fuel pump question

[dheavychevy38]

So I replaced the fuel pump in my 87 k5 with a tbi. The pump I used is a 60 psi pump but I am only getting 15 psi at the filter location and 10 psi at the rear of the tbi. Am I missing something ? Shouldn't I have 60 at the filter location and what ever the regulator puts out at the back of the tbi ??

 

[Bent77]

So I replaced the fuel pump in my 87 k5 with a tbi. The pump I used is a 60 psi pump but I am only getting 15 psi at the filter location and 10 psi at the rear of the tbi. Am I missing something ? Shouldn't I have 60 at the filter location and what ever the regulator puts out at the back of the tbi ??

If you measured the pressure deadhead, those would be normal pressure numbers. Does it run bad still?

Typically i like to see a tad more pressure at the regulator, but my motors are not stock.

 

[dheavychevy38]

It runs better but still would like to see it in the higer range of the tbi specs seeing as the pump is spec higher..

 

[bp71k5]

The stock regulator won't go much above 15psi. You can upgrade the spring to get around 18psi.

 

[dheavychevy38]

That i understand but if im still only geting 10 at the tbi would that mean the regulator isn't good ???

 

[Bent77]

It does make me feel inclined to pull it apart and have a look

 

[bp71k5]

Maybe I don't understand how you're measuring it, but the return line won't have much pressure. The actual pressure is measured at the inlet to the throttle body. I have read the regulators may not have been super accurate from the factory so 10 isn't that far out of the range. The regulator could certainly be an issue. You can change the diaphragm and convert it to an adjustable one at the same time. It's not difficult, but it won't adjust much over ~15psi.

 

[dyeager535]

The test for regulator is *supposed* to be pinching off the return line and watching pressure to ensure the pump can attain the PSI it's rated for, but I don't think that is a good idea with the braided lines, at all.

Testing TBI and TPI both at the fuel filter, I saw spec pressure. As I shimmed the stock TBI regulator spring I did all my testing at that location and could see the difference in pressure. Without being able to pinch the return line though, it could be a pump or the regulator if pressure is low.

Unless the pump is bad (or leaking in the tank) I don't see how it can be anything but the regulator.

Think of the regulator as a calibrated fuel leak. If pressure goes under whatever its set for (GM spec is 9-13PSI) it shuts, creating a "closed" fuel system that the pump is forcing fuel into. Once pressure reaches the regulators set point, the regulator leaks fuel back into the return line. Obviously it's not an on/off/on type deal, but you get the idea. This is how the fuel pressure is (theoretically) maintained at a constant whether the injectors are wide open under full throttle, or barely injecting fuel at idle. Little fuel is bypassed into the return line under heavy throttle, and much more is bypassed under light throttle conditions.

 

[bp71k5]

The test for regulator is *supposed* to be pinching off the return line and watching pressure to ensure the pump can attain the PSI it's rated for, but I don't think that is a good idea with the braided lines, at all.

Testing TBI and TPI both at the fuel filter, I saw spec pressure. As I shimmed the stock TBI regulator spring I did all my testing at that location and could see the difference in pressure. Without being able to pinch the return line though, it could be a pump or the regulator if pressure is low.

Unless the pump is bad (or leaking in the tank) I don't see how it can be anything but the regulator.

Think of the regulator as a calibrated fuel leak. If pressure goes under whatever its set for (GM spec is 9-13PSI) it shuts, creating a "closed" fuel system that the pump is forcing fuel into. Once pressure reaches the regulators set point, the regulator leaks fuel back into the return line. Obviously it's not an on/off/on type deal, but you get the idea. This is how the fuel pressure is (theoretically) maintained at a constant whether the injectors are wide open under full throttle, or barely injecting fuel at idle. Little fuel is bypassed into the return line under heavy throttle, and much more is bypassed under light throttle conditions.

I have a retrofit system so I'm not sure where the stock filter is. But anywhere between the pump and the regulator outlet (return line) should all be fuel pressure I'd think? Where's the filter on a stock system?

 

[dyeager535]

Your pressure would be anywhere BEFORE the regulator. After it will not be pressurized under any conditions I can imagine.

Stock fuel filter is inside the passenger frame rail halfway back. With stock fuel lines, either there or where the fuel line goes from frame rail to flexible lines are my preferred places to hook up the pressure tester, as they are much easier to access and you aren't potentially going to cross thread the fitting on the back of the TBI.