Bad gas mileage during winter months

[Rhinoculips]

It's on its way, fresh snow is on the peaks and slowly working its way down after each storm. Can't wait to go skiing, but can wait indefinitely for the poor MPG I will be getting.

My question is, how normal is it for gas mileage to drop in the winter months? My summer MPG for around town driving is 19. In the winter is gets as low as 13! This seems to be a regular cycle for as many years I have owned the '89 Westy. Is there something going on with my van or could it be "winterized" gas that I believe they switch to all across the Colorado Rocky Mountains in the winter?

By the way, since I live at 9,500 feet, I use 85 octane(as what has been used in it since it was new). If I go with higher 87 octane, power is uneffected and gas mileage goes down to about 17 MPG in the summer. I have never tried higher octane in the winter because of the drop I have experienced in the summer. Also, air/fuel filters, plugs, cap and rotor are changed regularly and tire pressure checked weekly.

Is there anything I can do to increase my winter MPG or is this just normal?

Many thanks for your input,

R

ps - I am off for a 3 week camping trip to Utah tomorrow. Can't wait!
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“May your trails be crooked, winding, lonesome, dangerous, leading to the most amazing view. May your mountains rise into and above the clouds.” - Edward Abbey

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[glutamodo]

There's a difference between winterized gasoline and oxygenated. Do you get the oxy-fuels up there? Along the Front Range, they have to sell oxy-fuels during the winter months. When I lived in Fort Collins, and they'd switch from straight gas to oxygenated, I'd notice right away because I'd drop a few MPG right away. (I never quite figured it out, because I'll buy E10 when I travel in the corn belt, and that doesn't seem to affect my milage noticably. There must be more to oxyfuels than just adding ethanol)

And the colder air temperatures don't help fuel economy either.

[scobax]

Interesting question, I'll take a stab at it. I spent fifteen years in Jackson Hole, WY and noticed the same thing. Here's what I surmised; Colder air is more dense than warm air, and as a consequence, the MAF meter in fuel injected engines is responding to the density as volume and instructs the injectors to add a correspondingly larger amount of fuel. As an illustration, I have seen "chips" being sold on the 'net that "fool" the ECU into believing that the intake air is a couple of degrees cooler, and as a consequence provide more fuel, equaling, according to the sales pitch, more horsepower. The part that goes unmentioned of course is the plummeting fuel economy that goes along with the richer intake mixture. So, if the air temp IS a couple of degrees cooler you should get the same response in fuel economy (unfortunately, not horsepower).
Additionally, the colder temps cause longer warm-up times, which cumulatively may be observed as lower MPG numbers.
Whatever the reason, the conditions at sustained higher altitudes tend to give more striking MPG variations than elsewhere, in my experience.
I will prepare myself for the inevitable scientific rebuttal from the esteemed panel!
_________________
87 Syncro Westy w/ Suby 2.5
RIP: 2.1 WBX. You were a good dog 'ol feller

“The word adventure has gotten overused. For me, when everything goes wrong, that's when adventure starts” -Yvon Chouinard

"It's not expensive, its priceless" -Unknown

[Rhinoculips]

Thanks guys,

What you say about ethanol and denser air makes sense and I kinda was leaning that way but....

Thing is, my 1990 Volvo 240 gets the same 34 MPG regardless of time of year. This is why I have been stumped by the drastic change in my Westy, but not my Volvo.

Hmmm.....
_________________
“May your trails be crooked, winding, lonesome, dangerous, leading to the most amazing view. May your mountains rise into and above the clouds.” - Edward Abbey

Click to view image

[scobax]

Hmmm... well... my first inclination is to blame the wretched digifant! The system is reknowned for its archaic complexity and its problematic nature. A logical assumption might be that these flaws in it's execution carried over to its precision of operation. Precision is a tall order when held up in comparison to the build quality of the venerated Volvo 240.
Solution= Subaru 2.2! Simple, effective, bulletproof.
Anyway, enjoy the trip!
_________________
87 Syncro Westy w/ Suby 2.5
RIP: 2.1 WBX. You were a good dog 'ol feller

“The word adventure has gotten overused. For me, when everything goes wrong, that's when adventure starts” -Yvon Chouinard

"It's not expensive, its priceless" -Unknown

[glutamodo]

My instances of the instant fuel economy drop were on carbureted bugs... so I think it's more than an EFI issue.

[Wildthings]

Part of the problem might be drier air and a build up of static in the AFM. I have never seen a drop in gas mileage as winter comes on, but I have always run a ground wire between my AFM and engine block on the recommendation of the dealer mechanic when I bought the van new. I also wonder is the "Vanagon" capaciter and the piece that plugs into the AFM harness might make a difference. I have run them since the first year or so I owned the van as well.

[scobax]

I should clarify before I get bounced...The Digifant has an air temperature sensor but no compensation for pressure. This was the basis for my logic. Perhaps the air temp sensor is at fault? The Digifant Protraining manual may shed some light here. I don't have it handy at the moment.
The 240 uses a Bosch k-jetronic system. Completely different animal. The k-jet is a mechanical system for starters, and as such, lends itself to diagnosis and repair more readily, IMHO.
The Digifant is a VW derivitave of the Bosch l-jetronic... similar name, wildly dis-similar execution.
Lots of good points in this thread though. Do the simple stuff first.
_________________
87 Syncro Westy w/ Suby 2.5
RIP: 2.1 WBX. You were a good dog 'ol feller

“The word adventure has gotten overused. For me, when everything goes wrong, that's when adventure starts” -Yvon Chouinard

"It's not expensive, its priceless" -Unknown

[Dogpilot]

I would do the Bently resistance checks on your temp II and the temp sender in the AFM. If the Temp II thinks the engine is perpetually cool it will stay in enrich mode. If the temp I in the AFM is reading bad, it to could cause the ECU to enrichen. Other enrichen culprits is the AFM itself and the O2 sensor. THe O2's go bad just for the hell of it and they fail to enrich side. The temp II is just insidious in nature and can hold the engine to enrich.

Bottom line, if you replace them both, our out under $100, which pays you back in fuel savings on the gas quickly. The AFM is another story, price wise.

On the weird side of gas milage. My Range Rover has gas milage readout. When I lived in FLorida, we took a trip out west. After crossing the mississippi river the gas milage started to creep up. By the time we got to New Mexico it was 5 mpg better than Florida. This stayed consistent for the duration of the trip. Once we turned home, and crossed the mississippi, the milage slowly got worse until it went back to normal. This reoccurred the following two years, and now that I am permanently in the west, it stays higher. Go figure.
_________________
Geology with a Syncro rocks!
86 Syncro Westy AKA "The Bughunter"
98 Disco I
08 Range Rover SC
08 VW Rabbit S
1951 O-1G

[tencentlife]

You're on it, scobax, although I have to disagree heartily as to the relative complexity of Digifant. It's one of the simplest electronic systems (just badly wired). Not as simple as K-jet but that's almost purely mechanical. And you're right, K doesn't compensate for intake temp (only block temp). But Digifant does, and I also think that that may be where the problem arises.

L systems use the AFM to measure air volume, which most people undertand isn't the same as mass, because volume of gases varies directly with temp. So the intake air temp sensor provides a correction factor for the volume the air flap measures, in order to approximate air mass so a stoichiometric mass of fuel can be added to it. If the temp signal from the IAT sensor is way off, all fuel calculations will be skewed in that direction.

If the IAT sensor's resistance is higher or at the high end of the resistance-to-temp range on the tables in Bentley, the ECU will calculate that air mass is greater and inject proportionally more fuel. Since the air mass isn't really so high, the extra fuel will be poorly utilised, and mpg will suffer.

However, at steady-load type conditions at least, the O2 Lambda system should be keeping air/fuel ratios close to the 14.7/1 lambda. So that's something to check as well.

Coolant temp sensor controls the progress of engine warmup mixture compensation. If it doesn't attain a low enough resistance as it warms, the ECU can run the engine as if it is almost warmed up, but not go into the normal hot-running mode. It will use too much fuel then, too.