While my truck is running my oil pressure gauge reads full. The gauge zeroes out when you shut the truck off yet it goes up to maximum when I start it again. Nothing seems wrong with the truck. What's up?

check the oil pressure first, it should read around 10 to 14 psi. If pressure is inside the range then your pressure sensor is bad. If lower or reads 0 psi your got a engine problem, further diagnosis needed. Good luck.

ur oil sending sensor is bad it reeds the oil pressure

Sounds like an electrical problem, make sure the ground leads to the engine are good and the battery terminals are clean. It might just be a bad connection in the wiring harness try jiggling the connections to see if it makes the gauges jump when it is running.

Start by verifying the oil pressure sending unit. It is quite common for a bad sensor to act erratically, such as you have described. Acquire an oil pressure gauge and install it in an appropriate location, perhaps in place of the oil sending unit if a more convenient location is not available, and run the engine. Observe the pressure on the test gauge at various RPM and over a period of several minutes as the engine warms. If pressure on the gauge is good, then you have a problem either with the oil pressure sending unit, in the circuitry, or instrument panel gauge. However, the problem is most likely the sending unit. Note: the sending unit often requires a special socket, which may be obtained at any auto parts store. Also, if you don't own a test gauge or unwilling to purchase, many auto parts stores now rent tools, others even loan tools provided an adequate security deposit is made.

because when the engine is shut off, the oil pump is also shut off, therefore no oil pressure. no problem here!

Two things...when it reads zero, is there any engine noise (tapping or knocking)? After you rev the engine does the pressure read normal at idle and function normally, or does it go back down? Either way, you should not drive the vehicle 'till you have a mechanical gauge (yours is electro-mechanical) put on there and the actual pressure is tested. The condition may be a bad sending unit, a bad bearing or possible obstructed oil pump screen. The repair is obviously different for each one and the price is as well.

sounds like possible oil starvation due to leak in pick up or supply galleries
suggest checking with proper test gauge to see if its the same
if it is the same check pressure relief valve in pump (to see if jamming open)
then check pick up in sump and "O" ring

That is probably a defective oil pressure gauge or a broken oil pressure sender, it reads 80 psi just because that is the maximum value allowed.

Pressure can be verified by reading it with a gauge tool.
If you really have pressure over 70 psi when engine is running then start replacing the oil filter, and check that the oil lines from pump are not obstructed, if there is nothing in there then replace oil pump.

If instead the gauge reads 80 with engine off, that is a defective pressure gauge.

Are you referring to the oil pressure gauge on the dash? If so, that is most likely the oil pressure sending unit having gone bad. The sensor runs about $40 - 50 (at least the last time I had to replace one on that engine). They are not terribly difficult to replace and I believe its a three wire sensor located back side of the engine, on the passenger side area. Its in a kind of tight spot and takes a bit to get it completely screwed out, but it can be done.

When they go bad, they typically will cause the gauge on the dash to read maximum oil pressure. You should be okay to run like that until you can get a new sensor as long as it is reading maximum oil pressure on the gauge and not low or no oil pressure on the gauge.

electrical problem. take the truck to the dealer and have the computer checked / reprogrammed. unfortunately, your average mechanic cannot reprogram the computer. have you noticed any other problem that would point in the direction of an electrical problem (lights, radio, lighter, etc.)?