2005 df115 random unable to start

[greatestfisherman]

i am troubleshooting a inttermitten issue... any advice is much appreciated. I have a motor that starts and runs most of the time perfectly fine... occasionally, after a hard run and stop to fish, i am unable to get it to start... i turn the key to on and do not hear the priming of the high pressure pump... it cranks and tries to start but as if their is no fuel pressure to the injectors... tow it home and test it and it primes fine afterward and starts like a new motor... i have replace everything is the VST tank and even confirmed the 12v when key on... i suspect issue to be related to some sensor or maybe even the ECM (hopefully not)... hopefully someone can help me better understand this motor...

I understand the pump gets grounded by the ecm for maybe 3 secs when the key is on positiion... and when running/cranking, gets grounded to adjust for the fuel pressure... their has to be some fuel pressure sensor or crank sensor that the ECM uses to determine when to ground... I checked my service manual and parts lists... only sensor i found is a MAP sensor... manifold air pressure sensor... is this what the ECM uses to determine when to ground the high pressure pump when cranking/running???

thanks in advance...

[greatestfisherman]

finally found something that might be what im looking for... CKP sensor under the coil is responsible for signaling the ECM when the motor is cranking/spinning and when to ground the pump and ignition coil for spark...

assuming thats all good, can some answer this other question... Does the high pressure pump ALWAYS PRIME ON KEY ON POSTION AT STARTUP or does it not prime when pressure in the rail is already in spec? I dont always hear the priming and just wondering maybe its by design...

thanks again.

[redlowrey]

finally found something that might be what im looking for... CKP sensor under the coil is responsible for signaling the ECM when the motor is cranking/spinning and when to ground the pump and ignition coil for spark...

assuming thats all good, can some answer this other question... Does the high pressure pump ALWAYS PRIME ON KEY ON POSTION AT STARTUP or does it not prime when pressure in the rail is already in spec? I dont always hear the priming and just wondering maybe its by design...

thanks again.

Yes it does, it will stay on for about 3 seconds and if the computer does no receive a tach signal it will remove the ground. When you turn the key on, the main control relay will energize and supply battery voltage to the pump and other devices, so battery voltage and current will flow too and through the pump back to the computer to ground for about three seconds, full ground, when you turn the key to the start position, the ground will still be at 100%, when the engine starts the computer will duty cycle the ground depending on engine load and rpm.

You may not have a pump problem, if it is only doing it after a long run, it may be a hot soak problem, when you check the pump circuit out also check the AIT sensor voltage, and the temp sensor voltage with the key on and come back with the voltage readings, don't disconnect the sensors.

[greatestfisherman]

AIT sensor, is that air intake temp sensor and where is this sensor located? I see a map sensor on the intake manifold... i was only able to locate 2 temp sensors... one top and one on the side next to the rectifier... how do i measure the voltage without disconnecting the sensors? i apologize if i come off as an idiot but i just want to make sure im understanding everything...

thanks again

[greatestfisherman]

figured out the AIT is the sensor connected to the intake (black plastic intake)...

[greatestfisherman]

just had a break through..... went to test the motor and its worked fine... ran it... stop and go and eventually was able to duplicate the random issue... immediately hooked up meter and confirmed 12v present at key on at the pump but pump not priming..... jump ground to pump and can hear pump trying to prime... sparks as i ground the black/white wire with the sound of the pump quickly whining and stopping a split second... after attempting to fully ground the pump to keep it priming, was able to build enough pressure in the rails to start the motor... once motor started, it ran fine... stop and go again and again... no issues... i have inspected my vst several times and its clean as a whistle... gas is fresh no debris... new filters low and high pressure... safe to say pump is the culprit... works most of the time and occasionally does not... new pump ordered... stay tune...