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Hi all,
My '98 V90 (153,700 miles and counting) recently experienced a relatively sudden drop in gas milege. I bought it amost three years ago with 74,000 miles, and it has typically gotten about 23-25 mpg in mixed city/hiway driving, and 26-28 on the highway. I use Mobil 1 synthetic oil (since about 100,000 miles), changed the fuel filter at about 100K miles, changed the air filter about every 35,000 miles (not as much dust in the air here in the wet Pacific Northwest), and changed spark plugs at about 100,000 miles. The plugs that came out were a tri-electrode plug, not severely eroded, and the color looked right. At the recommendation of FCP Groton, I put in Bosch copper-core plugs, a standard single-J-electrode plug, and very inexpensive. At the time I thought I could detect a slight reduction in mid-range throttle response with the new plugs, but general cruising and gas mileage didn't change noticably.
In September, my wife drove it to LA and back, and averaged about 25.7 mpg for the whole trip, with individual tanks ranging from about 24.5 to 26.4. That wasn't quite as good as the 27 and 28 mpg readings I have previously recorded, but I figured with almost 150,000 miles on the clock and sustained speeds of 75 to 80 mph, it wasn't bad. In-town mileage stayed at 23-25 when she got back.
We drove to LA again last week, and the week before we left, I saw that the in-town mileage was down to 21 mpg. In preparation for the trip, I had the timing belt, water pump (do it while the timing cover is already off; minimal extra labor), serpentine belt, air cleaner, and spark plugs changed, and changed the oil. My independent mechanic doesn't like Bosch plugs, so we installed an NGK lazer-platinum-iridium plug with a single ground electrode (spendy, about $15 per plug). The car worked well all the way down and back, but mileage for all 6 tanks of freeway driving down and back was 23.2 to 23.5 mpg, regardless of whether 87 or 91 octane was used. The one tank driven around the LA area was 20.0 mpg, the lowest I have ever recorded.
Any suggestions on what might cause this relatively sudden 15% drop in gas mileage? Thanks for any info.
Mike L ('98 V90, 153K; '82 245, 422K)
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