I have a 1997 f150 with 4.6 that won't start it will crank but has no fire. I have checked the fuses in the fuse box in the engine compartment they are all ok, but they all are hot all of the time even with the key off.
I have had the ign switch replaced not too long ago. should these fuses all be hot with the key off? if the pcm relay was bad would it cause this?.how do I check if the pcm is powering up?

 

Here's how I'd troubleshoot:

1. Check fuel pressure.
2. Check spark.

 

The coils and injectors are hot only when the ignition is On or in crank/start.
When ignition is turned On, the fuel pump should run about 3 seconds then stop.
This indicates the PCM has also powered up because the fuel pump relay is controlled by the PCM 'TIMER' 'the short run time'.
Does the fuel pump run at all?
Is there fuel pressure?
Is the fuel,pump fuse open if there is no pump run time? Is the crash sensor tripped?
Is there spark? There will be spark at cranking because the Crank sensor detects the crank rotation, signals the PCM, fires the coils and >>through<< the camshaft sensor, fires the fuel injectors.
With all this you should be able to see what is missing and look for the cause.
.

 

i had the same issue. It took me 18 months to get it figured out. My issue was the engine wire harness. I had it in many shops and no one could figure it out because most so called mechanics are now parts changers due to the codes it throws. I finally took it to a real mechanic with 45+ years of experience. It took him less than a minute to figure it out and fix the problem. He shook the wire harness saw the ark fixed it. Been running great since.

 

when it does not start the pcm has not given the injectors the 3 seconds of fuel,pcm did not power up. if I give the engine a shot of starting fluid it will start and run now pcm is powered up(strange). I have to change the pcm & fuel pump relays with no chance of what is going onI have clean the ground at firewall by the battery, it was ok.
I don't understand. what makes the pcm power up after I give the engine starting fluid to get it running.

 

Think this out.
A. You provided combustible fuel in the intake air stream >> in place << of fuel injection.
B. Fuel injection will not take place until the engine cranks to provide spark and fuel injection. If there is no fuel, the injectors still operate but with no fuel under pressure to offer.
B. You provided the temporary fuel source.,
C. Once cranking begins, the Crank Sensor provided a signal to the PCM to sync the spark fire order and the injector fire order >>and re-operate<< the fuel pump as long as there is 'no fault'.in the pump circuit.
The engine runs until the added fuel you sprayed in has depleted.
.
The above tells you, there is no fuel under pressure to feed the motor.
Let everything else alone but fix the fuel system.
You have proven the motor will run when it has fuel to burn.
..... Test fuel pressure at the test port with a gauge. Cycle the ignition On/Off several times and then read the gauge. It needs to be in the 35 psi range.
If none, the pump is not running, the pump may be faulty, pump relay faulty, power to the relay missing, crash sensor has tripped or is faulty.
......PCM ground to the pump rely can be tested with a meter to see if that short time function is missing when the ignition is turned on.
If pressure is too low, the engine will not run very well or long. Fuel pump faulty, filter plugged, wiring etc.
If too high, the return line is restricted or the fuel regulator is faulty.
There is always the possibility of two or more faults at the same time.
If you clear one fault and still not run, think a second fault may be present or you made a mistake somewhere.
Good luck.

 

sorry for the late reply been gone. my help me out and next time it would not start we checked the fuel psi while cranking it was 35 psi it had no spark to the sparkplugs. we installed a new pcm & fuel pump relay and it still did not start. we let the truck sit for about 30 min and it started right up. checked fuel psi with it running it was 58 psi. I have changed the fuel filter about 3 weeks ago so it should be ok.I am going to change the crank sensor to see if it help. at a loss

 

58 PSI is way too high. You probably need to replace the fuel pressure regulator. Normal range is 28 to 40, it should go up 5 to 10 when the vacuum line is removed.