: Bad MPG is freezing weather



weister42
01-19-11, 10:52 AM
I live in Wisconsin and I've noticed that my car gets less than 13mpg in the city and barely 20mpg on the highway when it gets real cold here, but I get much better mpg in the summer times, like 26 on the highway and almost 20mpg in the city. It was the same for the past two years.


I have no codes besides a P0442 which is small leak in the EVAP system, probably slit the hose under my car from when I ran over a deer carcass two years ago. I replaced the EVAP canister valve and that didn't help clear the code(I clear the code and it comes back in a few days), I can't see any leaks in the engine bay and the screw cap is tight. My Deville has 176k miles but it runs just fine, I can still spin my tires on pavement without much effort. I do have a set of snow tires on which is much softer, but that shouldn't cause the sharp drop in mpg.

Ranger
01-19-11, 01:11 PM
Have you checked for a leaky FPR? That will certainly lower your fuel mileage.

dkozloski
01-19-11, 02:04 PM
During warmup in the winter the fuel control is in open loop and operates off of mixture tables that are calibrated to deliver a very rich mixture at low temps. After the motor is fully warmed up and coolant temperature is up the fuel control switches to closed loop and everything leans out to normal. Driving short trips in -30F weather I can go through a tank of gas in 150 miles or less.

weister42
01-19-11, 03:56 PM
Have you checked for a leaky FPR? That will certainly lower your fuel mileage.


I replaced my FPR about 4 years ago or so, pretty sure it is still good but I'll check to see if any gas is coming out of the nipple.


I know the fact that gas engines need more fuel to stay running in really cold temperature but I wasn't expecting a drop of 5 or more miles per gallon. Now I know what's like to own an Escalade haha.

Ranger
01-19-11, 11:04 PM
The difference between summer and winter should be no more than about 2 MPG. That has been my experience. 5+ seems too high.

maeng9981
01-20-11, 02:26 AM
I am experiencing about none or ~1 mpg drop. Texas does not get that cold, so that might be a reason. But 5 mpg seems too excessive for me, though.

weister42
01-20-11, 06:34 PM
Just went for lunch a couple of hours ago and I used about a gallon of gas to warm up the car and drive a few miles to a restaurant and back.


It is going to be -15 degrees ambient tomorrow. God damn Wisconsin, you're trying to be the North Pole's little brother.

:bighead:

Submariner409
01-20-11, 08:54 PM
Just went for lunch a couple of hours ago and I used about a gallon of gas to warm up the car
There's your mileage problem - let the car sit and warm up and your gas mileage is exactly zero, and that factors heavily into both actual gallons used and the gas mileage DIC average.

As Cadillac recommends: get in, start the car, adjust your seat belt and drive away normally. Excessive "warmup" times and cold weather idling is death on an engine - the engine warms quicker under moderate driving conditions than sitting at idle, unloaded.

weister42
01-20-11, 09:46 PM
There's your mileage problem - let the car sit and warm up and your gas mileage is exactly zero, and that factors heavily into both actual gallons used and the gas mileage DIC average.

As Cadillac recommends: get in, start the car, adjust your seat belt and drive away normally. Excessive "warmup" times and cold weather idling is death on an engine - the engine warms quicker under moderate driving conditions than sitting at idle, unloaded.


I donno man, my engine doesn't idle too well when it's below 30 degrees and one of my pulleys makes noise when it's cold, getting that looked at this weekend. Remember this is Wisconsin, we're having -30 degrees for a wind chill tonight.

I only warm the car up until the coolant temp says 70 degrees so technically it is still "cold", I really doubt you should drive away like normal when the coolant temp is showing negatives. I had an old Ford Taurus that can start in single-digit weather and drive away like it's brand new but not my caddy, ol' roy is getting older and glitch happens.

dkozloski
01-20-11, 09:52 PM
I just got home from a trip to Sam's club, it was -40F, and that ain't that wind chill BS. The car goes through gas like sh!t through a tin horn. I'm lucky to get 150 miles on a tank of gas.

Ranger
01-20-11, 11:10 PM
Remember this is Wisconsin, we're having -30 degrees for a wind chill tonight.
Wind chills do not affect cars. Your car only feels the ambient temperature.

Sub is absolutely right. Give it 30 seconds, then go.

dkozloski
01-21-11, 03:05 AM
According to a GM Powertrain engineer that was part of the design and developement team, the Northstar was designed so that as soon as the engine started at -40F you could go immediately to full throttle and the engine would run smoothly. He didn't say how many times it would do it but he did say it made a lot of bad noises in protest.

Submariner409
01-21-11, 10:26 AM
I would guess that, particularly in the northern tier of states, there are block heater equipped Northstar cars lying around in junkyards. Installation is not too complicated - they're Calrod elements bolted to the lower sides of the block. Worth a try - look at www.car-part.com . Sure will help your electric bill.....:sneaky:

weister42
01-21-11, 08:43 PM
According to a GM Powertrain engineer that was part of the design and developement team, the Northstar was designed so that as soon as the engine started at -40F you could go immediately to full throttle and the engine would run smoothly. He didn't say how many times it would do it but he did say it made a lot of bad noises in protest.


Yeah GM engineers can do that because if it breaks then oh well, just pull another N* from inventory. If I go and do that and my engine blows up then I'm kind of SOL.

Oh well, I guess I'll just pay up.

rhslaura
01-25-11, 01:22 PM
I have a 99 STS that has just recently dropped 6-7 mpg. The temp gauge no longer reads straight up which it always did. It now reads a bit better than half way to straight up after 15 min of 70 mph highway driving. No codes, no head gasket issues, lots of power, heat is OK but maybe a bit less than normal. I think it may be a thermostat stuck open so the car is always running rich. Any thoughts? Thanks.

ejguillot
01-25-11, 01:53 PM
A stuck open thermostat is certainly possible, that happened to my 2000 STS a few years ago. A new thermostat isn't that expensive, get it changed out. Getting your gas mileage back will pay for the repair in short order.

Submariner409
01-25-11, 04:02 PM
I have a 99 STS that has just recently dropped 6-7 mpg. The temp gauge no longer reads straight up which it always did. It now reads a bit better than half way to straight up after 15 min of 70 mph highway driving. No codes, no head gasket issues, lots of power, heat is OK but maybe a bit less than normal. I think it may be a thermostat stuck open so the car is always running rich. Any thoughts? Thanks.

Take a look at your separate thread "99 STS running cold".

johnny kannapo
01-26-11, 03:41 AM
Sub said it, 5 minutes warming up just sucks fuel at idle pumping heavy oil. Oil filters will bypass at 10F. It's cold! I would not consider the thermostat right off.
You start up cold, get the heater going full blast & it will prolong the warm up period with cold air blowing across the heatercore. The transmission takes a long long time to get heat in the oil & good synthetic ATF will help. It takes 25 miles hwy to warm the trans. That fine Wisconsin E10 isn't helping either, running E10 in perfect weather conditions kills mpg 6.6%.

devistat00r
01-27-11, 06:21 PM
I also live in WI, 2001 Deville DTS is constantly getting about 3 mpg less on average in the winter. I never let the car idle for more then 60 seconds.