Should I feel the suction at the intake of my fuel pump



I couldn’t start my auto ('87 Toyota Pickup, carburetor, 4-cylinder engine). No gas made it to the carburetor. No gas made it to the fuel pump (mechanical, mounted on the engine). The tank has

gas (I loosened the drain plug enough to

test that). I took the fuel filter off

and it tested okay. Gas flowed out the

hoses to it when I did that.

I disconnected the hoses and had a neighbor crank the engine: I couldn’t feel

any suction at the fuel pump’s intake.

I took it off and cleaned it up in the

kitchen sink. If I put the intake into

a cup of water and pumped (it has a bellows

that a cam on the engine pushes) I got

a good stream out, but I still couldn’t

feel any suction at the intake. The specs

say it should draw 2.1-4.3 psi, which I

think I should feel.

What do you-all think?

What would be a good cheap device to

suck on the hose that runs from the

fuel filter to the fuel pump, other

than my cheeks? A suction bulb from

the drug store? I don’t want to buy

a compressor.

Thanks for your attention.

Random Troll


I use engine vacuum from another vehicle. I have a large pickle char that I have attached to 3/8? hose barbs to the metal lid. I run one hose to either a spare vacuum port on one of my trucks or I disconnect the PCV valve and use that port. I use an inline fuel filter between the pickle jar and intake manifold to insure that I don?t ingest any particles into the engine. The other hose barb I connect another hose to and use it for vacuum bleeding, cleaning out fuel tanks, testing fuel lines, etc.

This approach may turn on the check engine light on newer vehicles, but my old 86 K5 and 92 Dakota don?t mind a bit. The CEL should go out once the vacuum leak is removed though.


I think you are moving the pump lever by hand further than it is moved by the eccentric on the engine, your in the sink test is invalid.

On the other hand if it sprayed out it had to be sucking in, I still think your pump is toast.


Although it is rare, the camshaft may have the lobe that operates the fuel pump worn so that it won’t move the fuel pump lever far enough to pump the gasoline. This used to happen more frequently in the old days when we had a vacuum section in the fuel pump to boost the vacuum for the vacuum operated windshield wipers. However, it is still a possibility. If this is the case, the cheap way out is to buy an electric pump designed for carbureted engines. I think that these pumps are still available at J.C. Whitney.

I would try a replacement mechanical pump first, however.


A solid fluid coupling is far more pronounced than a cavity filled with air.

Submerge the pump in fluid and then, after flow is produced out the outlet, check for the suction that you seek.

Reinstall it and, with the outlet line disconnected and in a catch basin, crank the engine. If fuel flows out of the pump, you should be done there. Move to the carb. If it doesn’t, then rethink your problem sequence. Did the cap have a vacuum on it when you opened it? Did you remove the cap in this process?? ( I may have missed a couple of details)


Quoth geeaea: ‘Although it is rare, the camshaft may have the lobe
that operates the fuel pump worn so that it won’t move the fuel pump
lever far enough to pump the gasoline.’

The fuel line is clear, as a mouthful of gasoline sucked up hardly
trying revealed (I forgot my siphon bulb!), so I bought a new fuel
pump. I can’t feel suction at its intake either. The old pump was
mounted without the gaskets and spacer the instructions and manual
tell one to use, which may have been an effort to keep using a pump
after the camshaft’s lobe had worn down. I’ll find out when I mount
it. Fortunately it’s an easy job.


Replace the fuel hose from the fuel pump back to the fuel filter and gas tank. Install hose clamps securely. Old fuel hose can leak enough air that the fuel pump can’t draw fuel.


Someone in the past must have removed the gaskets and spacer. This was done either because the camshaft lobe was worn and this might have helped give a longer stroke to the fuel pump lever or because the fuel pump was almost worn out and it needed a longer stroke for the lever. Let’s hope it is the latter condition and a new mechanical pump is all that is needed. As I posted earlier, if the lobe is worn, then I would go with an electric pump.


Yes you should be able to feel a mild suction, approximately 10 inches of mercury. You could get fittings to attach a vacuum gauge and quantify that vacuum.

One problem I have seen with modern gasoline is that it rots out the fuel pump valves. When you take a failed pump apart, you will see how deformed and nonpatent the valve discs are. A good pump should be able to hold output pressure in suction side vacuum for a few minutes after operation of the lever.

Hope that helps.


The old pump did have gaskets and spacer, but they had
absorbed so much oil and gotten so stuck on and stuck together that I
didn’t recognize them. The new pump is a huge improvement: it starts
more quickly, doesn’t hesitate on fast acceleration, delivers more
power. I should have replaced it years ago. Failing did me a favor,
other than the inconvenience.