fuel gauge not reading consistently on fill-up

[rlaggren]

Been busy with building and installing a storage system in the "new" van in between dodging the rains, so the Maxima has had to just do the driving w/no attention other than an oil change. But here's a little puzzler:

Happened twice now. Fill it up and the gauge is only up to the "Full" mark where usually it rides a good 1/4" higher. Run the tank down and it goes like grease though the goose. Also, the weird fill-ups yielded _really_ great mpg. So...

It looks like the tank really _isn't_ filling, except that.... The last time (haven't refilled yet) I got suspicious and clicked it to the point where I could see clear fuel (not froth) in the fill pipe. Still couldn't the gauge to top out and my trip odometer shows me at 1/2 tank a lot quicker than usual.

I was parked at the pump with the front of the car _slightly_ down hill. Does this ring any bells with anybody? Seems possible there could be a froth bubble at the end of the tank away from the fill pipe, if the pipe is downhill. From my trip odometer, I'd say it was more than a gallon, maybe close to two.

I read Kassim's thread on his fuel gauge sitting high at all readings. Al suggested a possible VR cause (or a bad connection at the tank); but that would mean my VR is producing one voltage for the duration of a tank of gas, then switching the the other voltage for the next tank...! <g>

Rufus

[kassim503]

My guess is that the float is getting stuck from riding that extra 1/4 up sometimes, depending on the position the car is parked. Try hitting the trunk pan when she dosent go up and the bottom of the fuel tank, mabye itll dislodge it. It sounds like a sticking fuel float

[asavage]

I have a completely different explanation . . . but you have to read the Rabbit story to get the punchline.

I, too, got very odd MPG calculations -- until I figured it out.

[rlaggren]

Kassim, I don't think it's (only) a gauge issue because I _saw_ the fuel high up in the fill pipe which should mean a full tank. The gauge said only full (where it usually says "full +1/4inch", but what really puzzles me is the fuel mileage appears to vary greatly _and_ the gauge drops real fast meaning that even though I saw the fuel in the fill pipe, I'm down to 1/2 tank way before I should be (according to the odometer). IOW, the gauge could be flakey and that still wouldn't explain why I get great mileage one tank and mediocre the next - I drive quite consistently since the price went over $3 so I don't think that's the cause.

I'll start checking under the rear, see if the tank looks puckered, and I'll find the vent. I guess if it's squashing itself it could happen on the top just as easy so it looks like an interesting problem. I"m not sure the fuel pipe cap is actually air tight though, and w/out good vacuum in the tank _that_ bit of evil won't fly. If I can get ahold of the vent someplace convenient I might try putting a little (say 3-5 psi) pressure in it w/a bicycle pump and see where it goes - see if I hear a big PLONG from the back end. <G> Think that would upset the fuel system?

Rufus.

[kassim503]

Al has a interesting conclusion, I never would of thought of a plugged vent, I would definitley give it a shot, it could be your problem. I know you can pressurize the gas tank with no damage on any gasoline model, cause of the pressure that is there normally. You could drive with the gas cap off and not make sharp left turns, diesel isnt volatile like gasoline is.

[asavage]

I wouldn't say it's a conclusion, but it does fit the symptoms

[kassim503]

Well it fits the description better than a stuck float, they are pretty hard to jam

[rlaggren]

Well, according to the FSM the vent connects to the tank about the same place (front to rear) as the filler pipe which itself looks like it enters the tank toward the front. I didn't find the right moment to squish under the dragging rear and see things in real time. If I understand right from the book, that means the vent won't prevent a bubble at the rear of the tank, when the front wheels are slightly lower than the rear, even if it's free and clear.

Ergo, fuel the Maxima level or heading up hill if you want to fill the tank! If it tops off right with the car level I'll call it done - at least until I'm under there anyway for the body work.

Rufus