97 F150 4.3L. When try to start, it fires a few times and stops. New fuel pump, fuel pressure good, injectors removed and tested good. If you spray fuel in the intake with a spray bottle, the engine will run. With out that, it fires and stops...a fuel problem. Could the PCM be the cause of this...

 

You say that adding some combustable to the intake allows the motor to run.
Would not that indicate a lack of fuel?
Would you not check fuel pressure as the next thing to do?
If no pressure, fuel pump relay, fuel pump, regulator, crash sensor etc would be suspect.
The PCM would almost never be a cause.
Good luck.

 

Fuel pressure good. New fuel pump. Injectors taken out and cleaned. Have a good spray.

 

Fuel pressure needs to be in the 35 psi range.
Are there any codes set?
The cam sensor drive times fuel injection.
Fact that the motor will run indicates the PCM is good.
Does the pump run for a couple seconds when the igntion is tuned to run only?
It's still fuel related.
Good luck.

 

Depending on some details, the PCM COULD be involved.

The PCM turns on the fuel pump for those initial 2 seconds. Then it turns it on continuously once it decides that the engine is running. I'm not sure what signal it uses to decide that the engine is running - possibly the cam or crank sensor coming up to speed.

So, if the fuel pump doesn't come back on, the PCM might be at fault. It's possible that it can't decipher the engine start signal, or else that output is internally fried.

If the pump dpes come back on, the PCM should be pretty much out of the picture, given that it will run with a human fuel pump .

Good Luck,

hj

 

How the pump is controlled at start and cranking:
At inital PCM power up, the PCM Timer ckt grounds the lead to the pump relay running the pump until the PCM timer times out in about 2 seconds+/-.
At cranking the PCM detects crank rotation from the front Crank Sensor and regrounds the fuel pump relay via the internal timer.
After the pump relay is the main power feed from a fuse through the crash sensor then back to the tank.
As long as the crank is turning the pump relay is grounded to operate.
The crash sensor is there to open the main pump feed in case of hard forward hit to prevent the pump from feeding a possible fire if there is still power to the pump for any reason.
If the pump runs at inital key to Run, the circuit is good from/through the PCM.
The same internal PCM control timer runs the pump under both inital power up and cranking/running..
Good luck.

 

Will it run if you depress the pedal a snitch, to eliminate an IAC problem?

 

Ok, was using a pressure gauge from a friend. It was faulty. Went out and bought one. When I turn the key on, the pressure goes to 30lb. and with in a few seconds drops back to 0. Turn key off and back on, same thing happens. Is this a sign of a bad pressure regulator or could it be something else.

 

Usually it's the pump because it is supposed to hold pressure due to a one way check valve.
The check is not holding.
I would consider changing both due to age.
Good luck.

 

Fuel pressure should hold for a few minutes after
key is shut off

 

Is this what it did before you changed the fuel pump?

 

Is the check valve inherent to the pump or seperate. Just for info. I wonder what you mean by "both".....thanks..

 

The check valve is inside the pump.
It's there to hold pressure over the short term to make re starts faster without a lot of cranking and to begin to build pressure at first key on.
The regulator brings down the high pressure of the pump to the 38 psi range the rated flow rate of the injectors.
The fuel that is excess is returned back to the tank through a return line.
All injectors are rated for flow at 38 psi pressure.
At reduced pressure the injectors don't flow as much fuel when the PCM opens them on each pulewidth.
As well, if the flow volume and pressure is not up to spec, each time any injector opens that are all on common fuel rails the rest see the reduced pressure because the total system can't keep the pressure high enough fast enough.
The pump is capabile of as high as 100 psi.
Regulated down to 38psi insures the all the injectors on the rails don't go without enough pressure to meter fuel the correct amount that is commanded by the PCM..
As RPM rises, this becomes even more critical so the total system is overbuilt to handle all fueling requirements from starting, idle and high HP/RPM as long as there is no fault present..
Good luck.

 

Nice....