1998 L36 Poor fuel mileage

[beater98se]

One thing i've noticed with my Bonneville is the somewhat excessive fuel consumption since i put it on the road two months ago. I find I'm averaging 400-450km to a tank of gas mostly highway using regular 87, driving normally. Meanwhile, my 2001 Impala with the L36 gets around 600km to a tank with 91(tune) being slightly aggressive with the throttle. The impala has a 17gal tank while the bonne has an 18gal. There's no external fuel leak, nor is it throwing a CEL. I've cleaned the throttle body (was dirty af) as well as the MAF which did not make a huge difference. The cat and exhaust are about a year old, and the air filter is brand new. I've also tried swapping the tstat which did nothing. Only mod done was gutting the airbox. I filled up yesterday and drove 30km, which was enough to move the dial from F. In my impala i can drive a spirited 100km or more before the needle even budges. Only unnecessary weight are my two 10" subs in the trunk. I keep my tires properly inflated. Water pump and tensioner assy. are brand new, but I know a bearing inside the alternator is shot. I haven't noticed any raw gas smell or anything that smells like a rich condition.

I tested my FPR today, which showed 44psi KOEO which started to drop. KOER showed a steady 42psi and shot up to 50 with a snap of the throttle, and same when i disconnected the vac line. However, according to specs the pressure reading for 1998 should be 48 to 55psi. Could I possibly have a bad FPR?

[nos4blood70]

Does the car coast freely? Age of plugs and wires? Fuel filter? Maybe run some fuel cleaner through it.

[beater98se]

Does the car coast freely? Age of plugs and wires? Fuel filter? Maybe run some fuel cleaner through it.

The upper intake was done a couple years ago along with plugs and wires, and I did both a cleaner and the fuel filter recently. Brakes are all new so I couldn't see anything binding? I'm thinking a leaking FPR but I don't want to condemn it just yet.

[Jfridge92]

An easy way to check the FPR is to pull the vacuum line off of it and smell it. If it smells very strongly of gas, it's likely leaking. With the car running, you can also pull off that same line, Plug that line for about 45 seconds, and sometimes the FPP will visibly start leaking fuel from the nipple that the vacuum line plugs in to.