
CRD99 said:
Leaving your manual transmission Passat in gear and with the throttle at idle will shut off the injectors. That's why your MFA will show blank lines on the instantaneous mileage when you do this. The MFA is based on injector duty cycle.

The automatic transmission cars do not shut off the injectors at coast down.
Just asking, How do you know for sure?

Sounds like it is defintely more benifical to leave it in gear while coasting.
 

·
Registered
Joined
·
573 Posts
Hey, pass-variant, did you notice in your logging whether the injectors are shut down when coasting in both auto and TIP modes? My father's got a tip and I never knew what to tell him to maximize mileage when coasting down hills etc.
 

·
Registered
Joined
·
2,054 Posts
Cool information to know.. will leave mine in gear whil coasting - and now I know what the --- means! Cool Thanks :wink:
 

·
Registered
Joined
·
573 Posts
The --- doesn't necessarily mean the injectors have shut down, it just means the current MPG is higher than some arbitrary value (I think 99 mpg? I can't remember). It also reads --- on instantaneous MPG when you're stopped. (negative MPG :p )

But unless you're cruising down a hill in neutral at a high rate of speed, chances are it means the injectors are off.
 

·
Registered
Joined
·
6,172 Posts
pass-variant said:
automatic transmission cars do not shut off the injectors at coast down
My TIP V6, MFA, anad VAGCOM would beg to differ with you. There is no injector activity while coasting.
I agree with p-v on this. My tip 1.8T, MFD and VagCom show injector shutoff while coasting in drive (tip or auto); however, in neutral, the injectors stay on to keep the engine turning. In other words... popping into neutral on the downhills will actually use *more* gas than coasting in drive.
 

·
Registered
Joined
·
2,719 Posts
Meap said:
That's the idea, the fuel delivery is completely shut off becuase there is no output needed from the engine while coasting.

yes but if its in neutral

how does the comptuer keep at idel with no gas

or are you saying the passat can idle on no gas

i think its just the computer saying the mpg is out of range when it shows up ---
 

·
Registered
Joined
·
6,172 Posts
pete1 said:
...however, in neutral, the injectors stay on to keep the engine turning
crash893 said:
yes but if its in neutral how does the comptuer keep at idel with no gas
It doesn't shut the injectors off when you coast in *neutral*. Only when you coast in *drive*. ...for exactly the reasons you mentioned.

I hooked up the vag-com and watched the injector on time while driving home yesterday. With the car in drive, "---" on the MFD corresponds exactly to zero-on-time on the injectors. (Coasting in neutral will not produce "---" on the MFD) Also, while coasting in drive, at about 1200 RPM and below the injectors turn back on - i guess it's to "get ready to idle"?

nyibpass said:
Injectors only shut off when moving in gear with no throttle input- that's what this whole discussion was about. If you're in neutral, yes, the engine would be using gas to maintain idle.
^^Exactly.
 

·
4th Gear (Wax on Wax off)
Joined
·
4,974 Posts
On the Candian cars where the milage is calculated in L/ 100 KM when you are going down hill with your foot *off* the throttle the reading is 0.0 L/100 KM thus no fuel being fed.

Sitting still at an idle it will register 1.2 L/Hr etc because there is fuel being fed where on a US car at an idle there is zero miles per gallon and I think it goes to "---"
 

·
Retired PassatWorld Staff
Joined
·
6,164 Posts
The question is though... can you coast further in neutral with idle than no gas in gear... you're not pushing the drivetrain, so you'll coast much further.

It's not clear that you'll use more or less gas. The additional distance may more than make up for the fact the engine is idling.
 

·
Registered
Joined
·
318 Posts
Discussion Starter · #20 ·
Pete1,
I found the same thing, the injectors kick back on around 1200 RPM (while in gear and coasting).

pass-variant said:
The question is though... can you coast further in neutral with idle than no gas in gear... you're not pushing the drivetrain, so you'll coast much further.

It's not clear that you'll use more or less gas. The additional distance may more than make up for the fact the engine is idling.
Pass-variant, good question. Can the vag-com document Gas usage or fuel flow rate?
 
1 - 20 of 32 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top