Recently had my stock fuel pump go out, which was replaced. Also replaced was the fuel filter and the fuel pressure regulator. My gas mileage dropped from the ~200 to ~230 range on my weekly drive to ~160 to ~180. I drive an 80/20 mix of city/highway. The car drives fine, recently passed CA smog and no codes are thrown. My local repair shop can't find any issues and the air/fuel mix is fine according to them. I can smell gas on occasion days after pumping gas when running the heat in the car.

What could possibly be my issue with my gas mileage? Where do I start to try and resolve this? Any ideas?

Car: 96 Z28, A4 130,000 miles

 

Hook up a fuel pressure gauge. Without starting the engine, turn to key to "on" and watch the fuel pressure come up. Note the pressure, and when the pump stops, watch how fast the pressure drops. Rapidly falling pressure could be the result of a faulty fuel pressure regulator (but you have a new FPR), a leaking check valve in the fuel pump (but you have a new fuel pump), leaking injectors (but you wouldn't be likely to smell it through the heater), or leaking lines.

If the pressure drops off rapidly, start checking the lines, the filter and the FPR for leaks.

 

Thanks for the reply. What should my pressure be around?
If I have no leaks in the engine or the lines to the fuel tank, what could cause my fuel smell? I have noticed something odd, since the pump change, I never hear gas escaping when I open the gas cap. Like the tank doesn't hold presure, I found it odd so I bought a new gas cap. It did not help, I still don't hear the sound.

 

When the fuel pump primes, the pressure should be in the range of 40-45psi. Running, its different, but check the pressure when the pump primes, without the engine running.

Sounds like the fuel tank vent connection is off, or the rubber line is cracked. The pressure in the fuel tank is controlled by a pressure control valve, in the line from the top of the fuel tank to the charcoal canister in the driver's side rear fender. When that line leaks, it will smell at the rear of the car, and from my experience, will cause fuel vapor to come in through an open driver's window when you pull up to a stop at a light (C16 actually causes a nice buzz ). Not sure how it would come in through the heater, if the fuel vent was leaking back by the tank.

Courtesy of Shoebox:

http://shbox.com/1/evap_system.jpg

 

Ok, just put a gauge to the car.

When I turn the key to on, the pressure rises quickly to 40psi and stays there. I does not drop, even if I turn the key to off. With the engine running at idle, the pressure drops to 32psi, if I press the pedal down the psi stay at 32.

Just re-did the test again, because I forgot to disconnect the Vacuum line. I get the same readings, except when the key is turn on, the psi jump to 45psi quickly before dropping to 40psi and staying there. Car at idle is the same 32psi.

So does it seem like this are normal and does this eliminate the fuel system as being an issue?

thanks all

 

When you turn the key to on, the fuel pump should only run for a couple seconds, then shuts off. At that point, it won't make any difference whether the key is on or off.

At idle, with the vacuum line disconnected spec is 41 - 47psi. Ideally, you want 43.5psi. Reconnect the vacuum line and the pressure should drop proportional to intake manifold vaccum. With a stock cam, you usually see about an 8psi drop. If yours id dropping to 32psi, it seems a little low. Moving the accel pedal will only affect the fuel pressure when the pedal is moving, and intake manifold vacuum is changing. At steady RPM, it should to hold steady.

I;'d check the fuel pressure regulator. Was there any wet fuel in the vacuum line? Do you have a vacuum pump so you can pull a vacuum on the FPR and see if it holds the vacuum? A leak in the FPR diaphragm will pour raw fuel into the intake manifold.

All this assumes you're talking about the 96. If its the 02 the pressure numbers are way off.

 

I'll re-run the test again tonight.

Yes, this is for my 96 LT1. There was no wet or dry fuel at all in the vacuum line, it was just dry. I'll dig around to see if I find my vacuum pump and test that too.

Thanks