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Question DetailsAsked on 9/23/2017

my truck starts but then just dies. new fuel pum was put in

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So many possible causes - here are some I have seen, in no particular order except first one - of course see if any codes are popping using a code reader, though many times the codes are confusing or sometimes just dead wrong. That is a characteristic of failed fuel pump - will sometimes repeatedly start on the fumes but not get enough gas to run continuously.


0) out of gas - so is starting on fumes but no liquid to speak of to fuel the engine at normal idle speed


1) defective or not completely wired up fuel pump - or wiring to/from it is damaged so it runs intermittently. Will not necessarily throw a code.


2) vapor recovery canister is full of liquid or clogged so fuel pump is trying to pull against liquid, effectively the opposite of a vapor lock, where the pump is trying to pull a cvacuum on the tank so fuel flow stops after it has run a touch. Should throw a code but doesnot always.


3) this is a nasty one to track down - fuel cap sensor or vapor recovery canister sensor is faulty, so it is registering a cap off or canister fault - can happen in the case of an overly sensitive sensor as it gets older, where it will read only after a partial vacuum is put on the system but the fuel pump pulling fuel out of the tank. Took me a couple of days to find this one because the code it was throwing for a "cap off sensor" error said the transmission shift solenoid was bad - not even close !


4) if your vehicle has a separate fuel injection pump, that could be the problem


5) if you have central coil (as opposed to coil-on-plug on each plug) then a short in the high voltage lead or coil wire or defective coil can do this - work OK at startup but arc out when it is up to full operating voltage and operation. Coil test should check this.


6) vacuum leak on intake manifold or any of the hoses leading to the carb/throttle body - lets car start but the leak causes excessively lean mixture so it will not run - vacuum pressure test would find that


7) control board issue


8) low voltage in system - can cause failure of ignition when engine is running, but provide enough energy to start - simple voltage check should find that, plus would probably be cranking slow or hard


9) throttle body or air filter clogged up so much it is blocking most of the airflow - starts on the air sitting in the air intake system but does not get enough air through the filter, so it ckokes out


10) timing problem - at certain timing offset (due to jumped timing chain/belt) can start but not run, or at least not run well


11) camshaft or crankshaft sensor failure


12) this one was a real headscratcher (must explain my bald spot, right ?) for awhile once until I started tracing electrical function - an ignition switch that is engaging correctly when turned on, but then creeping from ON to OFF position, killing the engine. In my case was one of the one generating recalls (but occurred before the recalls startd) where the ignition switch turns itself off if you have a bunch of keys on the key ring


13) another head scratcher because you cannot test for it till the car is running, but if it dies bedfore you can test ... - a faulty diode in the alternator causing an electrical fault in the charging system - starts OK off battery but once the alternator started charging it shut down the computer because of the radically incorrect voltage - nd did not throw codes because it did not run long enough to generate a code


14) one i helped a neighbor track down - a defective fuse failing to burn out so a dead short - in the brake pedal position sensor in this case - so when he started the truck in park it started and idled OK, but touch the brake pedal and the shorted sensor shorted out the entire control board system and tripped the control board breaker - which auto-reset in a few seconds so would do this again and again. Found this one by accident by putting foot on the brake while trying a start so it did not even start that time.


15) water in fuel - blocking filter/inlet when engine is running but settles down and lets a bit of gas through (floating on top) at start


16) blocked fuel filter or lots of debris or sludge in tank - lets a bit of gas in but once it starts pulling it pulls up the debris or sludge and kills it - though more usually this makes for uneven run and missing rather than total dying i the engine will initially start. Hopefully tank was checked for debris/water when the fuel pump was replaced, but some vehicles the fuel tank does not have to come out to change the fuell pump so maybe not so in your case


17) another stumper that kept me busy for quite a few hours (mostly on my back in the snow and mud at a remote site) - at least some fuel pumps/floats (like some lighting systems) use a live wire to the device, but use the frame as the ground / neutral - so if ground/return lead from the tank pump (assuming in-tank fuel pump) / float gage to the frame did not get reconnected, will not work. Engine might start on fumes only. If electric fuel pump you should be able to hear it start to run when you turn ignition switch one to ON position without turning to START.


Probably others too - with shop computer and exhaust sensor in place a mechanic would be able to eliminate many of these by checking starting pressures, whether exhaust is putting out raw gas or not, whether timing is right, etc. Then a bit of process of elimination, if system is not throwing off correct error codes.


Hope you find the flaw quickly - I have heard of $1000+ bills for diagnosis for odd electrical faults like this might be, especially when they start changing out boards because they cannot find the fault and it is not throwing off reliable codes. Good Luck.



Answered 1 year ago by LCD




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