Can a weak fuel pump really cause misfire?

[avacado11]

You said that you bypassed the FPDM but did you also try bypassing the fuel pump relay? If you don't have a wiring diagram, I would get a few days access of Alldata so that you can look at diagrams and see what the theory of operation is for the fuel system. A wiring diagram will also see if there are any connectors that are in between the FPDM or fuel pump relay that you can access to check for power and grounds before you drop the tank. Do you know if the FPDM or the fuel pump relay is last in line before going to the tank?

My main concern at this point is either there’s damage somewhere in the harness not giving the pump 12v, or the pump is dead. Going to test voltage at the fuse and FPDM first, although it’s gonna be hard as I don’t think there should be 12v going to it after a few seconds. According to forscan the fuel pump doesn’t even turn on until the starter is cranking.

The truck also has an aftermarket remote start system. Could possibly have something to do with that depending on how it was wired up. But, no clue on that part yet.

 

[wtd]

My main concern at this point is either there’s damage somewhere in the harness not giving the pump 12v, or the pump is dead. Going to test voltage at the fuse and FPDM first, although it’s gonna be hard as I don’t think there should be 12v going to it after a few seconds. According to forscan the fuel pump doesn’t even turn on until the starter is cranking.

The truck also has an aftermarket remote start system. Could possibly have something to do with that depending on how it was wired up. But, no clue on that part yet.

This is why knowing the theory of operation of the fuel system is important. Does it only give a 2 second prime on time when the key is turned on like a lot of vehicles or does it not turn on until the starter is turned on like Forscan is suggesting.

That is why having some factory service info is important. If the FPDM or the fuel pump relay is not functioning, you will not have 12 volts at the pump. You need to figure out if these two components are functioning like they should be before trying to find a problem in the wiring harness. You also need to find out if your grounds to everything is good. The first thing you should be doing is verifying good power and grounds at all of the components in this circuit.

 

[avacado11]

This is why knowing the theory of operation of the fuel system is important. Does it only give a 2 second prime on time when the key is turned on like a lot of vehicles or does it not turn on until the starter is turned on like Forscan is suggesting.

That is why having some factory service info is important. If the FPDM or the fuel pump relay is not functioning, you will not have 12 volts at the pump. You need to figure out if these two components are functioning like they should be before trying to find a problem in the wiring harness. You also need to find out if your grounds to everything is good. The first thing you should be doing is verifying good power and grounds at all of the components in this circuit.

Dropped the tank this afternoon and tested the fuel pump connector. The connector gets 12V for 2 seconds then drops to 5-6V, which from what I read is normal. Everything else with the electrical system is connected(using old OEM FPDM). I did get a service advance trac after cycling the key without the pump plugged in - but I believe VSC is tied into pump to cut power to the engine. Not really what I’m worried about it’s a problem for another day.

I directly ran 12v to the fuel pump using my Milwaukee M12 battery. The pump works(unlike the old one I pulled out) but the pressure is so-so. The fuel comes out like an open water hose. It doesn’t spray out at what I would consider 60psi - unless it counts on the injectors being closed so the line pressure builds itself up. Can’t seem to find any sort of data that would verify this.

I will test and inspect ground points tomorrow. Spent about an hour and a half just dropping the heavy tank then pumping out 30 gallons worth of gasoline.

The only thing that has me thinking it’s the pump, or at minimum something really stupid, is that when this current pump went in the vehicle had cold soaked for 2 days prior. It started right up(despite also being below freezing) and the line held pressure afterwards. There’s no way a ground decided to break after just sitting for 12 hours.

 

[350Rocket]

I purchased forscan lite for my phone. Fuel pressure is showing anywhere from 59-60psi but nothings coming out. Not getting any fuel trim data as it’s not starting at all. Basically useless for me right now. MAF was showing 2.84 g/s despite not even cranking key. My friend has an ‘11 F150 5.0 and I was comparing data with his. His MAF showed 0 g/s until the key is turned. So, logically I replaced the MAF sensor thinking it’s not reading correctly. No luck. His fuel pressure was also showing 59-60psi.

Pulled the fuse box to inspect wiring and install a factory TSB for fuel pump fuse relocator - all is good. Haven’t tested voltage running to the pump yet but I’ll have to drop the tank to get to that connector. I ended up bypassing the FPDM and still had no luck even seeing fuel pressure come out of the tank. Swapped both new dorman and old OEM FPDM with no difference

since it’s too cold to be doing anything outside I decided to tear apart my old fuel pump(also a Delphi unit) to see how it works. Submerged the pump in water. Turns out, when I send 12v power directly to the pump it whirrs but nothing comes out of the output hose on the pump. The pump currently in the truck is also a Delphi unit. Since it’s Christmas Eve I decided to run to Autozone to pick up a spare fuel pump so I have one on hand tomorrow incase I find out it’s a pump issue(it also is a Delphi, all they have in stock). I hope this $300 pump is good.

At this point I highly suspect the pump is dead(I’ll test voltage to the connector first). I purchased it off Rock Auto. Funny part is, they retail for about $180 on their site right now. I only paid $48 2 weeks ago and it was pretty late at night. Possibly system error? No clue. But the Delphi pump(same part #) I got from Autozone is fully wrapped up in the box. My pump from Rock Auto was not packaged the same way despite being the same part. I almost feel like the Rock Auto one is a fake Delphi or was at minimum defective, tested, and resold.

Regardless, when that rock auto pump originally went into the truck it held fuel pressure overnight. I had to drop the tank a second time to get the filler neck back on and when I popped the fuel line off it sprayed gas everywhere. It doesn’t do it anymore.

Definitely sounds like a defective new fuel pump, especially considering that last paragraph.

 

[350Rocket]

Correct, there may be some sort of safeguard built into it to cut pressure if it detects no load. Sucks that Ford switched to a non-return fuel system. Everything has to be read through the ECM.

But, if that was the case wouldn’t the line still be pressurized if I cycle the key with the line hooked up, then popped it off? I mean when I cycle the key then pop the line, nothing comes out.

Yes you're right.

 

[brianl703]

The fuel comes out like an open water hose. It doesn’t spray out at what I would consider 60psi - unless it counts on the injectors being closed so the line pressure builds itself up. Can’t seem to find any sort of data that would verify this.

This is normal. The pressure won't be as high as 60PSI with the line open. EDIT: I want to say the flow rate when I had the pump in my Crown Vic running and emptying into a gas can was less than 1 gallon per minute. I didn't measure it with a stopwatch, though.

 

[weaponoffreedom]

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.

if fuel psi is the issue, then it is fuel filter or pump system..............you fuel trim changing could be from a number of thing such as a faulty MAF or IAT or MAF or hell, the computer...no telling without dealer equipment of good tech who knows the ins and outs of that particular vehicle.

 

[avacado11]

So while I wait for my new OEM pump to arrive I decided to go under and check/clean all of the grounds. Also replaced the Dorman FPDM with a new Ford OEM module. The grounds all look good.

Decided to test voltage at the pump connector again - doesn’t get 12V like it did yesterday(despite not touching a thing since). Tested the ground by connecting the black prong to the battery negative and sticking the red prong into the pump connector ground wire. 0.00V - which is good? I read that anything more than 0.05V on a ground means it’s bad. Anyways with my black prong still grounded to the battery using a set of jumper cables, the power line on the connector showed 0.00V when I turned the key. So I ungrounded the black prong and stuck it straight into the ground in the connector. All I got was up to 6V then dropped down to 0 after the 2 second window. This would explain why the fuel pressure is all jacked up. The pump isn’t running at full power.

I trace the wiring all the way to the PCM on the firewall. What I can see looks fine except after sticking a camera under the intake manifold there’s a old rodents nest in there. And the harness running from the PCM to the rear of the vehicle…is right next to the nest. I highly suspect the harness is damaged right there(all relays/fuses are new) The manifold is coming off tomorrow so I can inspect the harness and hopefully repair it.

Funny thing is I just repaired the coilpack wire in the fuse box yesterday after seeing it was all chewed up by a rat. The **** thing chewed the wire all the way up to the fuse box so there wasn’t enough line left to repair. Ended up running an add a fuse kit and rigging up the wiring to work since adding a new fuse connector was not a possibility.

 

[wtd]

So while I wait for my new OEM pump to arrive I decided to go under and check/clean all of the grounds. Also replaced the Dorman FPDM with a new Ford OEM module. The grounds all look good.

Decided to test voltage at the pump connector again - doesn’t get 12V like it did yesterday(despite not touching a thing since). Tested the ground by connecting the black prong to the battery negative and sticking the red prong into the pump connector ground wire. 0.00V - which is good? I read that anything more than 0.05V on a ground means it’s bad. Anyways with my black prong still grounded to the battery using a set of jumper cables, the power line on the connector showed 0.00V when I turned the key. So I ungrounded the black prong and stuck it straight into the ground in the connector. All I got was up to 6V then dropped down to 0 after the 2 second window. This would explain why the fuel pressure is all jacked up. The pump isn’t running at full power.

I trace the wiring all the way to the PCM on the firewall. What I can see looks fine except after sticking a camera under the intake manifold there’s a old rodents nest in there. And the harness running from the PCM to the rear of the vehicle…is right next to the nest. I highly suspect the harness is damaged right there. The manifold is coming off tomorrow so I can inspect the harness and hopefully repair it.

Funny thing is I just repaired the coil wire in the fuse box yesterday after seeing it was all chewed up by a rat. The **** thing chewed the wire all the way up to the fuse box so there wasn’t enough line left to repair. Ended up running an add a fuse kit and rigging up the wiring to work.

Did you have the circuit loaded when you did the voltage drop test on the ground side of the pump connector? Key on, engine off? 0.00v would be a good ground if it was loaded. I've been taught that anything over 0.10v is a bad ground depending on the type of circuit you are testing.

Voltage drop testing on the positive side would be having the red lead of the multimeter on the battery positive terminal and the black lead on the power side of the connector with the key on, engine off. Reading should be as close to 0 as possible to indicate little to no voltage drop.

 

[avacado11]

Did you have the circuit loaded when you did the voltage drop test on the ground side of the pump connector? Key on, engine off? 0.00v would be a good ground if it was loaded. I've been taught that anything over 0.10v is a bad ground depending on the type of circuit you are testing.

Voltage drop testing on the positive side would be having the red lead of the multimeter on the battery positive terminal and the black lead on the power side of the connector with the key on, engine off. Reading should be as close to 0 as possible to indicate little to no voltage drop.

Circuit was not loaded. Just a jumper cable straight from the negative post on the battery to the black prong on my volt meter. I will try it with the key on tomorrow and see what I get.

According to what I read(general fuel pump diagnostics) the pump ground is tested with the key off and the pump power is tested with the key on and should read 12V. Both times the black prong is grounded on the negative battery post. Maybe I did it wrong. Not too familiar with how to test electrical problems. But regardless only intermittently seeing 12V at the pump is my issue now. Atleast a bad FPDM is out of the picture. I mean the only thing left is either the wiring harness or the ECM is corrupted.

It’ll get new injectors anyways since it has to come off for the intake manifold removal process. Might go ahead and do manifold gaskets too.

Will post pictures of what I find

 

[wtd]

Circuit was not loaded. Just a jumper cable straight from the negative post on the battery to the black prong on my volt meter. I will try it with the key on tomorrow and see what I get.

According to what I read(general fuel pump diagnostics) the pump ground is tested with the key off and the pump power is tested with the key on and should read 12V. Both times the black prong is grounded on the negative battery post. Maybe I did it wrong. Not too familiar with how to test electrical problems. But regardless only intermittently seeing 12V at the pump is my issue now. Atleast a bad FPDM is out of the picture. I mean the only thing left is either the wiring harness or the ECM is corrupted.

It’ll get new injectors anyways since it has to come off for the intake manifold removal process. Might go ahead and do manifold gaskets too.

Will post pictures of what I find

All voltage drop testing has to be loaded. No load equals no voltage drop. A voltage drop test is to determine if you have any voltage loss in a circuit.
For example, say you are voltage drop testing the positive post on your battery to the battery clamp for the positive side and you read 2 volts. That means you have a 2 volt loss from the post to the cable end which is no good and shows you have resistance at the connection.

Have you tested the fuel pump relay?

 

[avacado11]

All voltage drop testing has to be loaded. No load equals no voltage drop. A voltage drop test is to determine if you have any voltage loss in a circuit.
For example, say you are voltage drop testing the positive post on your battery to the battery clamp for the positive side and you read 2 volts. That means you have a 2 volt loss from the post to the cable end which is no good and shows you have resistance at the connection.

Have you tested the fuel pump relay?

I have not tested relay yet, mainly because I haven’t looked into the relay diagram or looked into how to test relays. But, the relay is new ford OEM, and I swapped it out with the blower motor relay with no difference. So I think it’s safe to say it’s not that.

So ideally I should be seeing as close to 0V as I can with the voltage drop test on the power supply for the pump?

 

[wtd]

I have not tested relay yet, mainly because I haven’t looked into the relay diagram or looked into how to test relays. But, the relay is new ford OEM, and I swapped it out with the blower motor relay with no difference. So I think it’s safe to say it’s not that.

So ideally I should be seeing as close to 0V as I can with the voltage drop test on the power supply for the pump?

Yes, as close to 0V as possible.

 

[wtd]

I saw this video and it's something you can check out. It would be reason #2 in your case since you already did the fuse relocation repair.

Supposely the Body Control Module controls the FPDM and is powered by fuse 18 behind the kick panel where you will check for reason 2 in the video. I doubt that is the problem but you can check it while you are down there.

 

[350Rocket]

I saw this video and it's something you can check out. It would be reason #2 in your case since you already did the fuse relocation repair.

Supposely the Body Control Module controls the FPDM and is powered by fuse 18 behind the kick panel where you will check for reason 2 in the video. I doubt that is the problem but you can check it while you are down there.

I could see reason number 2 being a big problem for me as a land surveying party chief, I drive my own truck but frequently have a helper with me and they tend to be hard on the truck. We're in and out of it all day long. Hinge pins have been replaced twice on my 2005 Silverado in 235k miles.
I'm curious if this problem was fixed in the 2015+ body style? Our company owns 2 of those trucks. I'm the only one driving a personal truck.

 

[zzyzzx]

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.

Auto parts store rental tool?

 

[UncleDave]

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 hardest diagnostic challenge on my titan was a failing fuel pump that passed pressure and volume tests when cold, but failed when hot causing misfires that didnt trip any codes. The problem only manifested itself when towing.

 

[zzyzzx]

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

ZR8s OBD2 Code Reader with Live Data

Amazing deals on this Zr8S Live Data Obd2 Code Reader at Harbor Freight. Quality tools & low prices.

www.harborfreight.com

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

I would think that any scan tool that does live data would show fuel pressure. If not Forscan.

 

[wtd]

I think this thread has jinxed me. Yesterday my fiancé's younger son calls and says he is broke down. His truck won't start.

I go and pull the truck home. It's a crank, no start. I put a fuel gauge on it and it has 0 fuel pressure but does have fuel at the Shrader valve. When you turn the key on you can hear the pump prime if you are under the truck but no pressure.

We put a new Delphi pump in this truck a little over two years ago and about 11,000 miles ago and a new fuel filter.

I did some testing today and it has all of the correct inputs at the fuel pump relay and I swapped in the relay out of my 98 Chevy truck which uses the same relay but no luck. I can see the ground wire on the frame at the back of the truck and using a power probe, it shows a ground on the eyelet but I never could get it to light up and beep on the stud itself even after I took it off and cleaned everything.

I'm guessing the pump is bad but I'm still not sure about that ground at the frame so I will have to try some more testing tomorrow on that.

 

[avacado11]

I think this thread has jinxed me. Yesterday my fiancé's younger son calls and says he is broke down. His truck won't start.

I go and pull the truck home. It's a crank, no start. I put a fuel gauge on it and it has 0 fuel pressure but does have fuel at the Shrader valve. When you turn the key on you can hear the pump prime if you are under the truck but no pressure.

We put a new Delphi pump in this truck a little over two years ago and about 11,000 miles ago and a new fuel filter.

I did some testing today and it has all of the correct inputs at the fuel pump relay and I swapped in the relay out of my 98 Chevy truck which uses the same relay but no luck. I can see the ground wire on the frame at the back of the truck and using a power probe, it shows a ground on the eyelet but I never could get it to light up and beep on the stud itself even after I took it off and cleaned everything.

I'm guessing the pump is bad but I'm still not sure about that ground at the frame so I will have to try some more testing tomorrow on that.

Definitely test the ground before you drop the tank. Don’t make the same mistake I did.

What you could do is send 12v directly to the pump and see if your pressure goes up. Assuming you can reach the connector with the tank still mounted. 12v direct and No pressure = bad pump.

My old delphi pump would run off 12v directly but I suspected there was probably an internal failure with the backflow valve which is why I replaced it again. I think the delphi pump I got it with only lasted about 2 years though. It’s a 2014 and factory powertrain warranty would’ve only ended in mid 2019/100k. That pump I originally pulled out couldn’t have been that old considering it’s only been 19k/3.5 years since expiration.