Contour Enthusiasts Group Archives Forums General Troubleshooting Fuel Pressure -- AGAIN

Does anyone have the Powertrain Control/Emissions Diagnosis Manual? The service manual refers to this for diagnosing the fuel charging and control system.

I'm trying to figure out if this is normal: With my fuel pressure gauge attached, I see a spike to 39psi when the pump engages during key on, but then falls to 35psi when it shuts down after 1-2 seconds.

With the car running, fuel pressure is around 28psi, which I think is WAY too low. A blip of the throttle gives a spike up to around 40psi, but immedately falls back to 28. If I gradually raise rpms, fuel pressure stays at 28psi.

Also, what kind of voltages are to be expected at the fuel pump? I am trying to determine if there is a problem with the pump or if this is all normal. HELP!

-Lance

Lance,

What you are reporting sounds normal.
Fuel delivery is controlled by the duration of the enabled injector. Since our engines are SEQUENTIALLY injected, we should not require a high fuel pressure.
On a MultiPort injected Engine, Fuel pressures are about 30 - 35 PSI.
Throttlebody injected engine have sustained fuel pressure af abut 17 - 20 PSI.
This sounds low, but the throttlebody injectors are very large.
A small PSI is required with a larger injector.

As vacuum drops (open throttle) your fuel regulater restricts return flow to keep the pressure stabilized. That is the spike you are seeing. When you release the throttle, vacuum spikes, causing the regulator to release the fuel pressure, allowing the additional pressure to flow back to the return line.

Are you viewing this with a fresh Fuel Filter ?

Pete...

This all sounds quite normal to me. When you KOEO (Key On, Engine Off), the pump runs for a few seconds to pressurize the line in preparation to start the engine. If you are noticing an immediate pressure drop when the pump stops running, and that pressure drop continues and does not level off very quickly, I would suspect the fuel presure regulator.

The fluctuations you are observing in fuel pressure when "blipping" the throttle vs. slow and steady increase in rpm are normal also. Fuel pressure in your car is inversely vacuum regulated. The less vacuum, the more fuel pressure (greater engine loads cause less vacuum, hence the need for greater fuel pressure under load). The FPR acts as a valve for the return fuel system and is opened/closed by manifold vacuum. When you "blip" the throttle, the engine experiences a "load" as it tries to overcome the inertia of the crank and everything attached to it as the engine quickly increases in RPMs. This causes a large dip in manifold vacuum (hook up a vacuum gauge to a port directly into the upper intake plenum after the throttle body) which in turn causes the FPR to close the return path for the fuel. Bingo! There is your pressure spike, and is quite normal.

Now, when you gradually increase engine RPM, the sudden "load" never occurs, and hence there is no manifold vacuum dip, and hence no pressure spike. If you could introduce a load on the engine while increasing RPM, you would see the fuel pressure increase proportionally. HTH



PS, Fuel pump voltage should be the same as battery voltage. You need to measure the amperage to see if it is excessive.

RoadRunner
'CORRECTED' Secondary Stop Screw Adjustment
DID you try cracking the secondaries and didn't like the results?
Thanks Paul

Ok, makes sense.

However, I'm only seeing 9V on the fuel pump harness on one wire, and 9.8-12V on the other, depending on when the key is turned. I think it puts in 12V when the pump is active and 9.8V when idle.

Just wondering if the other one with a steady 9V is normal.

-Lance

Oh, and the 28 psi still concerns me, since the service manual says the normal pressure is 37-41 for key on engine running.

-Lancd

variation in voltage is how computer controls pressure. Decrease voltage, motor turns slower, pumps less fuel, pressure drops.

9v is likely the preset voltage value used during times when extra pressure is not required.

as for low press. check the regulator first, but I think that it may be the motor, especially if your car is high mileage. The motor is DC so it has brushes, and if you have high miles then the brushes are worn, therefor their pressure on the commutator is less and the brush bounces allowing for arcing between commutator blocks thus lowering operational efficency of the motor causing a lower RPM at 9v and therefor lower volume output, and with the resriction the same (press. reg.) pressure will be less.

But then again the regulator should compensate, unless the motor's performance has degraded to a point where the pressure regulator has compensated as much as possible and the regulator needle is at its seat and cannot close more allowing for further compensation for motor performance degredation. Which makes sense that the regulator would not have the ability to close completely, becaue if it did it would block flow and engine would stall, but if it has a limit as to how far it can close then if it were to fail and the electric pump were still operational the engine would still run, but not be able to produce it max power output due to lack of fuel.

you should have 12v at the pump at all times I had this problem on another car and it was a bad earth at the LH side of the rear panel in the lugage compartment. BTW check the earths behind the LH headlight as this can cause all sorts of weird problems. It rains a lot in Scotland !

Yeah, I am also under the impression that in Fords, the only thing that controls fuel pressure is the FPR, and that there should not be any variation in the voltage to the fuel pump except on/off. anyone clarify this one for sure?

Info I have at the moment Chiltons for all models 95 the PCM shows no involvment with the fuel system (except vapor purge). 96-97 PCM shows pins 80B/BL & 40PK/B go to Fuel pump relay. 98-99 pins 54 B/BL & 40 V/B goes to fuel pump module and 62 PK/B goes to fuel tank pressure sensor (Mitchell's says it's transducer and gets involved with ATX Park position, MTX with clutch switch and both go back to PCM (Gist only). 96-99 pin 40 monitors the voltage to the fuel pump, of course the inertia switch is in series with 95-99 and the fuel pump. ALL is 2.5l info Zetec similiar.
At the fuel pump Power: 95-97 should be battery voltage.
Fuse 15a no number? (got to love Chilton's?) to inertia sw and on to fuel pump all PK/O wires Ground at fuel area. (Check for clean ground connections)
98-99 Fuel Pump relay/Fuse f14 15a V/O to inertia sw V/O to fuel pump and ground at fuel pump area. My 98 after just two years was very rusty. I cleaned up and coated w/ Krylon's "Crystal Clear Acrylic Coating" 1301. I use on all grounds, power even battery connectors and all my ham radio outside grounds and antennas. Does good job slowing corrosion. Nil.
Whew! Paul
Need any explains don't hesitate I'll do my best.
Chilton's one note 35-40 psi. under normal conditions and 30 psi idle or high vacuum.
I have access to Mitchell's through my local library also Alex Peper OBD II guru his first was Mitchell's for what it's worth!