Christophe2m6

Hi guys!

Thinking about installing a Fuel Pressure gauge in my 69 Supreme, 350ci 4bbl - auto trans.

Can I install this gauge with an inline gauge BEFORE the fuel pump, so on the plastic fuel hose that goes from the fuel filter to the fuel pump, between them?

Will it read approx the same pressure as if it had been installed between the pump and carb?

As I've seen that fuel pressure on a 69 350 should be 5,5/7 PSI, I will use a 15Psi gauge, right?

Thanks for your help!

MDchanic

If you install it before the pump, it will read in negative numbers.

- Eric

67442nut

joe_padavano

To clarify, the line going into the fuel pump is a SUCTION line. There is no pressure to measure. If there were, you wouldn't need a fuel pump.

Koda

I wonder if he could install it facing backwards. It'd read as pressure then.

Fun71

I question the need for a dedicated fuel pressure gauge. Check it with an external gauge to see what you pump does for a reference point then remove it and don't worry about monitoring it on a continual basis.

redoldsman

There is no way I want a plastic fuel line running inside my car.

oldcutlass

I would not want a fuel pressure gauge inside my car period.

redoldsman

I would not argue with that. I have one that came with a set of gauges I bought and it is still up in the attic.

67442nut

Why not mount it under the hood near the carb like the racers do.
If you suspect a fuel pressure problem, simply raise the hood to check it.

midrange

I need to check my fuel pressure in order to see if the 'fall off' I have over 4800-5000 rpms is from low fuel pressure.
Where, or how, do I install a gauge without it being in the passenger compartment, but yet still see it when I'm test driving the car?
I've thought about temporarily zip tying it to the cowl screen, but not sure if that is a good idea either.

oldcutlass

Run it out from under the hood so its visible through the windshield temporarily.

MDchanic

Yeah, you'd rather have gas spray all over your windshield than all over your lap in the event of a failure.

The consequences of one could be bad.
The consequences of the other could be very bad.

- Eric

BlackGold

Before you g to the trouble of hooking up a fuel pressure gauge, you've got to ask yourself what you're going to do with the information. Don't get me wrong; more info is better. But it's not going to tell you a lot.

Under wide-open throttle, you'll probably see the pressure drop to 1 or 2 psi because the needle and seat will be completely open. That's fine, as long as the flow rate is adequate to fill the float bowl. Just like with your engine's oiling system, pressure is not what's important -- flow is. Pressure is a result, not a cause.

Let's say that under WOT you see the fuel pressure drop to zero. What have you learned? Maybe the pump's bad. But more likely there's a restriction somewhere or it's sucking air somewhere. So you're still stuck with tracking the problem down.

MDchanic

Sorry for the quibble, but no way, José.

It is slowly being realized in most fields that more information can be a bad thing.

It can trigger unnecessary concerns and diagnostic tangents, and waste tons of time and money.
It can also cause preoccupation with some detail (like water temperature or oil pressure) which can make you unhappy, and generally wreck your enjoyment of your car.

Just enough information is a good thing. I try not to seek out more information if I don't have something specific that I'm diagnosing.

- Eric

oldcutlass

It's diagnosing a symptom. More information is good as long as you know what your looking at and how to interpret it. While I agree sometimes ignorance is bliss, it doesn't apply to car engines.

Koda

Pardon me, but how is fuel pressure going to tell you anything about your fall off?

Fuel injection pressurizes the system and the injectors are just valves that open and pressurized fuel enters the airflow and atomizes. In a carburetor, your fuel pressure goes against the needle being held closed by the fuel bowl float. As that float drops, fuel, under pressure, leaves the needle seat, and then drops to zero pressure and sits in the bowl until needed. It is then sucked out the jets by the vacuum made by the venturis in the barrels, and other idle and off idle and accelerator pump systems, all running off various vacuums and pressures.

My point is that the fuel pressure coming out of the pump is something the rest of the carb after the fuel bowl never sees, unless your float malfunctions and you pressurize the bowl by overfilling it, which will then leak. What you should be concerned with is the volumetric flow rate of the fuel pump versus the flow rate of the fuel through the carb at wide open throttle. If the fuel pump cannot push enough fuel to feed the engine at WOT, your fuel bowl will become depleted and the car will run out of gas.

A bog is not indicative of running out of fuel. I have a car that was not run beyond around the block to warm it up for years. The secondaries were non functional. It would not go beyond 65, or when you punched it. It did not bog, it bucked, which is to say it repeatedly surged until you let off. It sounded like a guy revving a car in a parking lot vrooom...vrooom....vrooom. I believe it was leaning out and losing combustion and then picking back up.

90% of carb problems are ignition problems.

oldcutlass

A bog can also be indicative to much fuel pressure and over powering the needle and seat and flooding the carb. Fuel flow can be figured using pressure and the size line. The question was how to hook it up not why.

midrange

Thanks guys, I never gave it much thought about that fact that at WOT, the pressure would be low. I was only looking at the pressure end of things since a friend of mine asked if I knew what the psi was when it 'fell off'.
It makes much more sense to me now that it's more likely a volume problem rather than a pressure problem.