Primary pressure line

[Bitsyncmaster]

With our fuel gauge test set they read primary pressure and control pressure from the center hose (using a shut off to the WUR). Well I'm thinking I need to monitor primary pressure when driving (still have a problem with my car). Do I get primary pressure from the main fuel line from the accumulator? Or at the pump itself would be the same.

[Ron]

I would think anywhere between the pump and FD would be good.

[David T]

I would think anywhere between the pump and FD would be good.

Yes, if all you want to see is primary pressure. Just make triple sure that nothing is leaking when you hook up your gauge. Do NOT count on hose clamps, use the proper fittings, you are dealing with a lot of pressure.

[Bitsyncmaster]

Well I got a pressure transducer and set it up to read pressure at the fuel pump outlet with a "T". This transducer runs with 5 volt power so I'm using a USB outlet to power it. I tested it with air pressure from my compressor.

50 PSI = 2.40 volts
60 PSI = 2.76 volts
70 PSI = 3.16 volts
75 PSI = 3.34 volts

OK great but with my engine running I'm reading 3.96 volts which I compute would be about 90 PSI.

Guess I need to put a real pressure gauge on my setup to believe that pressure is or is not 90 PSI.

[Bitsyncmaster]

Put a standard pressure gauge on my setup and it reads 83 PSI engine running. Shutting the engine off it instantly drops to 30 PSI. Now this testing is before my check valve (I have a Tahoe fuel pump).

This pressure gauge and my air compressor gauge are not high quality gauges so accuracy is in question.

Now I will do the standard fuel pressure test at the FD center hose.

[Bitsyncmaster]

I think I know what the problem is. The pressure from the fuel pump is in pulses and my transducer is responding more to the high pressure pulses. Even the standard pressure gauge is reading higher because of those pulses.

I should have connected after the fuel accumulator to read an accurate primary pressure value. It's just that I did not want to cut those SS fuel lines. Not sure if I could get longer banjo bolt to add another banjo at the fuel distributor in line.

Anyway I can use what I have to determine if primary pressure is dropping under load (car accelerating).

The fuel accumulator does have two functions. One is to hold rest pressure. The other is to smooth out the fuel pump pulses. I plan to keep the accumulator after going EFI just to reduce those pulses.

[FABombjoy]

Have you verified your USB supply voltage quality on a scope? Most of them are not well regulated.
I've personally had trouble over the years attempting to use USB as reference-level 5v causing all sorts of trouble:

Some older examples:
https://www.righto.com/2012/10/a-dozen-usb-chargers-in-lab-apple-is.html

[Bitsyncmaster]

Have you verified your USB supply voltage quality on a scope? Most of them are not well regulated.
I've personally had trouble over the years attempting to use USB as reference-level 5v causing all sorts of trouble:

Some older examples:
https://www.righto.com/2012/10/a-dozen-usb-chargers-in-lab-apple-is.html

Well I checked the reading with and without the engine on and the reading did not change. The USB I'm using says 5.2 volts, it plugs into the cig lighter. When I did the test with the air compressor I used the same setup.

[Bitsyncmaster]

I moved my test setup to the other side of the check valve thinking that would read 75 PSI since the accumulator should be taking out pump pulses. But I still read high about 90 PSI volt reading 3.88 volts. I wonder if that same pressure is at the inline at the FD.

Pressure does drop to 40 PSI when the engine is shut off however it drops more at first and then slowly comes up to 40 PSI.

I guess you have to read primary pressure (for accuracy) with the normal gauge setup.