Fuel Pressure Test Q - This is what I found on the pump

[nvrenuf]

I currently have an OE type TBI pump, return style regulator and q-jet.

I’m getting a low pressure reading on my gauge and need to verify if it’s the pump or regulator.

If I bypass the regulator and connect the feed and return with a gauge fitting, will I get an accurate pump pressure reading? This would be a loop running from the pump to the engine compartment and back to the tank, there would be no load on the system.

 

[nvrenuf]

As I think about it I'm wondering if instead of a a straight run joining the in & out lines, should I use a " T " fitting and put the gauge opposite of the feed and connect the return to the 90° branch so the gauge directly feels the pump pressure. Any thoughts?

 

[Justin V]

Think of your garden hose. If you turn it on, will no nozzle or anything, you get tons of flow but not much pressure. Kink that hose and the pressure builds.

If you want a a good pressure reading, you need to have a backstop in the system.

 

[Justin V]

Leave the return out of it.

 

[nvrenuf]

So should I just dead head the feed line into a fitting with the gauge? (no return)

 

[sweetk30]

Yep need to test as if it was running . No restrictions no true test results.

 

[nvrenuf]

Ok, no return. I wasn't sure if that might build a false higher pressure reading.

 

[Justin V]

Before you do anything, pinch the return hose with the truck running. If your pressure builds (TBI pump should be able to hit 20+ easily), then it is the regulator most likely. If the pressure remains the same, it is likely the pump. Remember that it could be the short rubber hose inside the tank attached to your TBI pump.

 

[nvrenuf]

Yeah, I've read about the issues with the little hose. I really can't see something going wrong with the regulator, no moving parts but there is a little diaphragm in there. The regulator is off of my old K10 from way back so it's been sitting for 10+ years. The pump (Delco) is about 4 years old with very minimal run time but I know that doesn't mean much with electronics.

 

[Justin V]

Was thinking that there is the possibility of a clogged filter as well. If you determine it is a output problem, not a regulator problem, put a new filter first. You'll want a new filter with a new pump anyway, so do the easy thing before you pull the pump.

 

[rampage]

Clogged filter would be my first thought too. Easy and cheap things first.

 

[sweetk30]

most people forget to change fuel filters .

i just did one for a guy and he said the truck is like new . it will pass a vehicle in 1 gear drop now over the 2 gear down shift it took before .

 

[nvrenuf]

Well, I think I need a regulator. With the key on (motor off) I have 3 psi, if I pinch the return line the pressure jumps to 9 psi. While at both 3 and 9 psi, there is no change when I adjust the regulator.

I’m kind of baffled as there aren’t any moving parts in the regulator so I don’t know could have failed unless the spring on top of the diaphragm is too weak.

Top side of diaphragm, dry.


Bottom side with ball that seats in outlet.


Top cap with adjuster screw and spring.


Gauge I made to screw on to the feed line to the carb.

 

[rampage]

Again, how do you know that the filter isn’t clogged?

 

[nvrenuf]

I guess I really don't but if it were restricting flow I wouldn't think clamping the return would have an immediate jump, maybe a slow build of pressure?

Fwiw, the filter is about 4 yrs old but only have maybe 200 miles on it and the tank was clean when installed.

 

[rampage]

So you’ve gone through less than a tank of gas in 4yrs? Any chance of varnish buildup?

 

[nvrenuf]

Yep, the truck has been sitting in my shop since Nov '15 when I initially cut the roof off. This past weekend was the first time it's been on the road since but it has been running and pulled out of the shop occasionally.

There could be varnish but there were no signs of it inside the regulator and the carb seems to run fine.

 

[1-ton]

I may be wrong but, if you take a pressure reading before the regulator you are getting pump pressure. If you take a pressure reading after the regulator you are getting fuel system operating pressure.

 

[nvrenuf]

So I thought it was fixed but apparently not....

The idle pressure is really good and steady at 6 psi but I noticed today while driving that during normal driving it stays at 5-5.5 psi. If I accelerate hard it steadily drops to as low as 2 psi. It might drop more, that when I quit pushing it.

So how do I test this?

- TBI style tank and pump
- new AC Delco pump about 4 years ago with minimal run time
- factory frame mounted filter replaced with pump
- stock feed and return lines
- new Holley regulator
- quadrajet carb

 

[Bent77]

Replace the filter and check the rubbers on the pump output