TFitzPat87

Hi guys,

My gas mileage seems to be fluctuating. I was originally getting REALLY good gas mileage, and then all of the sudden it seemed like it was sucking it down. 30 mile drive to work would use a little less than 8th of a tank. I noticed my CCV was real loose, so I replaced that and the gasket, and now my mileage seems to vary.

Sometimes the 30 mile drive to work will only take about a 16th of a tank, and other times it will take just less than an 8th of a tank. Same drive, same distance, all highway.

Any idea's? I plan on taking out the TB and give that a good cleaning along with the IAC, I hope that helps, but I don't understand why it seems to fluctuate. Like I said, I was originally getting REALLY good mpg. Not sure what it was, as I didn't calculate, but it had to be like 20 mpg.

Mine is a 97 Sport, AW4, 67k miles, and I don't have a CEL. Did cap, rotor, and plugs before winter. Oil changes regularly.

Larry's XJ

Might be the "super accurate" gas gauge.

SG Mason

I had the same problem with my last tank of gas, actual mileage before was 18.7 mpg combined driving. The last tank it dropped down to 11.4 mpg. I was ready to start replacing injector o-rings, cleaning the IAC, and checking for cracks in the exhaust manifold. I filled up at a different station from my normal one on that last tank, filled up at my regular station a couple days ago and the fuel mileage is back to where it was, and I am getting 18.5 mpg again. I think my problem was just bad gas, but I am still going to clean the IAC and replace the o-rings once I get a nice enough day since i already bought everything I need to do the work.

TFitzPat87

Thats possible, but I filled up at my normal station before I noticed this. I did however change it up last time I filled up to see if that changed anything. I'll go ahead and go back to my normal one next time I fill up and see what happens.

I did get the TB and IAC cleaned, wasn't too dirty, but there was a layer of crud to clean off. Does seem to idle nicer now, wont know if that helps mpg since I haven't driven much since then.

Will O2 sensors throw a CEL if they are bad, or is it one of those things where they won't unless they are completely crapped out?

SG Mason

Normally they will, but sometimes they wont. I was chasing an injector CEL on my Stealth for a few weeks. Tested and changed the injectors, bought noid lights checked the wiring harness and everything checked out good. Finally I broke down and bought a data logger and I found the problem idling in the driveway. My O2 sensor was flat lined right in the middle of the range with no deviation. I replaced the O2 sensor and and the CEL for the injectors went away. Because my PCM was getting a signal from the fault O2 sensor it would change the fuel trims. When nothing changed on the O2 sensor signal the PCM logically decided that the injectors were bad

The Stealth is a 92 and OBD1 so that could be part of the misguiding CEL though.

tjwalker

Don't try to extrapolate gas mileage by your gas gauge. You can be easily mislead.

Do the math. Miles driven divided by gallons consumed. It's the only truly accurate way of determining gas mileage.

Get a number of samples (at least 3) before even attempting to come to any conclusion.

TFitzPat87

How am I supposed to know how many gallons I consume? I'm not trying to be ignorant, I honestly don't know.

tjwalker

Get a notebook. Note what mileage you are at when you fill gas or reset your trip odometer. When you fill gas the next time, check your trip odometer (or your notebook) and you'll know your "miles driven".

Gallons consumed can be taken from the gas station fueling pump.

Then it's just simple division. This way, you'll really know your gas mileage....

TFitzPat87

Got ya. I'll start keeping a log. After I posted I realized the pump would tell me how many gallons I used.

Thinking is difficult when you're hung over.

SG Mason

Be thankful you are not Canadian talking bout fuel consumption on a mainly American forum. First off we have to deal with liters per 100 km, then we can either convert that to imperial mpg or US mpg The imperial gallon is roughly 17% larger yet the mile are the same.

extrashaky

You're talking about the needle moving an extra 1/16th of a tank? These gauges are nowhere near sensitive enough for that to tell you anything.

Or, if you have a smart phone, you can get an app that does the math for you and keeps track of it on a single handy screen. I use an app for Android that is appropriately named "Mileage." It's free. I just have to remember at each fueling stop to enter the odometer readout and the number of gallons I pumped, and the app does the rest. If my fuel economy drops, it shows the number in red. If it goes up, it displays green.

It may do some weird things the first couple of times you enter your fill ups, but once it evens out it's pretty handy.

Yes, if an O2 sensor is bad, it should throw a code.

Larry's XJ

Also what I have seen is to use same pump each time and "fill" to first click-off of pump, thus filling to same level each time.

93_xjcherokee

My gas gauge is 100% accurate. Been driving for 50 miles on e. Lol

Fred/N0AZZ

I never let mine get below an 1/8 of a tank and most generally a 1/4. Even when on the trails I never let it get down to an 1/8 and out come the gas cans.


Do I trust the gauge no way not it or any other fuel gauge some say you have fuel when you run out, others show 3/4 tank when the tank is over flowing. Most of all never try to figure fuel mileage using it alone and no other considerations.

jeffcycles

How much influence will a cracked exhaust manifold have over mpg?