I recently had my fuel pump and sender unit replaced at the dealer. They also replaced the fuel pump relay. Before this all my gauges worked perfectly. The cluster is about 6 months old, factory new. Now, when the tank is full of gas, the fuel gauge reads about 3/4. At 1/4 on the gauge, it will only take about 6 gallons of gas. What could be the problem here?

Also, my oil pressure gauge is acting up. Before, starting the car cold from a anights rest, it would start out at about 45psi. After a few minutes it would level off at about 30psi, dead center. This fluctuates up and down about 5psi or so during driving. Normal for my new and old gauge. Now starting the car, it is at about 50psi. Sometimes it drops to about 35 psi, but giving it gas or driving hard it goes back up to about 45 or thereabouts. This all happened, gas and oil gauges, from the moment I drove my car out of the chevy dealers lot. I didn't pay it much attention at the time due to the car having been down for 4-5 days for repairs, and figuring it might need time to reset itself properly.

They were supposed to do some top end cleaning on the engine to remove carbon deposits and also clean the injectors. Not sure if they did this or not, but figured I'd add it in case it is significant.

Any idea what is going on here?

 

When you had the old fuel pump, did the gauge read full when you filled up? If so I am guessing they may have damage the fuel pump during the installation. Take it back to the dealer and explained to them the issues you have with the gauge not reading correctly when full.
As far as the oil goes, check to make sure the connection is on snug. It is located above your oil filter.

 

Before the pump keeled over dead, everything was working perfectly. Well, as perfect as gm gauges ever worked. When full, it always read above the full line a bit, and was relatively accurate. The oil gauge, I really dunno how they could have either messed the sender/gauge up or done something to the engine itself to cause that. But regardless, something did happen to either cause the gauge reading to go to hell, or to actually increase oil pressure by a boat load. I talked to them and am bringing the car back Monday to have em go over those 2 things to see what's up. The service guy said maybe the new fuel sender was bad. We'll see soon enough though.