Why does my fuel pressure vary so much?

[Dave]

I know fuel pressure will change with the air density and such, but c'mon by 15psi??? I like it around 48-50psi static, but today after hamerring on it, I get home, pop the hood, and the static pressure was at 33psi. No wonder it felt slow. I have the old style FM FPR unit, is that quite possibly the problem, or does everybody experience this?

PS: I just went out and started it, now the static base pressure is at 55psi. Does it change with the engine's heat?? WTF? lol

Thanks guys,
-Bryan

[tryingbe]

Do you have a stock fuel pressure regulator you can use for testing?

[MiniMopar]

Your fuel pump is getting tired. Sounds like classic thermal failure of the pump. You can use Simon's pressure test to verify (pinch the fuel return line).

[turbovanman²]

Or the reg could be on its way out, if pinching the return line yields around 90-100 psi.

[Dave]

I have a brand new 255 Walbro pump, and I've hardly ever ran it into the point of the fuel light coming on. The regulator is fairly new, but again it's old, being the FM style unit.

So basically fold the return line in half causing it to pinch and see if my pressure goes to 90-100psi? My gauge is only good to 60psi but I'm sure I'll see it overshoot it.

[turbovanman²]

I have a brand new 255 Walbro pump, and I've hardly ever ran it into the point of the fuel light coming on. The regulator is fairly new, but again it's old, being the FM style unit.

So basically fold the return line in half causing it to pinch and see if my pressure goes to 90-100psi? My gauge is only good to 60psi but I'm sure I'll see it overshoot it.

Use some pliers to pinch the line. If your guage doesn't read 100 psi, the test won't work.

[Dave]

Damn. Who uses a 100psi gauge on a normal bases!?!?! lol

[Una]

TD owners do.. heh.. 55psi base + 20psi boost = your gauge is useless.

[turbovanman²]

TD owners do.. heh.. 55psi base + 20psi boost = your gauge is useless.

Exactly,

[Dave]

But mine goes to 60psi.... that's all the higher I need. Anymore than 50psi I become way too rich and if starts to pop once I give it gas because it floods the chamber.

[Frank]

Uhh??? If you are running 55psi base pressure and you hit 20psi of boost, that is 75psi of fuel pressure.... how can 60psi be ok? My guage goes up to 130 or something like that.


Frank

[MiniMopar]

Typical cheap air compressor guage is 160psi. In any case, you should be able to made the needle wrap and peg against the stop by 0psi. Most guages can handle this OK.

Anyway, it doesn't matter. You are seeing 33psi static, over 20psi low. If it is a fuel flow problem, pinching the return hose will do nothing...or nearly nothing. If you see the pressure fly up, then you can save your pennies for a new FPR with confidense.

[gkcooper]

Gauge pressure = PSIG
uncorrected pressure = PSI

Our cars are rated at a nominal 55 PSIG. This means that it is 55 PSI as compared to the manifold, not ambient atmosphere.

Greg K. Cooper
Huntsville, Alabama

[Dave]

Okay I have my gauge mounted right next to the cam gear. Why would I need any larger gauge? I set the base static pressure to 48psi, plug the line back in and it usually goes to 54psi.

I'm not watching it rise as I hit the gas... really I can't.

[Una]

I have a nice 6" around one with a nice long hose. It goes to 100psi.. I don't set my fuel pressure, the stock regulator does. So it's 55psi base. Anyways, I tape it to the windshield and go for a drive. To make sure I have a good fuel pump, I find a nice hill to climb in a high gear. Fuel pressure should be 55psi+boost.. So at 20psi, that's 55+20, so it should be 75psi..
If your base fuel pressure is less than 55 because you turned it down, what's the point of having bigger injectors? My custom cal is adjusted for the bigger injectors, so I don't have to turn down the pressure, I get the actual rated flow out of them that way.

[Dave]

Una, I always thought the reason for larger injectors is for more fuel. Of course we have to turn the pressure down. Simon's the person who actually reccommended to me to start at 50psi and work from there, if I remember correctly.

[Una]

They're rated at a certain pressure. Our stock +40's are 52lbs at 55psi.. Less pressure = less flow.. Yeah, sometimes the custom cals from our vendors aren't quite right, and need to have the pressure turned down to come into line with reality. Just letting you know, they aren't really +40's when you have the pressure way down to start with..

[turbovanman²]

Okay I have my gauge mounted right next to the cam gear. Why would I need any larger gauge? I set the base static pressure to 48psi, plug the line back in and it usually goes to 54psi.

I'm not watching it rise as I hit the gas... really I can't.

Because you can't diagnose a 100 psi fuel system with a 60 psi fuel gauge,