Can a weak fuel pump really cause misfire?

Joined
Aug 12, 2015
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785
Location
Central NY
2014 F150 5.0 V8 119k miles

-hard to start first thing in the morning. Long crank, sputters, eventually fires up.
-after it starts the first time it’ll start perfectly as long as I don’t let it sit for more than 12 hours
-PO300 multiple cylinder misfire CEL(sometimes no CEL, just vibration at idle) occasionally but not every time
-exhaust smells like unburnt fuel for the first couple of seconds after long crank

Work around is to leave key in the “on” position for a minute or two then it’ll fire up fine even after sitting overnight. Otherwise runs beautifully once started up.

Battery is new. Fuel pump module is new. Pulled fuel line at the rail(no port to test fuel pressure) and put key to on position - nothing came out besides a drop of gasoline even though it’s supposed to be running 60 psi. Generic code reader showing 0 short term fuel trim. Not sure if the code reader just doesn’t work for Ford or if it actually is reading 0. Freeze frame data showed -0.8 long term fuel trim during my last PO300 CEL.

Everything is pointing to a bad fuel pump(original). But before I drop the tank is there anything else I should look at? Really dreading the fuel pump job because it just snowed and below freezing temps are forecasted for next week.

Coils and spark plugs are not Ford OEM so they were replaced at some point. Unsure of brand or when it was done. I just know they aren’t factory. No vacuum leaks either.
 
The fuel system is bleeding down. I don’t know if the fuel pump in that application has a check valve that has failed or what but somehow the fuel system is not holding prime.

Ok that’s what I figured too. Fuel is draining back into the tank plus the pump isn’t supplying gasoline quick enough to start up. It’s been getting worse and worse. 5 days ago it would start if I left it on for a minute. Now, it’s more like 5 minutes otherwise it’ll sputter.

I’d hate to drop the tank, replace the pump, then find out it’s not that and it’s like a leaking injector or just a bad ECU.
 
Do you have an 327elm obd2 dongle? My 07 f150 has a pressure sensor in the fuel rail that can be accessed with the forscan app. Idk if your generation of f150 has that sensor.

I don’t but I was just looking at getting forscan lite and ordering a BT OBD reader. The 12th gens have a sensor too. I guess I’ll go ahead and do the pump, if it still has a starting issue I’ll buy a set up to run regular forscan.
 
A marginally operating fuel pump can cause a lean miss but other things can too.If you prove the pump is the problem, it's easier to access the pump by unbolting the bed and either slide it back or remove it depending on the tank's location. The check valve is integrated in the pump. Don't skimp on the pump or you likely will be doing it over again.
 
If the pump was the problem it would have more than just a dribble or if wouldn't start at all? Maybe it's sensing the system isn't sealed and cuts fuel when you try to test it?
Another possibility could be coolant temperature sensor failing when cold although it probably would have set a code. Maybe leaving the key on lets have coolant sensor warm up a bit?
Still very well could be the pump but I'd want to know for sure before changing it.
 
C'mon man.... automotive forums are full of owners dumping wheelbarrows of parts into a vehicle then starting a thread on cel's or running issues.

I'd make sure all your 'software' is up to date. Means a trip down to a dealer, or Ford competent shop, to verify any ecu tsb updates for errors, drivability concerns.... If none, since its a pickup, take it down to the local chinesium parts store and load the bed up.

Genuine Ford or aftermarket fuel module?

Leaky injector, bad sensors, bad coil(s), or electrical glitch are worth troubleshooting. Get that fuel PSI prior to replacing pump. Otherwise, it becomes a shotgun repair.

 
Check your fuse 27 in the underhood fuse block to see if you are getting some melting of the fuse holder which could cause high resistance which could cause you to not get full power back to the fuel pump. Ford has made a fuse relocation kit to address this issue. Eventually the fuse starts to blow and leave you stranded. My fiancé's son's 2014 F150 with the 5.0L has experienced this fuse blowing but he just replaces the fuse when it blows instead of fixing it correctly.
 
I'd think it would be pretty obvious, but how about connecting a fuel pressure gauge to the system to see where it's at?

The newer 5.0s don’t have anywhere to attach a gauge to. Fuel pressure reading is shown through OBD2 but requires ford specific software to read it.
 
Check your fuse 27 in the underhood fuse block to see if you are getting some melting of the fuse holder which could cause high resistance which could cause you to not get full power back to the fuel pump. Ford has made a fuse relocation kit to address this issue. Eventually the fuse starts to blow and leave you stranded. My fiancé's son's 2014 F150 with the 5.0L has experienced this fuse blowing but he just replaces the fuse when it blows instead of fixing it correctly.

I checked fuse 27, it has a new fuse with no issues but is still at the original location. I will install the relocation kit once the weather gets nicer.
 
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The newer 5.0s don’t have anywhere to attach a gauge to. Fuel pressure reading is shown through OBD2 but requires ford specific software to read it.

The scantool I bought at Harbor Freight for $60 a couple of years ago can show fuel pressure on Ford vehicles. I think it's a standard OBD-II pid.
 
If the pump was the problem it would have more than just a dribble or if wouldn't start at all? Maybe it's sensing the system isn't sealed and cuts fuel when you try to test it?
Another possibility could be coolant temperature sensor failing when cold although it probably would have set a code. Maybe leaving the key on lets have coolant sensor warm up a bit?
Still very well could be the pump but I'd want to know for sure before changing it.

I decided to try leaving the key in for 5 minutes today. Same sputter/long crank, although tapping the gas pedal real quick fired it up.

MAF is reading 5.xx g/s which is about right.
 
The scantool I bought at Harbor Freight for $60 a couple of years ago can show fuel pressure on Ford vehicles. I think it's a standard OBD-II pid.

Do you remember what model is it? They’ve got a couple in the $60-70 range.

I’ve got an innova scanner from 2013 and a newer foxwell nt501 but its running BMW software. Never owned a Ford before so this is all new to me.
 
The newer 5.0s don’t have anywhere to attach a gauge to. Fuel pressure reading is shown through OBD2 but requires ford specific software to read it.
Then how do you verify the fuel pressure sensor is accurate? You can't tee in between the supply line and the fuel rail? Might need some adapters but hard to believe there isn't a way.
 
Do you remember what model is it? They’ve got a couple in the $60-70 range.

I’ve got an innova scanner from 2013 and a newer foxwell nt501 but its running BMW software. Never owned a Ford before so this is all new to me.

They no longer have the model I bought a couple of years ago, but the closest to it is this one:


The model I got WILL give me fuel pressure readings on my 2004 Crown Victoria.
 
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