Intermittent Failure to Start
 

I have a 2004 Ranger FX4 with the 4.0, 304,000 miles on it.

About 8 months ago I had a fail to start that was initially diagnosed as the fuel pump, not surprising considering the miles. Pump was changed and off I went, driving home I stopped for gas. Went to start the truck after fueling, and it failed to start. As my mechanic thought their might be an issue with the inertia switch, I checked that. It wasn't tripped but after pushing the button a few times, it started right up.
Since then I have continued to experience intermittent failures to start, sometimes more then once in a day, sometimes weeks between. Every time that it happens, if I depress the button on the inertia switch and turn the key, the truck starts right up.

I have: replaced the fuel filter and the fuel relay, had the fuel injection system cleaned, cleaned the intake manifold with the seafoam spray, fixed a sm vacuum leak (can't remember where that was), installed a new inertia switch.

I went 2 months since the swapping of the inertia switch, which was the last work I did on it, and this morning, no start. Pushing the switch, it fired right up. To me it seems obvious that the switch mush have some factor in the no starts.

Not sure if the mechanic swapped the inertia switch wiring harness. When it fails to start it cranks over nicely. Rarely when starting it does take longer then usual, sometimes 2 attempts to get her started and often a near start on the first attempt, but usually it cranks over like always. Except of course when it fails ;)

Any thoughts on where I should look next? Inertia switch wiring harness, PCM? I know that the inertia switch should never fail but its odd that in every no start event, pushing the button lets it fire right up.

Welcome to the forum

Not Inertia switch issue, never was, not sure what else you are doing to get to, or push, that button?

So what happens is this:
When you turn the key on the computer runs PATS key test, if key passes test computer closes Fuel Pump Relay for 2 seconds, If Anti-Theft Light flashes rapidly then PATS is your problem.

So with key on you should hear the fuel pump HUMMMM for those 2 seconds if you listen for it
You can turn key off and on and fuel pump will run for 2 seconds each time you do, doesn't hurt anything to do this

But what else happens is that the computer also has a wire on the inertia switch, when it closes the fuel pump relay it monitors if the inertia switch gets the 12volts for those 2 seconds.
If it doesn't then computer will turn on CEL(check engine light), or will turn on a dash light indicating "Check fuel cutoff switch"
Since you didn't mention CEL or codes, I think inertia switch is OK and getting 12volts for pump

So lets narrow down the field a bit
Listen for fuel pump, turn key off and on and listen, it is not quiet, you can hear it in the cab, get used to what it sounds like
Then at the next no start listen.
lets see if it is a fuel pump no start issue

Your 2004 needs about 25psi fuel pressure to fire up, normal running pressure is 60psi
And after shut off pressure should stay above 45psi, for MONTHS, not minutes, hours or days, months and months.
Each time key is turned on that 2 seconds of fuel pump run time = about 15psi of added pressure.
If fuel system was leaking, say a crack in the hose in gas tank, then there would be no smell of gas, but you would have 0 psi pressure after key was off for awhile.
And a no start on first crank with only 15psi
If you turned key off and then pushed inertia switch button a few times
Then turned key on again, you would now have 30psi so engine could start
key on and off 2 times

So weigh that scenario to what you were doing, to see if anything matches up


Crank sensor starts the ball rolling for spark and fuel injection, computer has no connection to starter motor, it has no clue you want to start the engine.
When the crank sensor starts sending out AC Voltage(crank is spinning) is when computer knows to start spark and fuel injectors.
Crank sensor is less likely to fail than inertia switch, lol, and there is also the Cam sensor.
Cam and crank sensors would both be sending signals when engine was turning, so if either one was not working or was out of "time" then computer would turn on CEL and set code accordingly.

One test you can do, when there is a no start, that can be helpful
Carry a can of Quick Start and screw driver
When you have a no start open the hood
Pop the top off air cleaner
Spray Quick start(ether) into the engines air tube
Try to start engine
If it starts up then fuel was the problem, injectors or fuel system
If it still doesn't start then Spark is the problem
Called the 50/50 test, instantly eliminates 50% of possible problems, lol

Thank you for your information, I will get a can of ether and try that and also mention your reply to my mechanic.