I have a 90 Topaz XR5 that has had a stumbling start since the first day I drove it (4/90). It recently has become worse, so I bought a fuel pressure guage and a vacuum gun to do some trouble shooting. First, I didn't find any vacuum leaks and the fuel pressure regulator test with the engine running works fine (~40psi with vacuum ~50psi without).
Now the problem. After the car sits overnight, the fuel pressure drops significantly down to 0psi - 10psi. The first key on, the pump engages and the pressure rises to about 20psi. I then key off and key on several more times before the pressure goes up to about 50psi and tops out. Sometimes, the fuel pressure actually drops back to 0psi. If I have good pressure and I start the car, it still stumbles until I give it just a little "gas". It then idles fine. I'm thinking that I may have pressure eventually at the fuel rail, but I'm thinking some of that is air and not fuel. Does the drop in pressure mean a bad regulator or fuel pump check valve? Any easy way to tell? If anyone has any suggestions, I sure would appreciate them.
Oh yes, there are no fuel leaks (on the floor, etc) that I can find.