The Stovebolt Site Forums The Shop Area Engine Shop Fuel tank level

A few weeks ago we hashed around fuel tank levels and different ways to determine the approximate amount. I drained the tank on my 54-3100 then put 5 gallons of high test in. Been working on gauge float. Put high test in and let it run out, getting the burps out of the valve train after setting for too long. Don't really want to go back into tank unless necessary. Where should 5 gallons of gas read on fuel gauge (+ -). Putting white gas in and run it out again to make sure ethanol is nearly out. If I put a 3/8 fiberglass rod down fill tube, how much should be on the rod? Doc.

The only general comment that I have seen on these old auto gauges is that when they get to the empty mark, you have about two gallons left. I have never challenged that theory but it sounds good to me.

I gave up on my fuel level gauge, and simply use an ohmmeter to determine fuel level. I have a 17 gallon tank with a 30 ohm float. Divide 17 by 30, and that equals .566 ohms per gallon. So if the meter reads 10 ohms, I multiply that by .566 and I know I have 5.6 gallons of fuel in the tank. You can easily calculate fuel mileage even on short trips. I keep a free HF meter connected to the float, and just check it as needed. 15 ohms is a half a tank.

I concur with this observation. When my needle stops wiggling from sloshing gas while driving, I need to find a gas station within 30 miles (2 gallons worth of gas).

With a bone-dry tank of 18 gallon capacity I added 9 gallons of gas, I then adjusted the float to read 1/2 full on the gage. Everything I have read about gages is that they are the most accurate mid gage.

Please remember the AD tanks include a horizontal baffle plate about 3 inches or so from the bottom. Even the replacement tanks include this from my experience. It wasn't a slosh baffle and interestingly with it in place your sender can't send a 0 ohm message to your gauge...the baffle prevents the wiper from reaching the lowest part of the range. One of GM's little mysteries, I suppose. So, with this baffle plate in place and a sender unit mounted from the top, your float can't touch the bottom of the tank nor register empty unless somebody has monkeyed with the float rod to make it register 0 when it hits the baffle. 4100 Fire Truck got your formula correct, and no matter which size tank you have at half tank the reading will be 30 ohms. As for the sender unit, the best ones by a very long shot are sold on eBay by a seller whose ID is renbusan. They measure exactly 0 when empty and 30 when full and the way they're made, they will last a long time. Image below. The wiper runs flat against the windings and the windings are molded into a nylon plate where they can't move nor touch each other, and it will take a long time before they wear out. Some of the senders sold by a popular vendor of parts frequently mentioned here are a completely different story. They are not 30 ohm senders but are 90 ohm senders onto which the assembler dribbled molten solder until the coil measured about 30 ohms. You can see a picture of this below. Obviously this isn't what you want.

Now about your original sender. What happens to it over time isn't pretty, and I believe this is the major cause of sender failure. The casing was made of pot metal and for a few years it was fine, but the single pivot point wears oblong and causes the wiper arm to travel all over the place. Images below. If GM had just made this with 2 opposed pivot points all would have been fine, but that would have cost more...

Great info guys. I think the ridges in my tank were throwing me off gallon wise. I know that I can never get my gauge to be absolute accurate. I also need to look at the manual, I always thought my 3100 had a twenty gallon back seat tank. Phew, these old truck are fun but persnickety but that's all part of the adventure. Thanks, Doc.

Doc,

As you probably now know, the 1954 3104 Pickup had a 17.5 gallon fuel tank (in the cab, behind the seat).

My 3106 1954 Suburban had a 16 gallon fuel tank (between the frame, under the rear floor).