Well I don't know how to explain this, but here it goes. Normally, I use the Fuel Used counter to determine when exactly to put more gas in the truck, but lately it has been very wrong. Normally, the gas pump would first stop a gallon before the number on the gauge and then I would top it up just past the number. The fuel gauge would accurately reflect that. But now the same gas pumps are first stopping PAST the number on the fuel used indicator and then I would go a gallon or gallon and a half beyond it before I would stop. When the fuel used gauge should say that I used half a tank (8-9 gallons), the fuel gauge is saying 1/4 a tank and when the car is on a hill, sometimes the gauge would hit close to the "E".

I normally refuel at half a tank to prevent fuel pump problems. I am not noticing any leaks anywhere, no gas odor, no rough running and no check engine lights, but where is the gas going? It seems that I am losing a gallon or more per week and it is going everywhere but the engine or on the ground. I have a cheap locking gas cap from Walmart. Could the gas be vaporizing out of the tank? It seems that sometimes the lock sticks and then it doesn't tighten properly until I tighten it like 4 times and then it is fine.

 

Sounds like the sending unit in the gas tank is shot. It has a little lever on it that sends info to the gauge to show how much gas you have.when they go bad the gauges bounce around,show more or less then what you really have. Could also be the gauge too. usually its the sending unit.

 

Sounds like a great theory and one that I thought of first, however, the gauge is correct. I had the pump and sender replaced last year and the gauge seems correct for the gas used since the tank is taking more and more gas but in relation to the gauge. It always goes lower when the car is on a downhill angle, but since the gas is lower than the fuel used gauge suggests is remaining, the gauge goes lower.

The fact that the tank is taking more gas and the fuel gauge is accurately reflecting the gas removed from the tank (wherever it goes), but the fuel used gauge is not means that all the gauges are more or less correct (gas remaining in the tank is accurately shown, gas used by the engine seems accurately shown) and the gas is going somewhere else but the cylinders since the truck's engine is using the same amount of gas as normal, but more gas is leaving the tank than is actually being consumed by the engine and there are no visible leaks.

Could my Walmart locking gas cap be the problem? Yes, I am blaming it just because it is from Walmart but since gas can evaporate it is the best I can think of. The key design on the Walmart version is totally different and looks cheaper (cheaper lock design and keys). But 1-1.5 gallons per week missing? That is almost $6 per week disappearing! The only other theory I can think of is that someone is somehow getting my gas cap open without breaking it and siphoning gas little by little in the college parking lot.

 

Could be colder weather, cold air = denser fuel/air ratio but a gallon a week seems extreme.
Have you cleaned the MAF, air filter? Does it have the same response & accelleration as before?

 

This fuel missing was the same as it was a couple weeks ago when it was cold, but it has been in the 50s and 60s in upstate NY lately. Unseasonably warm.

This truck has a CAI and I replaced the filter about 10,000 miles ago. It isn't that dirty and it has an open bottom (yes it is part of the filter, but that means more filter area, so it won't get stopped as easily.).

Acceleration seems okay. I was noticing a delayed 3rd gear (on another tread. Not really slipping, but happening at a higher RPM), but that is probably because the trans is getting overcooled (only 100-105F at the cooler with the IR thermometer. I can put my hand on it and it just feels warm.) by my trans cooler with these cooler temperatures lately and slow driving. Otherwise it accelerates just fine, but since I go to college in a small town with windy roads, I can't necessarily put that to the test. Plus I am honestly scared I will damage something if I do something stupid like go full throttle. I barely use 1/4-1/2 throttle all the time.

 

Whether the engine is using more (or less) fuel or if the fuel is disappearing into the ether, it will show by comparing manually computed mpg readings. You have done this at a previous time, right? If not, do it now and then then again after you have filled up a couple of times. Compare the two figures and see if there is actually a difference or not. Gauges and computers can become inaccurate.

 

The manual MPG readings and checking gas receipts for the same amount of miles per tank confirm what the gas gauge is saying, but the truck used significantly more gas than what the fuel used indicator said. I also reset my trip odometer when I get gas as a backup to the fuel used and gas gauge if those fail. I didn't just start this thread based on one tank. This has been the case for the past several weeks of noticing issues like this with the truck taking more and more gas per tank even when I nearly overfill the tank to the brim by continuing for more than a gallon after it first stops to try to match the fuel used indicator or just surpass it, but lately, it would be taking more than the fuel used indicator even before it stops the first time and I would continue and it would not overfill, confirming that there is gas missing somewhere even when the engine used the same amount of gas as usual. It is this latest tank that had the most gas missing from the tank that did not go to the engine that made me recheck the rest of the data and figure that there was gas missing, possibly evaporating. What makes it more confusing is that this is happening regardless of temperature or weather and it didn't do it last winter.

 

Don't get wrapped up in those "gauges". They are calculated values and NOT accurate to begin with so it doesn't surprise me that they are "off". It is a reference point, that is all. Nothing to see/do here.

 

Oh ok. Thanks. Sorry for the wild goose chase then.

I am going to replace the gas cap anyway since the lock is not working correctly now. The last locking cap I had broke thanks to the gas pump attendants in NJ in May. Too bad I can't get a locking gas door and a normal cap because it would be less trouble.

Lesson Learned: Never buy any car parts from Walmart.

 

I think the new cap might be solving the problem. It fits 100x more snug and the fuel wasn't missing at the last fillup. The gauge seemed more accurate and the fuel I added was very close to the fuel used counter. It stopped initially over a gallon and a half before the number and then I topped it off just past the number. I will wait until next fillup to be certain. The other cap came off very easily, so I think the gasket was bad as is.