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Old June 22nd, 2009, 04:34 PM   #1
Gr8wht2000GMC
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Default 98 ZR2 oil pressure/fuel pump switch

Is this all in the oil press. sending unit (behind the distributor) or is there a separate switch that kills the pump voltage on low pressure? My daughters truck runs super for 10 miles, then every time you get on it we lose pump power. I've already replaced pump, and upgraded CSFI to MPI spider. This switch has to be the perpetrator. Also oil pressure on mechanical is steady at 38-45 psi. Thanks in advance for any help.
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Old June 22nd, 2009, 04:45 PM   #2
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theres only 2 things that control the fuel pump. the computer and the oil pressure switch. the pressure switch is behind the distributor. check also for a loose or bad relay. i've seen water get into the fuse box and corrode the fuel pump relay causing problems with intermittent shutoff and or intermittent no starts
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Old June 23rd, 2009, 09:41 AM   #3
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Quote:
Originally Posted by Gr8wht2000GMC View Post
Is this all in the oil press. sending unit (behind the distributor) or is there a separate switch that kills the pump voltage on low pressure? My daughters truck runs super for 10 miles, then every time you get on it we lose pump power. I've already replaced pump, and upgraded CSFI to MPI spider. This switch has to be the perpetrator. Also oil pressure on mechanical is steady at 38-45 psi. Thanks in advance for any help.
I just went through something similar on a Suburban and assuming that the 4.3 is the same as the 5.7 the OP switch doesnít work the way youíre thinking. The OP switch canít actually turn off the fuel pump if the ECM is commanding the pump on. The ECM and the OP Switch are parallel paths that turn on the fuel pump. So, if either circuit is powered the fuel pump will run.

Unless the OP switch AND the fuel pump relay are both bad, I think you need to look for a wiring fault.

On a side note GM eliminated that part of the circuit sometime in 99 on the 5.7, of course my dadís Suburban was built 08/98 so the wiring diagram was wrong and it took me two weeks to find the short. In the end I rewired the pump to eliminate that part of the OP switch.
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Old June 23rd, 2009, 09:03 PM   #4
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Thanks for the input guys. This problem has been pretty difficult to track down. My brother had issues with this in Missouri, I bought the truck just knowing it was gonna be a simple fix. The fuel pump was upgraded at the same time as the fuel injection upgrade. I think I'll change the pressure switch anyway but will also rewire the pump. I guess I thought that the switch shut off the f-pump in case of low pressure. My symptons led me to believe that all was well until the oil was at operating temp. The symptoms also take too long to develop to be a open/closed loop issue.
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Old June 24th, 2009, 11:39 AM   #5
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When you replaced the pump did you replace the connector on the wiring harness? I replaced the old pump on the suburban when I did all of the other work and the new pump came with a flat 4 pin connector to replace the old square 4 pin. Apparently there is a TSB about burned out connectors on the square style plugs.
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Old June 24th, 2009, 05:53 PM   #6
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connecter replaced too.
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