Won't start with when fuel gauge is at 1/8 tank. Thought the guage was bad because when I add gas, it will start right up. However, It will only take 21 gallons in a 26 gallon tank, which indicates to me that there are still 5 gallons in the tank at the time it won't start. this has happened twice in the last week. Anybody have any suggestions as to what my problem is? Thanks...

Welcome to AF.
How many miles is on it?
I would run a fuel pressure check to make sure the fuel pump is up to par.
Are you sure it is a 26 gallon tank?
Have you ever added more than 21 gallon to it?
Is this a dual tank or single tank set up?
On dual tank setup when the secendary tank gets low it will transfer the rest of the fuel in it to the primary tank.!!!
They have a TSB about fuel gauges on them being bad and a new type replacement?