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Nissan Altima sudden fuel consumption increase?

My 1998 Altima suddenly uses around 30% more fuel than before. What could be the case, and how much would it cost to fix this?

First of all, how are you checking your mileage? Check engine light on? Driving habits or where you go changed.

In addition to answering the questions already posed, I think that the OP should disclose some information regarding the car’s maintenance.

When were the spark plugs and air filter last changed?
When was the last time that the air pressure in the tires was checked?

Right, don’t trust the OP to know if it is sudden or know how to check mileage. These might be issues, but there is a tendency around here to assume total ignorance.

Oxygen Sensors… Thats about it…


…or a thermostat that stays open, or a defective temp sensor…

Short of a leak I don’t see how a legitimate 30% loss of fuel mileage is not having an impact; either with the CEL being on, smoking, bucking, etc.

There’s a lot of empty blanks to fill in.

I’ll second @VDCdriver 's guess, the injectors are getting double pulsed b/c the engine coolant isn’t getting up to normal temperature, or the actual coolant temperature is ok but the engine computer’s temp sensor isn’t working.

Stuck brake.

Your best bet is to take it to a knowledgeable mechanic. As others have noted, it’s likely an O2 sensor or a fault thermostat or sensor. But that’s tough to diagnose from here.

Is there an engine (CEL) light on?
Is the vehicle driving any differently?
Did something change recently (tires, commute, etc)?

Any additional info you can provide us will help make it easier for us to help you.


Also check for dragging brakes.
Thermostat is overdue for a change IMHO, and not expensive,so that’d be one of the things I’d do first.

thanks everybody! Nothing has changed in the car’s routine - I don’t drive it much and do not commute. Nothing is leaking, at least as far as I can see on the garage floor. I use the odometer to see how much gas I use. A full tank used to take me about 350 miles, but now it is down to around 250 miles - mostly short drives around the city, or up to 30 miles highway driving. No interstate/cross country etc.

The needle starts moving after I drove 10 miles on a full tank - never happened before.

Take the number of miles you have driven after a fill up (don’t top off tank just stop when the pump shuts off) divide by the amount of fuel = Miles per gallon. Do that for at least 3 times and I think you will find you really don’t have a problem. Short trips and city streets will not get the best mileage.

In addition, the OP should bear in mind that the gas gauge is not a laboratory-grade measuring device, and the exact moment when the needle starts moving is not an accurate indication…of anything.