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Old 03-11-2019, 11:46 AM   #1
AstraFasta
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Default Haphazard Gas Mileage

I have noticed that my Astra Automatic has haphazard gas mileage. Sometimes it is a gas guzzler sometimes it sips gas.

This weekend I did 4 x 100 mile highway trips on somewhat hilly roads in cold weather with average about 35MPG. (36.2 MPG in 20F sunny. 35.1 MPG in 40F rainy. Average speed was about 62mph. All stats per on-board computer but look reasonable per gas fills.

One thing I noticed was that good mileage and better performance are when interior cabin heat is very hot. So hot I need to dial down temp selector to the midpoint and put fan on 1, even if evening outside temp is 20F.

When interior cabin heat is not so hot, I SUSPECT mileage and performance drop. But I just don't have definitive proof.

I have look at engine water temps per OBD port several times and they seem to be roughly 180F-190F on the highway so don't vary so much. But maybe they are closer to 190F when the interior cabin heat is very hot. Would just 10F decrease in water temps impact cabin heat so much?

Would this small variance in engine temps cause such a change in MPG and interior cabin heat? Could this indicate failing thermostat?

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Old 03-11-2019, 12:20 PM   #2
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Default Re: Haphazard Gas Mileage

Your current information on cooling system fluid temps is quite normal so the likelihood of a bad thermostat is low if not nil.

The need to reduce hot air temps into the cabin is not unusual. I've had to do the same thing with my car. I cannot leave the heat level at full for very long without perspiring in the car so the temperature has to be reduced. My HVAC control is manual so I don't know what temperature it's being dialed down to but it can be just barely above the mid-point for cold air/hot air mixing to remain comfortable. This simply indicates that the heater is remarkably efficient! Ambient cold air temps where I live don't get as low as yours, but I think that the same thing is true for your heating system as well.

The decreased fuel economy could be related to which vent settings are most regularly being used when the heater is on. Check your owner's handbook and verify whether or not the air conditioning is automatically on when vent selection moves towards the windshield defroster. Our 4 cyl. engines really take a hit on power and fuel consumption any time that the A/C is on.

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Old 03-11-2019, 01:09 PM   #3
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Default Re: Haphazard Gas Mileage

Interesting point about ac running in winter temps. Ensuring ac is off in winter temps should be checked in any vehicle when attempting to improve on fuel mileage. What may not be brought into discussions is the winter blend gasoline has less overall energy composted to summer fuel. Winter fuel requires more additives to help vaporize it in cold temperatures while summer fuel doesn't need it. The additives reduce fuel mileage a few percent or a little less mileage per gallon or km/liter. Most Saturns I'm familiar with use the intake air temperature sensor to monitor ambient temps. The body control module uses outside air temps to determine whether or not to allow ac use. Service manuals state 40F/4.4C as the cutoff to disble ac use. Practically speaking, anyone needing colder than 40F/4.4C air has a personal health problem. Rolling the widows down and/or opening the sun roof should allow plenty of cold air into the interior. Owner's manuals should state if ac use is automatically disabled in winter temps.

Heaters are the second radiator to the main cooling system radiator. Both remove heat with the heater a manual selection to provide personal comfort. Since Saturn heaters don't have shut off valves, hot coolant flows thru heaters all the time. The temperature door modulates the amount of air flow thru the heater core to provide zero to hot air into the cabin. I doubt running the heater on high contributes to better fuel economy. Most engine designs maximize heat at the combustion point. In other words, if compression were allowed to be maximized, free of emissions control, there can be better fuel mileage from higher combustion energy. More energy means more power with less fuel use but emissions are greater. The delicate balance is always a compromise of generating as much power as possible while reducing emissions as much as possible. The two are contrary to each other. The byproducts of combustion are in the form of heat; combustion of fuel and air generates power to turn the engine but the excessive heat generated must be removed. Oil, coolant and exhaust are three ways of eliminating excess heat. All three are necessary for regulating engine longevity. The heater is just an accessory for passenger comfort and doesn't contribute to fuel economy. While difficult to prove, shutting off heater use bypassing it is one way to see if fuel economy improves. The other way is to leave full heat on. No one has proven either method to contributing to better fuel economy.

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Old 03-11-2019, 05:28 PM   #4
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Default Re: Haphazard Gas Mileage

Thanks guys. The car is always set on dash/floor so AC compressor is not triggered.

Indeed the Astra's interior heating core flows coolant all year round (I looked at the plumbing and confirmed with mechanic).

Despite the seemingly tight water temperature range, the interior heat temp on highway trips seems to vary a lot week by week (lukewarm to uber-hot).

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Old 03-11-2019, 06:02 PM   #5
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Default Re: Haphazard Gas Mileage

Quote:
Originally Posted by AstraFasta View Post
...Despite the seemingly tight water temperature range, the interior heat temp on highway trips seems to vary a lot week by week (lukewarm to uber-hot).
This seems abnormal. I can understand the likelihood of delayed increase in heated air based upon very low outside temperatures as that's not unusual. Once the cooling system has reached a normal operating temperature then the heated air available should remain stable. However, if you notice that engine temperature is dropping while on these long trips then, yes, available heat will also be reduced. What were the outdoor temperatures on the days and hours when this swing "(lukewarm to uber-hot)" in the heated vent air would occur?

Lastly, it makes me wonder if there could there be problem with either blend air door (temperature setting), or the recirculation door with either one not maintaining the position which controls are set to keep them at. (Of course, looking at this from the outside all I can do is speculate...)

...
307,000 miles - Holy canolli!
The blessings of liberty erode in my country.
Gov't's grown bigger, but a chance exists that it will be reduced. I'm cautiously hopeful.

Last edited by pierrot; 03-11-2019 at 06:09 PM..

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Old 03-12-2019, 10:32 AM   #6
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Default Re: Haphazard Gas Mileage

Thanks Pierrot.

To clarify my previous posts, the interior cabin temperatures do not change during a particular long trip. They just seem to vary for weeks at a time; from scalding hot to lukewarm.

Interesting thoughts on the manual blend air door and recirculation door. I have inspected them in the past .

- The manual blend air door I checked with replacement of the cabin air filter last year and maybe 2013.

- The powered air recirculation door at the firewall was a nightmare as the plastic frame was misaligned from the factory and let water flow into the blower fan & cabin. I found the problem and realigned the recirculation door housing, adding a lot of silicone. Door worked fine in 2018 both before and after reassembly. We always have fresh air running into the car so that button is not used.

I will check both the manual blend & recirculation door functions this week.

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Old 05-02-2019, 03:10 PM   #7
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Default Re: Haphazard Gas Mileage

Ok so I did a total of 5 similar round trips (10 x 100 mile trips) this winter/spring.

The mileage reported by the on-board computer ranged from about 32 to 36 MPG. Generally the highest MPG was reflective of "hyper-mileing" with lower speeds and very gentle acceleration. Lower MPG related to driving a bit faster (maybe 5 MPH average) and a bit less careful on acceleration, albeit still gentle.

Obviously the on-board computer estimates are not perfect and probably biased high but these results are both consistent and good. I think the Astra just gets much worse gas mileage in the city on short runs.

The cabin heat has been consistently hot for months. Also engine coolant temps are generally mid 180s F. So I think cabin heat was not a thermostat issue but maybe the interior cold/hot air mixer door was not properly closing but is now.

Thanks for all your tips.

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Old 05-20-2019, 01:15 PM   #8
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Default Re: Haphazard Gas Mileage

Did another identical round trip but with a passenger. Warm weather and I suppose winter blend gas is no longer. Also welded small leak at exhaust flex pipe earlier this week so exhaust and 02 sensors should be more efficient.

First leg had some traffic jam, maybe slightly more aggressive driving and AC on the whole trip. 32.1 MPG.

Return leg at night with no traffic, no AC use, cooler ambient temps, and slightly more relaxed driving. 34.2 MPG.

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Old 05-21-2019, 10:28 AM   #9
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Default Re: Haphazard Gas Mileage

Sounds like a good resolution.

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Old 05-21-2019, 12:51 PM   #10
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Default Consistently Good Highway Gas Mileage

I should change the title to something like:

"Consistently Good Highway Gas Mileage"

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