View Full Version : Fuel pump vacuum

09-19-2016, 11:30 PM
If I'm not using the vacuum connections on top of the fuel pump do they need to be capped for the pump to work?

Joe Johnston
09-20-2016, 07:53 AM
No "need" to

09-20-2016, 09:30 PM
Well I can't get fuel to the pump. This pump worked before I took everything apart. I installed a filter in line before the pump, just in case, along with the filter at the carb. There was also some pressure built up in the gas tank when I added more fuel this afternoon. What do I do next?

Joe Johnston
09-21-2016, 09:03 AM
Well I can't get fuel to the pump

This is not the pump's fault and you must have fuel to it for it to pump to the carb. You will need to check the following:
1) Since it has been apart the internals MAY have an air leak and/or not assembled properly which is not creating enough suction to draw fuel to it.
2) The pump may not be installed properly as the lever can be installed on top of the camshaft lobe instead of under it. (Never seen it happen but have only heard it is possible.)
3) The "pre-filter" you installed may be restricting the flow. It may be directional and in backwards and stopping the flow. For testing purposes, I would remove it until after you get things running well and then re-install if you want.
4) Check for air leaks at all the fittings in the fuel lines to the pump.
5) If you have a Mity-Vac hand operated vacuum pump try and draw gas through the lines to the pump. Inexpensive versions are available at Harbor Freight stores if you don't have one and Autozone may even have one in their "loaner" tool selection.
6) Pressure in the tank can be caused by temperature changes. At this point it would have been a good thing to help push gas to the pump, BUT your tank should not be pressurized and you may have a problem in the venting or the cap itself. No more than the level would drop from getting gas to the pump, I doubt that improper venting is the problem preventing getting gas to the pump, but the cause of pressurization must be eliminated.
7) By today's standards, these pumps are a bit weak and they have the disadvantage of having to suck fuel all the way from the tank, instead of pushing it as our fuel injected cars do. They operate at about 5 pounds pressure at the carb. That said they have worked quite well and dependably in millions of cars for who knows how many millions of miles and hours.
There is a cause, just have to do a bit of checking to find it.

Joe Johnston
09-21-2016, 10:07 AM
forgot to add - do you have the fuel line FROM the tank to the INPUT port on the pump?

09-21-2016, 11:25 AM
Very often the car can't crank fast enough to draw the fuel from the tank when it's dry. Pour some gas into the carburetor so the car runs for a few seconds. If that doesn't work do as Joe mentioned and try to draw fuel from the tank using a vacuum pump. You can also try pouring gas into the pump to prime it. Make sure that your gas cap is vented.


09-21-2016, 03:51 PM
Thanks for all the great info!

If I did put the pump together properly and all the seals are good, would I be able to feel the vacuum with a finger while turning it over?

I'm sure I got the lever under the cam. It was a pain getting it bolted up while having the arm pressed against the cam lobe.

I probably do need a new gas cap. I don't think the one I have is vented. Have to check when I get home.

I believe I have the lines connected properly. There's a hardline from the pump to the carb. The other port has a nipple screwed in with a rubber line connected from there to the main hardline going back to the tank.

09-21-2016, 04:15 PM
When you move the lever you should hear it pumping. When you put your finger over the inlet you should feel vacuum.


09-22-2016, 12:38 AM
My gas cap does seem to be vented. Apparently not enough to relieve pressure in the tank. There was a lot of pressure built up again this afternoon.

I was able to feel some vacuum at the pump input. Still not sure if it's enough and it doesn't seem to be pushing fuel to the carb. I was able to create a vacuum to get fuel from the tank to the pump to give it some help. Need to troubleshoot that some more.

I filled the bowl in the carb and was able to get it to run for about 15 seconds but it spits and sputters and shuts down. I'm sure I need to get the carb adjusted properly but have no real experience in that. Any help would be appreciated on that as well.

09-22-2016, 10:27 AM
One thing you can try is disconnecting the line that goes to the tank and run a short line to a gas can or any container you can put some gas in. If it won't push or suck gas from a short distance then it's not going to work all the way to the tank. As for your carburetor has it been rebuilt. If it's the original it at least needs to be cleaned out and a new rebuild kit installed.


09-22-2016, 11:35 AM
If your game, try this: locate a suitable container to hold fuel, say about maybe a pint-quart should be sufficient (a Tall-Boy will due), punch a hole in the bottom just large enough to press (for retention & sealing) fuel hose into and be retained, fabricate a hangar to position your container above the carburetor (gravity fuel delivery) filled with fuel (coat hangar to under-hood framing?), route your fuel supply hose to the carburetor inlet fitting. Place original fuel supply hose from pump into a container (may need more fuel hose; secure appropriately), start engine. With engine running on your remote fuel supply (remember to maintain fuel quantity in remote container), is the mechanical fuel pump pumping fuel? How much? Time engine run duration (1500-2000 R.P.M.) and then measure fuel volume delivered. Note, this would be a "free flow" value; one could fabricate a regulated restriction (using an adjustable fuel pressure regulator w/ gauge?) or just an orifice restriction (maybe just Vise-Grips pinching hose w/ gauge teed in line between pump & pinch?) to create back-pressure (approx. 5 p.s.i. +/-) for a more accurate value as intended. Also, fuel flow sums will vary with engine R.P.M.s.

Understand that this procedure is probably not E.P.A. (open containers with fume escaping into the atmosphere and/or spillage in the environment), or O.S.H.A. (to much fume or spillage about hot surfaces may make for fire!) approved.

So remember, if your game, it was all your idea, I had nothing to due with it! Scott.

09-22-2016, 12:21 PM
I did rebuild the carb. It's a 2100 I believe. At one point last night it spit out a ball of fire. I set the needles to 1.5 turns out.

Joe Johnston
09-22-2016, 12:38 PM
Fire is good! Shows you have Air, Fuel and Spark. Now to just get them in the right ratios and INSIDE the engine!

09-22-2016, 04:40 PM
I think I should probably just replace the pump at this point. Need to recheck TDC to verify timing too. Any pump for a 352 will fit right?

09-22-2016, 05:26 PM
Any FE pump will work 352 or 390. If you aren't using the vacuum ports get a pump without them. They're much cheaper.


09-26-2016, 06:25 PM
Well I got a new fuel pump. The other one was getting fuel to it but just wasn't pushing it up to the carb. Hopefully I can get that installed this week and give it another shot.

I'm still concerned about the pressure building in the gas tank. I confirmed I do have a vented gas cap but it doesn't seem to vent anything. By the end of the day there's a bunch of pressure built up. What's the proper way to fix this problem?

Joe Johnston
09-26-2016, 07:00 PM
1) buy a new correct cap
2) don't tighten it which would be OK for test drives and around the block
3) drill a 1/16" hole through it

09-26-2016, 07:26 PM
I thought I did buy a correct cap. What's the difference between the vented cap I have and a correct vented cap? And where do I get one? I also bought a second vented cap that is supposed to fit but the tabs don't go deep enough. Drill a hole? Is that how the factory would fix this problem with a tank that doesn't have any vent?

Joe Johnston
09-26-2016, 08:25 PM
If you bought the cap at the local parts store, there is a good probability it may not be correct, as evidenced by the second one you bought that doesn't fit properly. One of our Thunderbird specific parts suppliers should have a correct one.

No, drilling a hole is not how the factory would do it, but it would be a quick backyard mechanic fix to circumvent the issue. I am having a hard time understanding how pressure is building up in this tank for you when you can't be driving the car since:
The other one was getting fuel to it but just wasn't pushing it up to the carb. With very limited run time, the only way I can think pressure can build up is by extreme temperature changes - like having the cap off when very cold and tightening it and then opening it when it is very hot for a day. What is causing the pressure to build up??

09-26-2016, 10:02 PM
Honestly I wish I understood what was going on. The car is sitting in the garage while I'm fumbling through these problems to get it started up. I have been leaving the cap on and there's probably a 20 deg temp change from day to night.

09-26-2016, 10:46 PM
Even with a vented cap all the old Fords that I've had build up pressure. It seems to be more prevalent in hot weather. It never seemed to effect the way the cars ran.


09-28-2016, 10:17 PM
Update! Apparently I need professional help. New fuel pump seems to be working great. That's one problem down. I guess I won't worry about the fuel tank pressure right now. It has been extra warm lately. I double triple checked that I was at TDC on the compression stroke and that the distributor rotor is pointed at number one on the rotor cap as well as checking my firing order (15426378). I turn her over and she seems to start up for a few seconds but just spits out the carb enough that it blows the choke plate wide open and dies. I have no clue where to go from here. Please help!

09-29-2016, 08:49 PM
Final update on this thread. I was backwards on my timing. I was aiming for 6deg after TDC when I thought I was aiming at 6 deg btdc. All is good. Now that it runs it seems to want to kick into reverse. That'll be a new thread unless I can figure it out.

09-29-2016, 11:24 PM
Glad to see that you got the timing sorted out. If the car slips into reverse try adjusting the manual linkage.


09-30-2016, 07:25 PM
Final update on this thread. I was backwards on my timing. I was aiming for 6deg after TDC when I thought I was aiming at 6 deg btdc. All is good. Now that it runs it seems to want to kick into reverse. That'll be a new thread unless I can figure it out.

Wow, it is a wonder you didn't have some overheating issues with the timing that much retarded!
Still, it's good that you've sorted it out, onto the next problem!

10-01-2016, 12:57 AM
Well the thing wouldn't stay on long enough to heat up. I adjusted the linkage as suggested so I'm good to go. I didn't have that set up right either so it had room to pop itself into reverse. Just need to work on some carb adjustments and I should be able to take a test drive around the block. Can't say thank you enough for all the help I get here.