George Dickel

1993 LT1, auto. Gas mileage has dropped into the toilet. Went from 24 -25 hwy to 17. Codes 55 and 64 have come up. Low fuel pressure and a lean condition. Changed the fuel filter, wasn't clogged. Hooked up my fuel pressure guage and tested according to my Helms manual. Manual says to connect the guage and disconnect the vacume line to the fuel pressure regulator. When I start and run the engine the pressure is 48 psi. When I connect the vacume line to the regulator it drops to 38 but according to the Haynes manual that is normal, no mention is made in the Helms manual on this. When I drive the car with the vacume line connected the fuel pressure will bounce back and forth between 40 and 50 psi. The needle is a blur bouncing between 40 and 50. Is this normal? Is this caused by the pulsing of the injectors? The Helms manual says the accumulator valve is supposed to prevent fluctuation in pressure. When I let off the gas to slow down the pressure drops to 38, but when I accelerate it goes back to bouncing between 40 and 50.

If I disconnect and plug the vacume outlet the pressure is at 48 when it is idling but goes back to bouncing between 40 and 50 when I run at higher RPM. Also when I get on it hard, when it is in 2nd it seems to be quite sluggish and when it hits 4,000 RPM to 4,500 RPM it starts missing and then at 4,500 to 5,500 it stops missing and really pulls hard until it hits about 5,500 and shifts to 3rd and the same thing happens again.

I changed the fuel pump about 6 months ago so it is not an old pump but it could be defective. I'm stumped at this time so any help will be greatly appreciated.

olman

High vacuum at idle or cruise will hold the pressure at 38. As you accelerate the vacuum drops allowing the pressure to increase. I can't explain the fluctuating needle unless the regulator is going south. Rough running in a specific RPM range may be in the dist. Maybe someone more knowlegable than me will chime in.

REDC4CORVETTE

Your running to rich and the fuel presure for a stock LT1 is 43.5 psi with vac disconected and pluged at idle.
If you have an adjustable regulator turn it down,if not trash the one you have and get a new factory regulator.

Nathan Plemons

I have a fuel pressure gauge on my A-pillar so I know how it's supposed to behave. Essentially your fuel pressure should only really depend on the vacuum applied to the regulator, the higher th vacuum the lower the reading. Now I have a cam that costs me a little bit of vacuum at idle so mine idles around 40-41 PSI instead of the factory 38 but this is fine. At wide open throttle the vacuum goes away and pressure spikes up to 48-49 and stays that way until I let off the throttle.

For a full on high RPM decel the pressure will drop down to 36-38 which makes perfect sense if you think about it. You're turning a high RPM and the engine is sucking a lot of air. The throttle blades slam closed and it still tries to suck a lot of air but it can't, this creates a larger vacuum and pulls down the fuel pressure.

The key is that it should never fluctuate under a steady throttle position. If you're holding a constant RPM with a constant throttle position the gauge should be rock solid. The pressure should rise as you give more throttle and lower as you let off. You should be able to basically watch your pedal movements in the gauge. If it is moving and you're not touching the throttle you've got an issue.

How long does the car hold it's fuel pressure after you shut it off? It should hold for several hours, if not you've got a problem somewhere. You could have an injector that is leaking, causing very erratic readings. You can test that by removing the fuel rail from the car (don't worry it's only 4 bolts) and putting a jar under each of the injectors. Turn the key on and look. You shouldn't see any fuel from any of the injectors. If that's cool try cranking the car a few times. You should have have about the same amount of fuel in each jar. If one is significantly more or less than the others you have an injector problem.