General Maintenance & Repairs - Faulty fuel sending unit?

Recently Iv been getting crap mpg. Like 150/tank I changed o2 sensors and everything I could think of. When I go from full to empty tank nd fill up it only puts in 12 gallons. A friend with an 02 ss does the same he gets around 230 in a cammed ls1. He fills up and puts in 14 gallons. So could this be a bad sending unit? My only option left is to just run the car til it won't start cuz no gas. Bad idea I know but what else is there to do??

Check your exhaust for leaks. Air will leak in pre-O2s and the car will get a false lean and add way too much gas. My car had been running pig rich the past few weeks w/ shit performance. Found an exhaust leak on the manifold, even w/ a brand new gasket. Put some Ultra High Temp Red RTV on my finger and smeared it over the leak, let it cure overnight, fired up this morning and ran great.

Iv got a small one after the o2

But that doesn't explain why when I fill up it only puts 12 gallons.

You may have a bunch of crap filling your tank. Check your fuel filter for debris.

Also, is it after both sensors or just the first? If it's close enough to a pre-cat sensor, it can still potentially leak in to cause a false lean and make you run rich.

doesnt sound like the sending unit is the problem. 12 gallons only getting you 150 miles is appr. 12mpg. something is definately wrong.

i am getting over 20 mpg with 50/50 driving (18miles city/18miles highway per day).

if i try to run 87 or 89 i get mid to high teens, it is cheaper for me to run premium.

It's in the slip fit part of the ypipe on the passenger side after the first o2. My rears are deleted.

Let me be more specific. It's after the collector to ypipe. It's the ypipe to the part the goes to the merge.
What else could it be?? It doesn't smell like gas except on start up cuz !AIR. no smoke every. New air filter new PCM with a bolt on tune by frost MAF cleaned. Both new o2s I ran some seafoam thru the tank last week just to check it off the list. I'll throw another fuel filter on soon. I'm all city driving.

I've been trying to solve a similar problem myself, though my mileage is about 14-15mpg in all city driving for me. I've already replaced the fuel filter, IAC, O2 sensors (the 3 oldest ones, 1 is only a few years old), PCV valve, PCV elbows & connectors, cleaned the MAF, checked the plugs, Seafoamed the gas tank, Amsoil Power Foamed the intake, and my mileage is just as bad from what I can tell so far. (If you follow my posts, you'll find more detail about it all.) I've also been having occasional hard starting for several months, so I'm thinking it's the fuel. It still smells rich on startup, so I'm guessing a clogged injector(s). The old fuel filter was on there way too long, so that may be partially my fault. My plan is to test fuel rail pressure to rule out the fuel pump, and then get a set of rebuilt injectors.

In your case, it could be many things, but the fact that you have a tune makes it even more complicated. The AIR system helps to heat up the cats, so it wouldn't prevent the smell of gas at start up, nor would its absence create one. The car is in open loop then anyway and the cats are cold, so if it smells like gas, it's something else. I won't get to my fuel injectors for about 3 weeks, but I'll let you know if that helps. All it takes is one to be clogged and the whole bank sprays more fuel as the computer tries to overcompensate. Since my car runs smoothly with no other symptoms than the hard start, rich exhaust at startup, and bad MPG, I'm thinking I'm on the right track (fingers crossed).

mine only smells like gas on start up cuz the AIR pump is gone. i dont have hard starts or any other symptoms other than bad mpg.

Any other ideas guys?? And I'm putting shell 91 or texaco 91 only.

Could be a leaking injector. Pull the rail w/ the injectors on it, turn key to on w/o starting the engine, and see if any squeezes out.

Ok. If some comes out?
If none comes out what next? Oh Iv got a wideband that should be here tomorrow granted UPS doesnt drop the ball like FedEx did. What should my afr be at idle cruise and wot?

If one of the injectors is leaking, then that means time to replace it. If several are leaking, check your fuel pressure regulator. It could be allowing too high a psi through the rails. As far as the tune, it's possible that it went screwy sometime. But I don't know much about tuning. You might want to go ask about it in the tuning section.

Ill be installing my wideband tomorrow so I'll see if I'm running too rich. I trust frosts tune but you never know.

idle and cruise should be 14.7ish in closed loop.

WOT 12.8-13.2

Thanks Lee. I looked around a while and only saw built engine people asking what their cammed or boosted afr should be. Didn't think it applied to just a Bolton car.