When my 92 305 tbi., bird is cold starting for about 10 minutes, it runs horribly. It sputters, hesitates, and stalls, and is worse with the ac on. After about 10 minutes this condition clears up and it seems to run normally. Have had a complete tune up, and my mechanic seems to think that the fuel pump is the problem? He says that the fuel pressure is low causing a bad cold start. Dont these fuel pumps just normally die sudden death when they go bad, or can they die a slow death and perform poorly over a prolonged period?? Please advise, because I do not want to change the fuel pump unless i am sure it is going to solve this problem?? Please advise!!! Thanks, Rob.

 

Fuel pressure output from the pump can reduce gradually if the pump is failing. There are other causes for reduced fuel pressure, such as clogging filter, ruptured pulsaton damper (in the tank), or an internally leaking pressure regulator.

Exactly what is the fuel pressure? Pressure should be 11-13 PSI with the engine running. Low pressure can result in a lean condition in open loop (cold starts, WOT) and closer to normal mixture in closed loop, when the oxygen sensor is feeding back to the ECM.

 

Thanks so much for your reply. My mechanic said that my psi fuel pressure is running at 8 1/2 to 9 psi, causing the terrible cold running lean condition I get for 10 minutes or so. Is this poor running condition consistent with what you would think would be the case if the fuel pump is weak? If so, would you agree that the fuel pump is probably bad? Can the fuel pressure be increased thru a mod on the fuel preasure regulator, possibly helping this condition? Thanks again for your help, Rob.

 

The first thing to do is test the voltage as close to the pump as possible. Electric pumps are very voltage sensitive, and a gradual loss of pressure may indicate a slowly degrading connection. There should be a connector under the rear seat, and it may be good to probe the wires after that connector to verify good voltage to the pump.

The fuel pressure regulator can be tested by monitoring pressure while blocking the fuel return line. If the pressure increases with the return line blocked, the regulator is suspect.