Hard start/low fuel pressure

[SeanC]

Hey guys,

I've been having a starting problem where you have to crank the car a couple of times until it starts. If you turn the key to the on position a couple of times before you start it, it pretty much starts right away (builds up pressure). I put a pressure guage on it today and when I turned my key to the on position, the pressure would jump up to about 42 (hard to tell exactly since I was running back and forth by myself...) and would drop back down to 0 in less than a few seconds. In addition, at idle my fuel pressure is about 36-37 psi.....I'm guessing my fuel pump check valve is leaking? Any suggestions or thoughts on this? Thanks!

[transamz28]

fuel pressure regulator? you could take the vacuum line off and see if it smell of gas bad, also leaking injectors, i d check the oil , take the cap off and see if it smell of gas

[SeanC]

How can I test if the fuel pressure regulator is leaking, compared to the fuel pump?

I havn't replaced the fuel filter since I got the car and am wondering if that can account for my low fuel pressure at idle. Obviously there is something else wrong if the system pressure drops that dramatically (more than 3-10 psi)...

[Injuneer]

How did you measure the fuel pressure at idle? If you measured it with the vacuum compensation line still connected to the fuel pressure regulator, 36-37psi is the correct pressure at idle. If you remove the vacuum compensation line while its idling (cover the end of the hose to prevent a vacuum leak) the fuel pressure should be 43.5psi. GM accepts anything between 41-47psi as within specs. The purpose of the vacuum line is to maintain a constant 43.5 DIFFERENTIAL pressure - the pressure measured between the fuel rails and the intake manifold. When there is vacuum present in the intake manifold, the pressure regulator drops the fuel pressure in the rails, to compensate for the vacuum in the manifold.

A leaking check valve in the fuel pump will only affect the pressure when the pump is not running. Once the pump is running, it will develop the full pressure it is capable of, whether the check valve is working or not. But as noted, the loss or pressure when the pump pshuts off could also be due to leaks in the fuel lines, leaking fuel injectors, or a ruptured diaphragm in the fuel pressure regulator. Each of those possibilities needs to be ruled out.

[SeanC]

Thank you very much for the help so far!

Injuneer, I did test idle fuel pressure with the vaccum line on the fuel pressure regulator. I forgot that fuel pressure drops around 3-10psi when the engine is running and there is vaccum.

I know of the possible things that could be causing this loss of pressure, but i'm not completely sure how to go about eliminating each problem. Would you guys be able to help me through it? I bought Ben Watsons Chevy fuel injection book and I have my shop manual, so i'm going to be doing some more reading (as well as Shboxes's website). I'm really unsure about clamping the lines, but I don't have a fuel line shut off adapter...what do you guys think?

[MN6WS6]

The easiest way to check the fuel pressure regulator is to pull the vaccuum line going to it. If there's fuel in the line (either wet or a strong odor), then the regulator has failed.

I do have to tell you though, my car did exactly what yours is doing, and even after checking everything, it ended up being the check valve on the fuel pump. If you think about it, that valve is just about the only way you could lose that much pressure that fast. A fuel leak would be obvious because it would spray or pour. An injector leaking that bad would puddle fuel or if the valve was open could possibly hydrolock the engine (had that happen on my motorcycle). I tried to avoid the inevitable and borrowed parts, swapping out the regulator and all the injectors PRAYING it wasn't the pump, but it ended up I was just being stubborn.

And the pump isn't that bad. I did it in the driveway on jackstands and correctly (pulling the exhaust) by myself in about 4 hours. I just couldn't bring myself to cut a huge hole in the bottom of my WS6.

[SeanC]

Thanks for the response MN6WS6! I really think it is my pump, I just feel like I need to be completely sure before going through all that trouble of replacing it. I am going to use the "trap door" method because I feel like i'm always going to have my car and if I ever have to do anything with the fuel pump again, it wont be a big deal. I am just a little hesitant about cutting around fuel lines....

I will pull the vaccum line first thing in the morning and see if there is any presence of fuel.

[Injuneer]

Because gasoline in non-compressible, like other liquids, it only takes the loss of a few drops or so to cause the pressure to fall to 0. You aren't going to lose enough fuel to hydrolock a cylinder. If a leaking injector was flooding a cylinder, it would show up as the engine being easier to start with the accel pedal fully on the floor, putting it in clear flood mode. Pulling the rails and pressuring the fuel system will show if an injector is leaking badly.

If its the pump check valve, cycling the key a few times before trying to start it may work. That would only make a leaking injector problem worse.

To me its worth thinking your way through all the possibilites before deciding to replace the fuel pump.

[SeanC]

Without a doubt turning the key to on a few times helps. I pulled the vaccum line on the fuel pressure regulator and when you put it up to your nose, it definitely has a pretty strong smell of gasoline. Does this mean its the component leaking pressure?

[transamz28]

Without a doubt turning the key to on a few times helps. I pulled the vaccum line on the fuel pressure regulator and when you put it up to your nose, it definitely has a pretty strong smell of gasoline. Does this mean its the component leaking pressure?

i would replace that and take it from there, its not expensive.

you could lift the fuel rails and turn the igniton on to pressurize the system and see if any of the injectors are leaking.
can find that connector under the hood that will turn the fuel pump on to keep it running for longer.

i did get hydrolock from a really bad leaking injector, but there was a no start condition too . and that killed the pinion thing in the starter, inside the thing that engages onto the flexplate. Yours is not that bad , dont see any possibilities of getting it to hydrolock.

there a re a few write ups about coutting the trap door to replace the fuel pump, gives you measurements where to cut too. i used the snips to cut that, quite soft metal there , i cant see that causing any structural probelms

you can upgrade the fuel pump if you replace it too