Malfunctioning fuel gauge
 

I'm in the second month of owning a 1989 Corvette. When I first got the car the fuel gauge read less than 1/4 tank and was working normally. I would add fuel 5 gals. at a time and again everything was working well. Last week I refueled to 3/4 full which registered on the gauge. I drove it and the fuel level showed the normal usage. Now after driving approx. 45 miles the gauge has not moved and the distance to empty is stuck on the same number range. After watching Motor Week Pat Goss did a segment on fuel additives. BG 44K was suggested when the sending unit is stuck. I would like to know it this stuff really works? Any other suggestion? If needed, is replacing a fuel tank sending unit a DIY job?

A fuel gauge doesn't often react incrrementally with any precision. You've driven 45 miles and you're expecting what type of deviation in the gauge from the 3/4 mark? You've never filled it? I don't know that you have issues or NOT but it would seem you don't either with just 5 gal purchases at random intervals! BG44K isn't going to hurt anything but the money maybe could be used for something more useful.

I don't know how to assume it's functioning accurately without a "fill up"! Fill it, set the tripometer, drive 100 miles, fill it, check mileage and fuel gauge level. Fill it, set tripometer, drive 150 miles, fill it, check mileage and fuel level gauge. You'll need to establish one (that it's full), two (some sort of expected mileage per gallon) and then evaluate the gauge with a more accurate set of "knowns".

Make a habit of fill ups and setting the tripometer, buy gas a regular calculate mileage intervals. You need to keep substantially more than 5 gals of fuel in the tank for sure. The fuel is the "coolant" for the fuel pump. If you want to buy lesser quanities of fuel at a time, determine the miles per gallon expected buy in larger than 5 gallon increments I believe and use the tripometer and not the gauge.

Advertised capacity is 20 gallons but it seems something nearer 18+ is the accepted norm.

Yep..

you have to establish a baseline against the trip meter to learn the actual usage and how the gauge responds. Most times it won;t move when its full until its used at least 25% of the fuel. You document this by the trip meter. That will show miles driven and you can easily est what the usage should be. Once you DO get the gauge to show 1/2 or so, it tends to move more with the actual level.
These senders are rheostats of sorts that are simply a series of contacts that a point sweeps across. The gauge simply shows where that point is in contact with at that moment. BUT, as they wear the spashing of the fuel shorts sevral contacts together so the gauge does not move until there is a great difference. Just one bar or 2..may not be enough difference in the signal to generate a fresh gauge read. It WILL show acurately each time you shut down and start up,. since that resets the gauge.

The system also has a dampner electronically so the gauge is not constantly reacting to fuel sloshing around. Its dampened to be SLOW to show a change so its not bouncing around like a kid on crack.

Some folks swear by Techron to clean the sender contacts. I do it manually by taking it all apart as needed. A new sender assy is around $250 without a pump. They sometimes go on sale for as low as $175 no pump... Look on FleaBay for used ones. You just have to wait for a nice used one but they will want near as much as new usually.

Donlt let that price shock you....
back in the 90's....that sender assy sold for $650 all day long. They've really dropped in price since aftermarket got involved in production.

I had a bad fuel level sensor for a while and was using the trip odometer as a gas gauge. I found that I can get (just barely) 250 miles on a tank. The dashboard says I'm getting about 15 miles per gallon average. I was filling up when the trip odometer got to about 220 or 230 miles just to be safe.

I've used the tripometer as a fuel gauge for many, many years in everything I drive. The tripometer has certainly been a more reliable reference than some/most later fuel gauges. I always have a reasonable cushion in mind and damn sure don't run it near or below quarter.