1993 S10 Blazer 4.3 Vortec w/ CMFI (Eng Code W)

I've been fighting a hard starting problem and have tracked it down to the fuel pump prime. After the truck sits for a few hours it has to crank for about 20 seconds or so until the voltage appears on the high side of the fuel pump relay. My Chiltons Manual shows "To Fuel Pump Prime Connector" going to the normally closed contact on the relay. There is no connection to the that contact. I have +12 V all of the time on the Orange relay wire, the Normally open contact. The manual shows a gray wire on the relay "common" that shows going to the fuel pump. After the 20 sec of cranking +12 v appears on the relay coil. presumably from the PCM, closes the relay contact to the +12V (Orange) and fires up the fuel pump, then I get fuel pressure and the engine starts right up. Seems like the Chilton manual is miss leading. I believe that the gray wire that says that it goes to the fuel pump must also feed a connection to the PCM that is supposed to provide a temporary Fuel Pump Prime voltage.
Thanks!!

 

The fuel pump prime connector is a diagnostic connector only and is not meant to be connected all of the time. There is something else wrong causing your problem.

 

The only code that shows up is a 54 which show low voltage on fuel pump circuit or egr system depending on which web site you go to. It's not the egr, I tried a new one but no difference, so I started looking deeper and found the fuel pump prime voltage not there. This just started all of a sudden, no problems or check eng lite before, just one morning I had to crank it excessivly (20-30 sec). It used to fire right up just barely touching the starter. Could be it be something else? Is something causing the "prime" voltage from coming on? Does the "prime" voltage come from the PCM? My Chilton book shows the gray wire that connects to fuel pump also connect to an oil pressure switch and the PCM. The other side of the oil pressure switch connect 12 volts to the gray wire apparently when enough oil pressure is developed by the excessive cranking. Does this 12 volts to the PCM cause 12 volts to go via the PCM to feed the wire going to the fuel pump relay coil? You used to hear the fuel pump whine for afew seconds when you first turned on the ign. That doesn't happen now unless you jumper 12 volts to the prime wire. When I do that fuel pressure immediately jumps up to60 lbs so I don't suspect the fuel pump. Once the pressure is up the engine fires right up.

 

There should be a timed +12V signal to the fuel pump relay for starting which is supplied by the PCM. I am unsure as to the logic which is used in conjunction with the oil pressure switch in these trucks, but I always thought that the oil pressure switch did not come into play until after the engine was running. It should not impact the initial key-on running of the fuel pump.

 

OK that makes sence. With the 20+ seconds of cranking enough oil pressure is developed to energize the fuel pump relay. So the PCM should be supplying the 12 volts to the relay coil. Is there an easy way to check that the voltage is coming from the PCM (Which connector, which pin)? Maybe I have a wiring problem between the PCM & the relay.

 

With a pinout from the PCM, sure... I do not have my references with me to pull such information though...