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73-87 Chevy _ GMC Trucks => Engine/Drivetrain => Topic started by: Chuck Step-a-side on December 06, 2017, 06:24:33 PM
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I have a 1984 Chevy Stepside with a ’87 centerbolt 350 and edelbrock carburetor with saddle (duel) fuel tanks and I have some concerns about my fuel pressure being at .5 psi. The truck runs fine down the road, but it idles a little on the rough side at a stop. I know that a mechanical fuel pump psi should be between 7 an 9 psi. I have replaced the fuel pressure gauge with another pressure gauge and it read the same at .5. My next thought is a faulty fuel pump and I’ve replaced it with no change to the psi even after switching from one tank to the other. My only option now is to replace the fuel filters that I have installed at each gas tanks before the fuel tank selector switch. I don’t remember if there were any fuel filters originally at the tank and I may have installed the Fram G15 3/8” can type filters as a prevention from any particulars from the tanks before gas flows to the tank selector switch (as recommended by the tank selector switch manufacture) and now I’m debating whether they’re clogged or they shouldn’t be in the gas lines at all. Can anyone tell me if there are any fuel filters on the ‘80s Chevy trucks and/or your own experience with the duel fuel tanks repairing and fuel filter. Thanks, Chuck
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How many lines go to your fuel pump, 2 or 3?
If it's three, pinch the smaller return line and see what the pressure is then. You need a point of resistance to create the rated psi.
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You think the gas pressure is bleeding through the vapor line, correct? I'll try that and get back to you this weekend. Thank you,Chuck
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From the factory there are screens in the tank and a filter between the pump and carb. The tank selector valves were problematic, that may be your problem. Try bypassing it or run a hose from a can to the pump.
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When I purchased my 1984 Silverado truck it came with an '87 centerbolt 350 engine and an Edelbrock carb and manifold. The EGR valve and charcoal canister was removed and the steel gas vapor line located on the driverside chassis by the radiator was left open. Since then I had replaced the fuel pump and the gas tank selector switch with the same factory switch for dual saddle tanks which included the vapor return lines outlets. I have recently installed a fuel pressure gauge near the carburetor and my pressure is reading at .5 psi (it should be between 7 and 9 psi). The truck idles a little rough, but drives fine. I was told to pinch the vapor lines to see if there any difference in fuel pressure and there is none. Can my fuel pressure be low because the vapor line connection hoses are old and cracked? Should I capped them off and close the vapor line circuit? I'm open for any suggestions.Thanks, Chuck
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What are the vapor lines connected at both ends.
Vapor lines are 5/16 which have nothing to do with pump pressure.
Do you have 1/4 return lines also. These are tied into the 3rd smallest outlet on the pump.
Should have more than .5, gauge correct?
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From what I've seen when replacing the tank switch there are 2 -3/8" gas lines for the tanks and then there's 4 other smaller lines (vapor and return) coming from the tanks and going to the front end of the truck. I'll need to do a thorough check on the gas lines to find what is going where. I do know that I have replaced all the rubber hose connection for all the 3/8" gas lines from the tanks to the carburetor. I did not do any changes to the vapor or the return lines.
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Remove the filters from between the tanks and pump. Perform a fuel pump evaluation using a proper test gauge. While cranking, the fuel pump should draw >20 "Hg of vacuum, deliver 1+ quart of fuel in 30 seconds, and build ~7 PSI of pressure. How many hoses/lines connect to the existing fuel pump?
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Removing the filters was going to be the next thing to do. I'll need to drop the tanks this weekend. There are 3 hose hook up onto the fuel pump. Thanks
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Gas pressure is the wrong terminology, just getting that out of the way. Fuel pressure is the term to use, but with that being said you need to be more concerned with fuel volume than fuel pressure in a carbureted system unless of course it is excessive pressure.
So if you think your problem is fuel delivery or starvation, do a volume test from each tank as bd pointed out and ditch the extra restrictions. One filter is sufficient after the pump. What valve manufacturer did you use? If they require more than oem to protect their part, they are just covering their rear or their parts are garbage.
Is your whole reason for looking into this simply a rough idle?
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It boils down to the fuel pressure reading and rough idle. I believe both are related. Moving down the road the engine runs fine. The filters were recommended by the Pollack fuel selector valve manufacture; one (Fram G15) for each tank before the switch. I will remove those filters this weekend and see if that makes a difference. Thanks
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Did you do a pressure check? I've seen fuel lines in the past with a pinhole in the top of the steel line along the frame, that doesn't leak but allows air to get in. You could blow air thru the line and see if you hear any leaks. Hope that helps