I have a 97 chevy K1500 5.0L with approx 200K miles. Occasionally I would get a rough idle and stumbling at low speeds. I previously (bout 2 months ago) checked fuel pressure and it was a bit low at about 52psi. If I pinched the retrun line a bit, the pressure would spike up to 75 psi (I then shut it down so I didn't spike the pump with excessive psi). So I figured the fuel pressure regulator may be bad since it appeared the pump can supply the needed pressure. I replaced it and the pressure reading after was approx 58 psi and it would hold. About 2 months later, the stumbling started happening again and this time set an engine light of P1351 High ICM voltage. I was a little suspicious that this may be the result, and not the cause. I checked it out. It had good ground. I had approx 2.7vac between the ICM and the 12v while cranking (spec is 1-4 vac). I had continuity to the ecm connector, even while wiggling the harness. On a hunch, I checked the fuel pressure again since even though it felt like a spark/timing issue, it might be fuel related. With key on, it will run to 60psi and then as soon as the pump stops, it drops down to 0psi. The strange thing is that the truck will start. Has anyone seen a pump pressurize to 60psi, then drop to 0psi and still start? If the regulator was stuck open in bypass, would the injectors even fire? Once the engine starts and is running, I have approx 50 psi and it will jump towards 60 psi if I rev the engine a bit.