Engine off coasting a TDI.

ranger pete

Veteran Member
Joined
Jul 29, 2011
Location
connecticut
TDI
2011 JSW 6MT
I have a '11 JSW 6MT.


Have owned since april. I have put a bit over 30K miles on it. A lot of work driving.

I have gotten between 42-46 mpg (measured by odometer/pump readings).



I have found that warm weather does yield better mpg. I am sure part of this is the decreased air drag of warm air, I suspect that another factor might be that during engine on coasting when the engine is in fuel cutoff, I suspect it cools the head fairly quickly. The result is that you have an engine running at less than optimum temps for a bit.

I also wonder what the effect is to the cat/dpf. What does a minute of relatively cold air going through them do, as you would have on a long downhill while in gear under fuel cutoff?


I have been thinking more and more lately that maybe EOCing might be a good thing, especially in cold weather.



I have a lot of experience EOCing gassers, but have been hesitant to do it because cycling the ignition on off causes a lot of glow plug use and it is also a PITA sometimes to get my bluetooth to sync back up.


The solution to these issues would be adding some sort of fuel cutoff.

Has anyone done this with a newer tdi?



There is another thing this might help.



I experienced the infamous intercooler icing problem last week when it got down into the high teens/low twenties. I have installed a lower grill block using pipe insulation. I am thinking that a good bit of this icing might be occuring during fuel cutoff. Doing more EOCing could address this.

Would love to hear from anyone who has done this mod to a 2010 or newer tdi.
 

[486]

Top Post Dawg
Joined
Mar 1, 2014
Location
MN
TDI
02 golf ALH
I suspect that another factor might be that during engine on coasting when the engine is in fuel cutoff, I suspect it cools the head fairly quickly. The result is that you have an engine running at less than optimum temps for a bit.
If you watch a scan tool rather than the gauge needle, even the coolant temp drops pretty sharply while coasting to a stop. You seem stuck on two options of engine off in neutral, or in gear overrun.
What I do is put it in neutral and let the engine idle. Still have 200F or so EGTs, so things still stay warm, and you don't have to dink around with the ignition while driving.
 

Vince Waldon

Top Post Dawg
Joined
Apr 25, 2009
Location
Edmonton AB Canada
TDI
2001 ALH Jetta, 2003 ALH Wagon, 2005 BEW Wagon
As well, using a scan tool will likely show you that the engine already cuts the fuel to 0 during deceleration, and can approach zero during coasting. :)
 

hskrdu

Top Post Dawg
Joined
Oct 17, 2003
Location
Maryland and New England
TDI
2003 Golf GLS 4D 5M, 2015 GSW SE 6M
It was unclear to me if by "coasting" you meant in gear or in neutral. If you scan your fuel usage, you will likely find that less fuel is used if coasting in gear rather than neutral, and generally better FE than EOC. Of course, on some grades you may lose more momentum when in gear than when in neutral, but you'll likely see something similar to fuel cut off in 6th gear when there is no input from the fuel pedal. Lots of threads here discussing these issues for the older cars, a few for the CR cars. Warm weather also means summer fuel, less warm up time for the engine, and (as said) less drag. My observations are that the first two play a larger role than the last.
 

ranger pete

Veteran Member
Joined
Jul 29, 2011
Location
connecticut
TDI
2011 JSW 6MT
I do realize that fuel use at idle with a tdi is almost nothing. Perhaps plenty of coasting in neutral with engine on is the best strategy, especially in cold weather because coasting in gear will quickly lower combustion chamber temps while in fuel cutoff mode.
 

GoFaster

Moderator at Large
Joined
Jun 16, 1999
Location
Brampton, Ontario, Canada
TDI
2006 Jetta TDI
I have to say the following. I am obligated to.

Engine-off coasting on a vehicle that does not have designed-in provision for it is unsafe. In your particular case, your power-assisted brakes rely upon vacuum from an engine-driven vacuum pump. There is a vacuum reservoir that holds enough vacuum for a couple of brake applications, but after that, you will lose power assistance, and it will then take enormously more pedal pressure to stop.

Vehicles with hydraulic power steering will immediately lose power steering assist. Electric power steering is capable of operating engine-off for some period, but I don't know what the underlying logic is.
 

ranger pete

Veteran Member
Joined
Jul 29, 2011
Location
connecticut
TDI
2011 JSW 6MT
GoFaster,


Everything you say is spot on.



That being said, I think responsible hyper-miling, including EOCing is perfectly fine, IF you thoroughly understand your car's systems.

I have successfully hypermiled manual trans cars in the past. I still do my son's '05 Focus. My 91 Accord was an awesome EOCer. Its fantastic steering was still better than most car's with their powersteering on.

Cardinal rule one- If you don't have a clutch pedal, don't EOC. End of discussion. You have no reliable, almost instantaneous method of spinning up your power systems to restore full braking/steering.

If you have a manual and are considering being a hypermiling geek, get to know how your car functions while EOCing. The first time you should try it, should be on a deserted, straight road. Turn ignition off. Pump the brakes. How many pumps do you get before your brakes go away? Try turning. Does your steering become very heavy? Some cars do, some don't.

As to the question of whether or not this should be done, here is my argument that it is perfectly safe.

What is one of, if not the most important factor in accidents?

Inattentive drivers.

A person who is actively hypermiling is very attentive. If he isn't, he isn't very good at it. And he is actually thinking about driving. Contrast this with a person rolling down the hiway with the cruise set to 72 mph. That person is far more likely to space out or even knod off.

So the question I still don't have a good answer to, is does it ever make sense to EOC a tdi. I can see reasons for and against.



For- It saves some fuel. Far less than a gasser, given a diesel's extreme efficiency at idle.



When the engine is off, it retains combustion chamber temps for a while, whereas a spinning motor in fuel cutoff, quickly cools. The result is lowered efficiency and possibly sooting issues or increased dpf/cat clogging.



Against- tdis will idle forever on a few ounces of diesel. The savings in fuel are negligible. More than offset by increased clutch/drivetrain wear of pop-starting. An idling tdi maintains combustion chamber temps.



The one other issue I wonder about is the coldweather intercooler icing problem.



When does it occur? Does it occur during high load situations when the egr valve is wide open? Does it happen at idle? Does it happen during fuel cutoff?

Would having the engine shut-off for short periods help/hurt? Or not really matter.

I don't have the answers to these questions, which is why I am here talking to all you smart folks.
 

piotrsko

Veteran Member
Joined
Aug 11, 2013
Location
Reno Nv
TDI
2013 Golf, 2000 F-250 (7.3)
From what I understand, icing is caused by driving in wet slush and is mostly independent of engine running. I do not recall anybody icing a running car unless they were attempting boating their VW.

With all the other nonsense going on computer controlled, AND, only using a quart of fuel every so often, EOC is pointless. You gather bigger savings with aero, belly pans,tires, and adjusting the attached loose steering wheel nut. MY $0.02 YMMV
 

ranger pete

Veteran Member
Joined
Jul 29, 2011
Location
connecticut
TDI
2011 JSW 6MT
From what I understand, icing is caused by driving in wet slush and is mostly independent of engine running. I do not recall anybody icing a running car unless they were attempting boating their VW.

With all the other nonsense going on computer controlled, AND, only using a quart of fuel every so often, EOC is pointless. You gather bigger savings with aero, belly pans,tires, and adjusting the attached loose steering wheel nut. MY $0.02 YMMV


The icing issue happened to me the first cold morning this year. I drove about 25 miles to the place I was working that day. It was maybe 25 degrees when I left and clear.

When I tried to start that afternoon, it started then immediately stalled. It acted like a low battery on restart, then was fine.

I have been told this is text book case of intercooler icing.

It ices while driving when EGR valve is open. The exhaust gas is high in moisture and causes ice in the intercooler.

After you park, it melts because the engine bay is warm. On restart, you get a lot of water which causes hydro lock.




Sent from my iPhone using Tapatalk
 

Purringvw

New member
Joined
Apr 5, 2022
Location
Middleburg
TDI
2015 Golf TDI
I have to say the following. I am obligated to.

Engine-off coasting on a vehicle that does not have designed-in provision for it is unsafe. In your particular case, your power-assisted brakes rely upon vacuum from an engine-driven vacuum pump. There is a vacuum reservoir that holds enough vacuum for a couple of brake applications, but after that, you will lose power assistance, and it will then take enormously more pedal pressure to stop.

Vehicles with hydraulic power steering will immediately lose power steering assist. Electric power steering is capable of operating engine-off for some period, but I don't know what the underlying logic is.
 

Purringvw

New member
Joined
Apr 5, 2022
Location
Middleburg
TDI
2015 Golf TDI
I have done these hypermiling techniques with my 2015 Golf TDI 6S and it yielded 65.19 MPGs at the pump , actual calculations .
 

Mongler98

Top Post Dawg
Joined
Mar 23, 2011
Location
COLORADO (SE of Denver)
TDI
98 Jetta TDI AHU 1.9L (944 TDI swap in progress) I moved so now i got nothing but an AHU in a garage on a pallet.
I have done these hypermiling techniques with my 2015 Golf TDI 6S and it yielded 65.19 MPGs at the pump , actual calculations .
good thread... good idea but when actually watching the GPH real time consumption going downhill or at idle at operating temps on just about all TDI's they all have about 0.08 to 0.15 with a usually average of 0.11 GPH or gallons per hour. If you shut it down for every downhill for the next 30 years of driving 50K a year... it would still be less than a tank of fuel... tis even less when you are going at any speed and let off as its a defuel map vs an idle. i cant speak for all TDI engines or your 2011.... but its still less than idle... AKA not even worth turning off!

TDI's have a very interesting dynamic vs gas cars and that is running VERY LEAN... like 1,000 to 1 at idle.... or something equally crazy... in other words hypermiling isn’t the same for gas as it is a TDI. the myth that shutting down a car at a light or down a hill is outdated and has no actual use now, it all came about with the carburetor and was a 10 second rule... but now with fuel injection metered by a computer... I don’t care how long you have to sit at a light or coast downhill... this has NOTHING to do with actual gains... that being said I have also got 62mpg on my massive 762 gt2052 230 build AHU mk3 Jetta. Why? It’s not difficult when you do highway cross country!
 
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