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Old 03-24-2013, 20:31   #1 (permalink)
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Sudden Poor MPG?? Navigator

I've got a 1999 Navigator that, as it usually goes with the Navigator, does not get the best mileage. I usually can get around 12-13.5 mpg (tested, not just on the trip computer). But just recently, I tested my mileage again and found I'm only getting 9 mpg!! To me, that seems like a considerable drop to just all of a sudden happen like that. My trip computer is reading 13.2, but tested at 9.

Does anyone have any ideas as to what might be dragging me down so much? I don't have any extra or lots of weight in the car and am about 70/30 city/hwy. It has not had the plugs changed out recently and is probably getting close to due, but any advise on how to improve this mileage would be greatly appreciated. 9mpg with a 28 gallon tank at $4/gal is pretty brutal!!
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Old 03-24-2013, 20:36   #2 (permalink)
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Re: Sudden Poor MPG?? Navigator

Spark plug changes do help the mileage. Are you checking the miles driven and fuel put in, or are you using the message center?
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Old 03-24-2013, 21:00   #3 (permalink)
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Re: Sudden Poor MPG?? Navigator

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Spark plug changes do help the mileage. Are you checking the miles driven and fuel put in, or are you using the message center?
I'm checking the miles driven/fuel put in. The message center is still reading 13.2 mpg, but my miles/fuel is coming out to 9.
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Old 03-25-2013, 01:23   #4 (permalink)
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Re: Sudden Poor MPG?? Navigator

I would calculate the mileage over maybe three fill-ups. It sounds like you just figured the 9 MPG based on one fill.
I suspect that if your calculations are correct, then differences in such things as angle of pavement where filled, temperature of fuel remaining in your tank, temperature of fuel entering tank, calibration of pump, etc. may be involved.
To get a reading that is even close to accurate with one fill you would have to fill at the same pump headed in the same direction with the temperatures the same and assume that the stations tank was not recently filled with fuel of a different temperature.
Better to just calculate over a longer period.
BTW I have found that my digital mileage meter is pretty accurate, so much so that I don't even bother checking it any, more.
Then if you find 9 MPG over a longer period you definitely need to look into a possible missing cylinder etc.- Brian
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Old 03-25-2013, 12:29   #5 (permalink)
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Re: Sudden Poor MPG?? Navigator

the only way to check the mileage properly is to do the bottle test both highway and city.
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Old 03-26-2013, 14:01   #6 (permalink)
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Re: Sudden Poor MPG?? Navigator

On boards are a good reference, but not gospel. As Noleaks stated, calculate over a few fillups. Check tire pressure, plugs and wires, as well as bearing in mind the winter mix for the fuel. BTW, are you running regular or E-85?
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Old 03-26-2013, 14:06   #7 (permalink)
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Re: Sudden Poor MPG?? Navigator

Checked the air filter ? is the car reaching working temp o.k and not running cold ? If so thermostat could be stuck open which wont help mpg .
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Old 03-27-2013, 06:22   #8 (permalink)
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Re: Sudden Poor MPG?? Navigator

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Checked the air filter ? is the car reaching working temp o.k and not running cold ? If so thermostat could be stuck open which wont help mpg .
FORGOT TO MENTION: this is not the only time I have registered under 10mpg on this car. Other recent fill ups (the previous two) were 9.6 and 10mpg. These fill ups are a 50/50 city/hwy.

I have not checked the air filter because its not that old, but I will. As for the working temp, it doesn't seem to be running cold. After running a while, water temp gauge shows the line under the halfway mark and it does that a little while to get it warmed in the morning (interior heat especially takes a while to heat). I will try to check the thermostat later.

To catskinner, I am running regular fuel.
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Old 03-28-2013, 17:44   #9 (permalink)
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Re: Sudden Poor MPG?? Navigator

A cat that's beginning to foul will drag down mileage, but you'd also notice it choking on acceleration, particularly hard acceleration.
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Old 04-01-2013, 20:58   #10 (permalink)
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Re: Sudden Poor MPG?? Navigator

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A cat that's beginning to foul will drag down mileage, but you'd also notice it choking on acceleration, particularly hard acceleration.
I actually have been having issues with acceleration and have another posting on this issue (link listed below). While in 3rd gear on slight incline and low rpm's, I'm getting a hard thumping-like sound/feeling that is very noticeable. I've been thinking it had something to do with the transmission, but now am wondering if it could be a misfiring. Aside from this, I do not have any other acceleration problems or choking.

Take a look at my other post on this issue for details on what I' describing. If you have any other suggestions on this as well, I'd greatly appreciate it, as it is pretty emparasing when other passengers are in the car. Here's the link:

Navigator Transmission Thumping?
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