Mechanical Fuel Pump Help!
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4th Gear
Joined: May 2023
Posts: 4
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Mechanical Fuel Pump Help!
So I'm new to wrenching on my 1967 F-100. Got stranded last week due to a failed fuel pump. I replaced said fuel pump, and stuck a clear filter inline so I could see flow.
All was good until I got stranded again just a few days later. A few minutes into my drive, the engine died as fuel wasn't getting to the carb. Inspection of the clear filter confirmed that flow was all but a trickle.
I tested my setup in the garage and found that fuel flow is poor and almost non-existent when hooked up to inlet of the carb, but great when dumping into a Gatorade bottle.
What am I missing? Is the replacement fuel pump faulty as to not be able to overcome the gravity of the fuel going uphill to the carb?
First part of attached video shows fuel pumping into a gatorade bottle. Second part shows no flow when attached to carburetor.
All was good until I got stranded again just a few days later. A few minutes into my drive, the engine died as fuel wasn't getting to the carb. Inspection of the clear filter confirmed that flow was all but a trickle.
I tested my setup in the garage and found that fuel flow is poor and almost non-existent when hooked up to inlet of the carb, but great when dumping into a Gatorade bottle.
What am I missing? Is the replacement fuel pump faulty as to not be able to overcome the gravity of the fuel going uphill to the carb?
First part of attached video shows fuel pumping into a gatorade bottle. Second part shows no flow when attached to carburetor.
Cargo Master
Joined: Jan 2016
Posts: 2,672
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I'm guessing you're expecting too much flow with engine cranking but not starting. My wild guess is the engine uses about a gallon (probably less) per hour while idling in a driveway. At 12.5 miles per gallon, my truck uses 5.6 gallons per hour on the freeway.
However, from the sound in the video, your engine is not running. So even though it is cranking (at just a couple hundred rpms), I doubt it's using as much as it would idling at 600-700 rpms. With engine cold and the choke open it will suck even less fuel than if it were warm and cranking (but not starting).
However, from the sound in the video, your engine is not running. So even though it is cranking (at just a couple hundred rpms), I doubt it's using as much as it would idling at 600-700 rpms. With engine cold and the choke open it will suck even less fuel than if it were warm and cranking (but not starting).
Tuned
Joined: Apr 2023
Posts: 363
Likes: 108
if your float bowl is full, it won't allow anymore fuel from the fuel pump to enter the carb. but what you are describing sounds a lot more like vapor lock rather than fuel pumps dying (although its not uncommon to get a new broken fuel pump) is your carb sitting directly on the intake or is there a phenolic spacer separating it? or maybe a sheet metal spacer deflecting heat? if its right on the intake, i'd add a phenolic spacer and see if the issue continues. it's cheap and a good thing to have anyways even if your issue is elsewhere
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4th Gear
Joined: May 2023
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However, from the sound in the video, your engine is not running. So even though it is cranking (at just a couple hundred rpms), I doubt it's using as much as it would idling at 600-700 rpms. With engine cold and the choke open it will suck even less fuel than if it were warm and cranking (but not starting).
Tuned
Joined: Apr 2023
Posts: 363
Likes: 108
when you get vapor lock the fuel in the carburetor/fuel lines boil from the heat of the engine/engine compartment and cause those gases to expand pushing back on the line trying to advance fuel to the float bowl. your engine will turn over just fine as it's not locking the block but merely preventing fuel from flowing to the float bowl.
when you are filling the Gatorade bottle, there is absolutely nothing to restrict the flow, but even without vapor lock, fuel won't be flowing to the carburetor if the float bowls are full and the needle is seated.
you have good flow. its extremely unlikely given that amount of flow you aren't exceeding the minimum PSI for that carb and that flow is certainly exceeding the fuel needs of that engine so there has to be a restriction somewhere... many carbs have a screen on the inlet almost like a filter... those can get clogged which could be causing your issue as well; however, it sounds like your issue is after driving a while and the engine and engine compartment has gotten good and hot which is why i think it's vapor lock.
when you are filling the Gatorade bottle, there is absolutely nothing to restrict the flow, but even without vapor lock, fuel won't be flowing to the carburetor if the float bowls are full and the needle is seated.
you have good flow. its extremely unlikely given that amount of flow you aren't exceeding the minimum PSI for that carb and that flow is certainly exceeding the fuel needs of that engine so there has to be a restriction somewhere... many carbs have a screen on the inlet almost like a filter... those can get clogged which could be causing your issue as well; however, it sounds like your issue is after driving a while and the engine and engine compartment has gotten good and hot which is why i think it's vapor lock.
Thread Starter
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4th Gear
Joined: May 2023
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if your float bowl is full, it won't allow anymore fuel from the fuel pump to enter the carb. but what you are describing sounds a lot more like vapor lock rather than fuel pumps dying (although its not uncommon to get a new broken fuel pump) is your carb sitting directly on the intake or is there a phenolic spacer separating it? or maybe a sheet metal spacer deflecting heat? if its right on the intake, i'd add a phenolic spacer and see if the issue continues. it's cheap and a good thing to have anyways even if your issue is elsewhere
Tuned
Joined: Apr 2023
Posts: 363
Likes: 108
I do have a spacer between the carb and intake so I'm on the fence about vapor lock. Things ran fine (enough) with the old pump before it completely died on me, so a new pump shouldn't suddenly be affected by heat soak? But what you're saying about the float bowl being full is sounding like it could be the culprit. I didn't think to check the fuel level/float adjustment- could too high of a float cause this problem?
too high or low would be a problem with any pump... how long does it take to die and what happens after it died? i'm leaning towards the pump delivering slightly too much pressure and it flooding it when it blows the needle off the seat and floods over the vent?
I've just never heard of intermitted issues with a mechanical fuel pump... they either work or don't. they could give too much or too little pressure. i guess they could lose suction or get blocked, but from your video that doesn't appear to be the case
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Fleet Mechanic
Joined: Apr 2017
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From: COSMOS
What fuel pump did you get 
It MAY have too much pressure . Are u 100% sure you are connecting everything back the same way while testing .
It is easy to get hoses crossed
among other things.............Your other pump may have died because of trash , now new pump might have trash in it .Flush pump out with carb cleaner .......good flush...........This should b simple , take your time and stay calm 
& U will get it..............
Is filter backwards
Have u rode or sat in a Chevrolet recently...........any GM vehicle.........oh nooooooo
It MAY have too much pressure . Are u 100% sure you are connecting everything back the same way while testing .It is easy to get hoses crossed
among other things.............Your other pump may have died because of trash , now new pump might have trash in it .Flush pump out with carb cleaner .......good flush...........This should b simple , take your time and stay calm & U will get it..............Is filter backwards
Have u rode or sat in a Chevrolet recently...........any GM vehicle
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Fleet Mechanic
Joined: Apr 2017
Posts: 1,391
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From: COSMOS
First it is pumping , now see what pump you have and the rating it has , gal/min , and/or psi . You need a pump that pumps 7 lbs. max . for a 302 , not sure what engine u have , so 9 lbs or so may be good for a bigger engine . Unless something got knocked out of wack , , I think it is either dirt somewhere , or too much pressure , filter problem , or connection/vacuum leak . Start & make sure you have the right pump , if it has a built in filter , you do not need a second one . If you have an extra filter it may be restricting the gas flow.........
FTE Legend
Joined: Nov 2015
Posts: 30,919
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From: Angier, NC
I have seen posts of new motor driven fuel pumps putting out way to much PSI.
And yes I had one on a non Ford motor that put out 18 PSI.
I have replaced it with a different brand but have not tested it yet.
BTW even 7 psi max on a Eldel carb is too much for the needle / seat.
PSI has nothing to do with the amount of fuel needed fore motor size.
Dave ----
And yes I had one on a non Ford motor that put out 18 PSI.
I have replaced it with a different brand but have not tested it yet.
BTW even 7 psi max on a Eldel carb is too much for the needle / seat.
PSI has nothing to do with the amount of fuel needed fore motor size.
Dave ----
Fleet Mechanic
Joined: Apr 2017
Posts: 1,391
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From: COSMOS
I do have a spacer between the carb and intake so I'm on the fence about vapor lock. Things ran fine (enough) with the old pump before it completely died on me, so a new pump shouldn't suddenly be affected by heat soak? But what you're saying about the float bowl being full is sounding like it could be the culprit. I didn't think to check the fuel level/float adjustment- could too high of a float cause this problem?
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