Low mileage with my '97 Civic EX

Guys
Purchased a used Automatic '97 Honda Civic EX 4 months ago with just 56K miles on it. I drive about 1500 miles/month - 90% highway miles (60-75 mph). I do oil changes promptly at 3k miles. Last week my car
hit 60K miles and I changed the Spark Plugs (Firestone replaced with Bosch Platinum plugs). they even said that the wires, rotor and distribution cap are good. Tires and Brakes are good. Before I changed the Spark plugs I used to get 28mpg and now i am getting only 22mpg. I was expecting it to hit atleast 30mpg. I am very concerned now. Can anyone help me understand what the problem is?
One of my friend suggested that I can change the Air filter - Planning to do that soon. Should I replace the transmission fluid, fuel filter or anything else? I use 87octane either at Wawa or Shell.
Thanks in advance for your time SK
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SK wrote:

Changing the air filter might help a bit. But I think the biggest problem is the Bosch spark plugs. Get rid of those and get OEM plugs by NGK or ND.
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Thanks for your reply, High Tech Misfit!!
One other question I would like to ask the Internet Experts - My turn lights do not cancel automatically.. What needs to be checked/replaced?
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turn switch mechanism has a broken piece - the switch assembly has to be replaced. I don't know how hard it is to get the switch loose enough to inspect the inner edge, but it may be about as much work as the entire replacement depending how the connector comes out of the steering column.
Mike
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SK wrote:

-----------------------------------
Use honda recommended plugs, and only honda Z1 tranny fluid when you drain-n-fill it. No 'power flushing'. Air filter is a two minute DIY job. NAPA is good. NAPA can give you the correct plugs too. Don't idle to warm up your engine . . it gives Zero MPG.
'Curly'
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As the weather cools cars get lower mileage for a number of reasons. These will vary somewhat from driver to driver and one car model to another.
Things over which you have no control which will reduce fuel mileage (in no particular order): -- Engine computer set to a richer mixer (more fuel) in winter. -- Use of a different fuel (per federal law and local custom) with a lower heating value in winter. -- Denser air in winter, due to lower temperatures, and so increasing wind resistance, the #1 robber of fuel efficiency at highway speeds, summer and winter, etc. -- Oil viscosity is higher at lower temperatures.
Things you can do that cost nothing and may help improve fuel mileage: -- Check tire pressure every two weeks and particularly during the change of seasons. Also, higher pressures may result in slightly better mileage. My 91 Civic's tire pressure is supposed to be 26 psi, but for the past few years, I have maintained it at 28 psi, for better fuel efficiency at the expense of a more comfortable ride, which I don't really notice anyway. It's a Civic, after all.
-- Check the PCV valve. Wrap the hose connecting to it with cloth, squeeze there with pliers to cut off flow. Listen for a click, which should happen within 30 seconds. No click, then spend the bucks and replace the valve. Click, and the valve may or may not be operating optimally. Consider cleaning the PCV valve with WD-40 or PB Blaster. The engine control system relies on a particular metered flow through this valve for different operating conditions. If something's clogging it or the spring is old, then it won't work correctly, blah blah.
-- Check the level of the coolant in the reservoir, per the owner's manual. Consider doing an air purge of the cooling system. Certain engine control components are cooled by this coolant. Wrong levels mean improper cooling mean sub-optimal operation. Strongly consider a complete coolant drain, flush, and refill, using only OEM coolant or Havoline Orange Dexcool.
Things you can do that are not expensive and are probably worth the money for a car you bought used and don't know well: -- Replace fuel filter and air filter -- Replace distributor cap, rotor, ignition wires, and plugs with OEM ones. (I don't know if the Bosch ones are the problem, but they sure do seem suspicious, given your description.) I wouldn't bother with platinums unless the owner's manual said to. -- Wait until the gas tank is near empty and add a bottle of the fuel injector cleaner "Chevron Techron." Fill tank.
For a car that does mostly highway miles with no other adverse conditions, I would consider far less frequent oil changes. Like no more than every 5k miles It's the starts and stops that are hard on an engine, generally speaking.
Some resources for your new used Honda are listed at:
http://home.earthlink.net/~honda.lioness/id9.html
At a minimum, consider getting an owner's manual, per the directions at this site, and at least study the maintenance schedule in it.
One other thing to consider, but not unless you have other symptoms along with lousy mileage: New O2 sensor. It would surprise me if a car this young needed one, but it's something to consider if this problem persists into the warm seasons.
I don't know about the transmission fluid's effects on mileage, but if I had the spare money, I think I'd start fresh with my new used car and replace it. Good investment, since the transmission is one of the most expensive items on the car that will mess up if not properly lubricated.

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Good point. I don't recall whether the thread was here or in another group, but at least one poster reported improved throttle response after replacing an O2 sensor. It is easy to assume the "check engine" light will tell us if the O2 sensor is bad, but they apparently can get pretty "soft" before the ECU complains.
Mike
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Yes, so a number of consumer and other car sites report. Last year it was enough to persuade me to replace my 150k, 13-year-old O2 sensor pre-emptively. (That it was only about $45 for an OEM one helped, too.)
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wrote

Mike
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Yes, my 1991 Honda Civic LX, 1.5 Liter. Bought it at www.automedicsupply.com . Its prices go up quite steadily for more recent years, though (and conversely, somewhat down for older years). This site's prices appear to be a lot less than Majestic's and slhonda's OEM sensor prices.
While I realize one can still drive a car with a bad O2 sensor, the car will run poorly. I didn't want to be stranded somewhere in that situation. Not when the fix was $45 for a part that would probably last the rest of the car's life. I kept the old one to pop in and milk out a few more thousand miles etc. as needed.
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replaced just once sometime, why not do it when convenient and get more years of benefit?
Mike
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Thankq guys for your valuable time and responses. Need to take it to the mechanic sometime this weekend.
SK
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