When my 92 C4 base model sets for more than a couple of days, the fuel drains from the fuel rails back into the tank causing me to have to re- prime. It doesn't seem to be draining past the pistons as i see or smell no gas in the oil. I don't see any puddles of gas nor do i smell it anywhere. Pressure checked the check valve on the fuel pump and holds pressure. Pressure checked the fuel system at the fuel rails. I get a good 45 psi when running but pressure drops to 0 over night. I replaced the fuel filter seals and the little fuel valve at the pressure check point. Any other suggestions what might be going on.

 

As far as I remember, its normal for it to drop to zero or essentially zero overnight.

 

Yes, it is normal to drop to zero over night. You should hold close to full pressure for about 1-2 hours and then it will start to drop slowly after that.

There is not a problem.

 

If the pressure drops rapidly, like to zero in 5 minutes, then that indicates a leaky injector.

It's possible the fuel pressure regulator diaphragm has a small hole in it. Remove the vacuum line while the engine is running. No fuel should come out of the FPR vacuum port.

Longer starts after a couple of days is normal. You can somewhat get around this by turning on the ignition and letting the ECM pressurize the fuel rails. Then turn off the ignition and turn it back on again. That will run the fuel pump again and you'll be sure the fuel rails are pressurized for starting.

To clarify the rationale for this: the ECM runs the fuel pump for 2 seconds at key-on. Then it shuts off the fuel pump so it won't run continuously if you turn on the ignition but don't start the car.

 

How do it start after sitting for a couple days? Does it crank, crank, crank or does it start quicker?

 

Yes, the system is designed to de-pressurize when not running. It would be extremely dangerous if it didn't.

The fuel pump 'primes' the system when you turn the key to the 'on' position. That is the 'buzzing' sound you hear from the back, just prior to starting the vehicle.

 

Cliff always hits the nail on the head..

 

Yeah, it may depressurize, but it shouldn't loose the volume of fuel in the rails.

 

If I am losing volume in the rails, not smelling or seeing fuel, and not seeing fuel in the regulator vacuum line, I would have to assume a leaky injector.
Thanks for all the help guys.

 

The fuel will flow past the FPR and back to the tank return line. If your maintaining pressure even if it drops 10 psi in 2-3 hours...it is not an injector.

Think your looking for a problem that does not exist.

••••• TESTING FOR LOSS OF FUEL PRESSURE

... Here are the sure fail tests for the fuel system to save you time and money.. Take a fuel pressure gauge attach it to the Schrader valve.. Turn the key on and off a few times to build up the pressure in the system... If you are having starting trouble the pressure will probably drop as soon as it hits 38 or 40 and the pump turns off it will probably leak back to zero within a few minutes. Ok now right after you prime the system pinch off the return line near the tank if the pressure drop stops it is a bad fuel pressure regulator.... Next pinch off the feed line if the pressure holds then the leak is before the point where you pinched the line IE fuel pump check valve..hole in the metal line on the sending unit caused by it rubbing on the inside of the tank you should visibly be able to see this if you pull the pump and the sending unit...(this was my problem) or the pulse dampener which is the piece between the pump and metal line on the sending unit.. the diaphragm tears.. and last if you pinch both the feed and return line near the tank and the leak down of the pressure continues then a fuel injector is your problem.. This is the GM test.. saves time and money..
•• disconnect the vacuum line from fuel pressure regulator

 

what makes you think you have a leaky injector?


From everything that you have said, I am NOT seeing ANY sign of a problem. The car is functioning normally. The system should NEVER hold pressure longer than it is warmed and ready to start. Once its cold, the system THEN re-charges the fuel system and begins the cold-start process.

Its only designed to hold tension on the rails for as long as it takes to cool it off. Its made that way for a reason. Gasoline under pressure for long periods is more apt to leak and cause a FIRE under the hood and ruin your day. The car don't like it much either...

 

I'm with Leesvet on this. There is fuel return line on the system. As the system is shut down, fuel is pushed back into the tank. So, you do, and should lose volume in the lines.

Fuel pressure when charged should 40-42 psi +/-. Once the system is shut off, it should hold pressure, dropping incrementally over about an hour or so. Now, if it is dropping rapidly, then a leaky injector might be indicated. But from your description, it doesn't seem like that is happening. Also if you had a leaky injector, depending on the severity, you would have a rough, hard start condition, as well as significant gasoline in the oil, from the flooding caused ny the injector.

 

A leaking injector or a bad check valve in the pump would cause the pressure to drop to zero in less than 2 minutes.

Your looking for a problem that does not exist.

 

Thanks again..