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Carhelp4u
Carhelp4u, ASE CERTIFIED TECH
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Working on a low fuel pressure problem with a 1994 s10 Blazer

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Working on a low fuel pressure problem with a 1994 s10 Blazer 4.3(W), changed out the fuel tank, pump, and filter, prior to the change all was working well, tank developed a rust leak. Put it back together and ended up with a low pump pressure, looking at my Chiltons as it pertains to a 4.3 CPI system I determined that the oil pressure switch is bad and fuel pump relay is good, the oil switch could be partially grounding the current to the pump. However the Chilton's suggests the following for low pressure, restricted fuel line, checked, restricted filter, checked, or a "disconnected fuel pulse dampener (pulsator)", What is that and where is it??? And any other thoughts? Can a bad oil switch cause the fuel pump to run slow?
Submitted: 5 years ago.
Category: Chevy
Expert:  Carhelp4u replied 5 years ago.
If you are getting 12 volts at the fuel tank, I would not figure that the oil pressure switch would have anything to do with this. My main concern here is that you got the correct pump. If by chance you were sold a fuel pump for a 4.3Z instead of the 4.3W you would have this problem. The fuel pulsator is a device that was put into the fuel tank on the pressure side of the pump. This was designed to take the shock of the pump pressure to make the pump last longer. They leak and allow for fuel to be dumped right back into the fuel tank. Here is what I recommend. The first thing I would do is check to verify that you got the correct pump for this engine. Then I would verify that you are getting 12 volts to the gray wire at the fuel tank sender. I would also evaluate the fuel tank ground to make sure that this is clean and tight. If you are getting 12 volts to the tank, we would really need to look at the chance of this being a pulsator problem, or even a fuel problem at the intake end of this. Both the regulator and injectors can cause for a low pressure problem if they are failing. Feel free to contact me with any findings! good luck!
Customer: replied 5 years ago.
According to the specs for the assembly, I purchased a complete unit, pump, sending unit ect., the unit was for 4.3 W fuel injected and the 4.3 Z fuel injected
Expert:  Carhelp4u replied 5 years ago.

The 4.3 Z is only throttle body injection. It requires somewhere around 13psi (I did not check, just an estimate) where the 4.3W model uses 55psi+. There is a big difference there for sure. Here is the actual testing for both model. The 4.3Z is first and then the 4.3W.

graphic

graphic

Customer: replied 5 years ago.
My pump psi is 20 with the key on and engine off. Could low volts to the pump be the problem, the ground is clean and checked, I have not checked the volts but with the relay disconnected and dumping fuel in the intake the engine runs but not the pump which suggest the bad oil pressure switch the switch should run the pump after reaching approx 3psi oil pressure, could the bad switch lower the volts to the pump?. It would seem that one assembly could not work for both W & Z
Expert:  Carhelp4u replied 5 years ago.
Try disconnecting the oil pressure switch all together. This is not a safety device, this is just a backup to the fuel pump relay. The oil pressure switch is wired in a way that if the fuel pump relay fails the switch will still supply voltage to the pump as long as oil pressure is found. So if the oil pressure switch is the problem this would eliminate it. I could see a problem if the switch was grounding this circut but I would figure this would pop a fuse.
Expert:  Carhelp4u replied 5 years ago.
Another thing you may want to try. It sounds you have a fuel pressure tester. If you cycle the key from off to on, you see your 20psi, how fast does the fuel pressure drop after the pump stops running. If it drops quickly this would indicate a problem within the mechanical end of the fuel system rather than electrically.
Customer: replied 5 years ago.
The pressure holds within a couple of lbs, I don't think there is a leak inside the plenum. I will disconnect the switch tomorrow and give it a try, if the pressure is still low I will pursue the specs for the pump to make sure it is capable of producing the required psi, if not out comes the tank and assembly, ugh, start from scratch. thank you for your time
Expert:  Carhelp4u replied 5 years ago.
No problem. Let me know how you make out if you would not mind. I honestly cannot say I have ever seen an oil pressure switch create this, but I guess it is possible. Thanks!
Carhelp4u, ASE CERTIFIED TECH
Category: Chevy
Satisfied Customers: 5203
Experience: ASE CERTIFICATION
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