I have a low rpm miss and I am trying to fix it. The car runs good at wot, but the idle is a little off. I changed the spark plugs today, the old spark plugs looked fine though. After changing the spark plugs there was no change in the idle. So I tested my fuel pressure; I found that the pressure builds to 45 psi with the key on, but bleeds down to zero as soon as the pump stops. Is this a sign of a bad fuel pump or could it be the injectors. I am thinking it is the pump because the plugs were all clean when I replaced them. Will a fuel pump that loses pressure cause a rough idle but run good at wot. Thanks

 

Usually if the fuel pressure bleeds off rapidly, the only problem will be extended cranking on starts. The bleeddown can be caused by 1) faulty check valve in the fuel pump, 2) leaking injector(s), 3) faulty fuel pressure diaphragm.

The injectors can be checked by pulling the fuel rails with the injectors intact, and pressuring the fuel system to look for leaks. The fuel pressure regulator can be checked by pulling the vacuum line off and looking for wet fuel in the line. Hardest problem to track down is the check valve in the pump. To check that, you have to block off the return line, and that's hard to do. Some people crimp the plastic return line with pliers, but I would not recommend that.

The rapid bleed off should not affect the pressure when the engine is running. Recheck your fuel pressure, with the engine running:

As idle, remove the vacuum compensation line from the fuel pressure regulator. Cover the end of the vacuum line, so you don't have a vacuum leak. The fuel pressure should be 43.5psi. GM specs accept anything in the range of 41-47psi.

Reconnect the vacuum line, and the fuel pressure should drop proportional to intake manifold vacuum. With a stock cam, you will see the pressure drop 6 to 8psi below the "no vacuum" pressure.

 

I finally got the chance to remove the fuel rail and pressurize the fuel system. The injectors are fine and the fuel pressure regulator is good. I also checked the fuel pressure with the car running. The pressure was 45psi with the vacuum line off and 38psi with the vacuum line on. I must have a problem with the fuel pump check valve. I am thinking the opti is the problem. I had the rotor blow apart on me month ago so I replaced the cap and rotor. But just before the rotor blew apart the car developed a low rpm slight miss and sometimes it pings at high rpm's. The car does have the hot cam but I think my mpg is also off at about 8mpg on the streets.