Fuel Pressure Testing questions

My 1990 still will not start.

If I were to get a fuel pressure testing guage and hook it to the Schrader valve and just hold the key to start so the engine turns over and over and over, will that give me an accurate reading to the fuel pressure in the rail?

Thanks

Cottontail said:

My 1990 still will not start.

If I were to get a fuel pressure testing guage and hook it to the Schrader valve and just hold the key to start so the engine turns over and over and over, will that give me an accurate reading to the fuel pressure in the rail?

Thanks

Click to expand...

Yes

Yes, but you don't even have to go that far. Hold the gage in one hand, turn the key to ON with the other (or have a helper do it.) The fuel pump runs for 3-5 seconds without a crank to pressurise the system. You should see, as I recall, ~39psi without vacuum at the regulator (~31 w/vacuum at idle.)

5-90

5-90 said:

Yes, but you don't even have to go that far. Hold the gage in one hand, turn the key to ON with the other (or have a helper do it.) The fuel pump runs for 3-5 seconds without a crank to pressurise the system. You should see, as I recall, ~39psi without vacuum at the regulator (~31 w/vacuum at idle.)

5-90

Click to expand...

Just remember that if the check valve is leaking down but the pump is good, you might have to cycle the key two or three times to get the pressure up to 39 after it's sat for a while.

I know this is old but i have a few related questions.

What does the regulator do - I ask this becuase if i idle the engine and connect or disconnect the vac line to the regulator the engine tone doesnt change ?

My vacuum pressure isnt very stable meaning on a guage it moves back and forth (enigen on-idle). When i connect a fuel gauge and idle the engine, the fuel gugae also moves a bit around.

My question is, is the fuel press problem due to a problem with vacuum or the opposite ?

Maybe someone can clarify this for me.


pete

The fuel pressure regulator (when rail-mounted and subject to manifold vacuum) is there to maintain a fixed pressure drop across the injector for consistent fuel delivery.

The fuel pressure regulator has been deleted on several more recent fuel injection systems, allowing a slight variation in fuel delivery. Therefore, the systems are designed to allow for an adjusting variation in pulse width (narrowing the pulse width helps to make up for increased pressure drop across the fuel injector,) but leaving the regulator in place tends to simplify programming the fuel delivery curve.

5-90

Ok let me ask this then....If i have a vacuum problem that causes a guage needle when placed to move back and forth, how does one know if this is being casued by the fuel pressure or actaully bad vacuum? The way i look at it they are both related, so how can one distinguish a problem with on from the other ?

A bad vacuum supply can cause irregularities within the regulator causign the presure to wander up and down a bit. On the other hand a flawed fuel pump can cause a unstable pressure and cause the engine to create a wandering vacuum ?

Pete

Unplug the vacuum and see if fuel pressure is a steady 39#

On the Renix system, putting a jumper wire between Diagnostic Connector D1-5 (+12v) and D1-6 (fuel pump +) will result in a constantly running pump (for test purposes). Fuel pressure should be 31psi at idle with the vacuum line connected to the fuel pressure regulator and 39psi with the vacuum line disconnected.