Bad Fuel Mileage?

[EloManga]

Well, my parents have a 99 Altima (2.4L 4spd atx) and recently I've noticed the mileage go down hill.
Last year, I drove down to Florida, going 120Km/hr and got like 175Km's per 15L's using Chevron, 155 on Sunoco. Also, the oil that was in the engine was Esso XD-3 with a Napa silver line filter.
Now, I've got Mobil 7500 clean (and I'm a bit over full), with a Napa nascar filter, the fuel filter is brand new, tires are set, air filter's clean and driving it to work, where top speed is 100Km/h, but not there for very long, the mileage is at best 130ishKm's/15L. I think on the last tank, I calculated 8.4L's/100Km's.
So, does anyone have a clue as to what may be causing the poor mileage?

I don't drive it hard, at all. It's only got 70K Km's on it. My only guess is the oil, but not sure.

 

[Matt_S]

Oil? No. But a marginal O2 sensor will throw mileage off. It sounds like you've got the rest covered. It might be worth cleaning the intake, too, and using a good fuel system cleaner.

 

[EloManga]

Ok, I forgot to mention, I changed the plugs less than 20K ago. But I am going to check them this weekend and clean and re-gap.
The O2 sounds the most likely. Can that be checked with a scan tool?
Also, I tried cleaning the intake before with tb cleaner, but it threw a code. Appearantly you can't do that on these engines with obd2.

 

[Quest]

tuneups, tuneups, tuneups.

Also get a computer ECU code check to see if anything is at fault.

Check your tailpipe (or have someone drives the car for you and you'll tailgate it) for smokes. If excessive black, powdery smokes should ever happen during acceleration or coasting, something definitly not right while the engine is in closed loop operation. Check O2 sensor, thermostat, thermo sensor, etc.

 

[EloManga]

I told my pops to go get the car scanned for a code and buddy the mechanic says we'd see the engine light if there was one. Do you guys agree with that?
Also, buddy said that we need to use japanese parts on it, or it won't run good. We've got Bosch Platinum plugs in there now, you think this could be the culprit?

 

[ConfederateTyrant]

To my understanding it is true that there would be a check engine light if you had a faulty O2 sensor. As for the plugs, did your gas mileage make that instant of a drop right after replacing the plugs? I have heard of some people having problems with Bosch plugs (particularly the Platinum +2 and Platinum +4 ones) in longevity and performance. I know a lot of people are fond of NGK and Denso in Japanese import cars.

 

[EloManga]

No, it didn't drop right away. I drove it to Fla last year and got great mileage, it just started happening about a month ago. They're the regular single tip platinum's also.

 

[ConfederateTyrant]

If the plugs are easy to check, I'd recommend checking them. I'm not trying to badmouth Mobil, but I recall reading that some users had a drop in fuel economy while using Mobil 7500.

 

[Vilan]

I've heard that O2 sensors can get "lazy" and not work very well without tripping an error code. If the sensor for their car isn't too expensive, you might try changing it/them. But no guarantees.

I'm surprised Buddy told your dad has car was OK. Most mechanics would've grabbed the car and billed him for O2 sensor replacement, fuel injector flush and a new Johnson rod.

 

[Quest]

O2 sensor does get lazy or intermittently defective (if it is due to factory workmanship).

For lazy O2 sensor, replacing it is the best way to deal with it.

also, use good quality gas.Certain brands/blends of gasoline with excessive maganese inside (Ethyl made and sold them as octane booster, I think it's called MMT(?)) will definitely ruin your day, if not your O2 sensors and/or your cats.

Here certain parts of Canada are now selling gasoline that is maganese-free, at least Chevron does.

 

[slalom44]

Are you comparing the fuel economy you got on a road trip to daily driving fuel economy? If so, there's your problem. You can't compare fuel economy with long distance highway miles to city/highway driving.

I have gotten into the habit of calculating my highway mileage for every tank of gas I buy. On the receipt, I write down the odometer reading, subtract the mileage from the previous recipt, and divide by the gallons of gas I buy. In time, you get a very good idea of the effects of temperature, driving habits, the use of A/C or windows down, etc. If you or your parents had done this prior to your trip, you'd know exactly how much your mileage had changed. Unfortunately, you have nothing to compare your current fuel economy to except an isolated trip.

 

[EloManga]

My comparison is actually between my 2.5L V6 10:1 compression Mazda KL engine, to this 2.4L Nissan engine. I'm getting better mileage in the Probe than the Nissan, and mine's a pig!!!! That's my comparison. I'm driving it to work and back, and I'm getting under 9L/100Km's, while the Nissan is getting over 9L's. This is pretty bad by my standards. My driving is not aggressive either. I barely hit the limit, since I'm in traffic, and actually trying to conserve fuel. The Altima actually has a 4spd o/d atx, and rpm's @ 100Km/hr are ~2000. While my Probe in 5th at 100 is 3100rpm. So, I'm gonna bet the farm something is definitely wrong.
On the weekend, I cleaned the plugs, not too bad, but the idle is still smooth, I don't smell anything bad out of the exhaust, and the gutless power it came with, hasn't gone away.
I guess I could try to test the o2's on a bench, but actually my dad said that atleast 1 of them was replaced. They run about $100 each, so changing them for the fun of it isn't a cheap fix.
1 thing that came to mind was that I now recall that the metal wire wrap around the flex pipe had been burning off and I believe it's all gone, you think that could affect mileage? I don't hear an exhaust leak though.