Can a stuck open thermostat make gas milage go down?

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Can a faulty or stuck open thermostat cause a car to get bad gas milage in the city, but great (normal) milage on the freeway?

My brother's civic gets bad mgp in the city; about 9 gallons to every 140miles, but average, about 26mpg-32mpg on the freeway. This is on a 1994 Honda Civic EX (sedan) automatic, approx 97,000 miles. I have used redline si-1 fuel system cleaner within the past 10,000 miles, and he usually uses chevron gas, which is pretty good.

I have reason to believe that the fuel pressure regulator may be going bad as well, which can cause bad milage or the car to run rich; there seems to be quite a bit of black 'carbon' soot on the tailpipe. Or this could be from a faulty 02 sensor.

I will be changing out the thermostat tonight, the oil, and the tranny fluid as well.

Thanks for any input.

EDIT: I just thougth of something, could a bad PCV valve make the car get bad gas milage as well? The honda one costs $22 from the dealer, and I am having trouble finding it at parts stores.
 
An open thermostat would cause greater fuel efficiency loss on the highway because the engine doesn't need to generate much horsepower to keep the car running at that velocity, plus the radiator is seeing maximum airflow. With those factors, an open thermostat would cool an engine too much, causing the water temp needle to nose dive to the bottom of the gauge, resulting in bad fuel efficiency.

Speaking of gauges, is the car's temp gauge moving at all or is it broken? That would be a starting point.

I once had a thermostat stuck open. It would read "middle" when I was idling (red light, traffic, etc.) Once I started moving, it would slowly creep down to the cold zone. On the highway, it would go in cold zone.
 
Temprature gauge sender works. I have tested it with a multimeter before. There are three 'switches' in the cooling system on (most) civics, the temp gauge sender, the fan thermo-switch, and the coolant temperature switch. Temp sender is that dash gauge, fan switch is for the radiator fan, and the coolant temp switch is for the ECU i believe.
 
The car will get bad gas MPG.

I bought a Purolator PCV valve for my wife's 98 Civic EX from Discount Auto Parts for $6.00, but they had to order it and I picked it up 3 days later.
 
quote:

Originally posted by ChrisW:
Can a faulty or stuck open thermostat cause a car to get bad gas milage in the city, but great (normal) milage on the freeway?

My '84 Chevy does the exact thing when a thermostat is stuck open.
 
It could be your o2 is cooling down in city traffic and the car is going into open loop operation. I'd stick a scantool on there if at all possible and look at all the values.

I'd check all the vacuum lines, particularly to the MAP sensor if it has one, and generally go over the car from stem to stern.
 
When I had a stuck open thermostat in my accord, it would get cold on the highway but warm up in the city.

It's funny - you exit the highway, shift to neutral and the engine idles high and starts to warm up.
 
Culprit found (at least I hope so), stuck open thermo. The rubber seal thing around in middle of the thermo was badly deteriorated causing the thermo to stick open. I will try and take a pic later.

I flushed the system 6 times using distilled water (I was unable to reach the engine block drain easily with the tools I had [too close to exhaust manifold], which I am sure would have made the job faster), and refilled with Zerex G 05, which I believe to be a better coolant to honda coolant in both terms of price per gallon, and longevity. Funny, I bought two gallons of the zerex, and total cooling system capacity is 1.16 gallons, or 4.7L.

Car warms up much faster now.
 
quote:

Originally posted by ChrisW:
Culprit found (at least I hope so), stuck open thermo. The rubber seal thing around in middle of the thermo was badly deteriorated causing the thermo to stick open. I will try and take a pic later.

I flushed the system 6 times using distilled water (I was unable to reach the engine block drain easily with the tools I had [too close to exhaust manifold], which I am sure would have made the job faster), and refilled with Zerex G 05, which I believe to be a better coolant to honda coolant in both terms of price per gallon, and longevity. Funny, I bought two gallons of the zerex, and total cooling system capacity is 1.16 gallons, or 4.7L.

Car warms up much faster now.


uhhmn, you mixed coolant 50/50 with water, right?
 
When I had my 88 Dodge Shadow Turbo, it used to have a problem with thermostats sticking open. I seemed to have to replace one every year. I always saw my gas mileage drop pretty significantly when it stuck open, as it would drop by about 3 or 4 MPG!
 
quote:

Originally posted by andyd:
uhhmn, you mixed coolant 50/50 with water, right?

Of course.
grin.gif
I just didn't think that the system capacity was that small on a honda civic until I looked it up in the factory Helms manual. Nearest place that sells zerex G 05 is a town over; pepboys, so I didn't want to have to make another trip if i guessed too little. Total capacity is 4.7L, so I poured in about 2.3-2.5L of coolant and topped with water; my guess its at least 50% but no more than 70%.
 
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