Fuel pump runs continuously!

[vagt6]

Just installed a new Moss SU fuel pump on my MK III '74 Midget. It's the points-type pump, Moss says it's the correct replacement.

I hooked it up, turned on the ignition and the pump just runs continuously, won't stop clicking. And, it's much louder than the old one!

What could be the problem? Does the "inlet" fuel hose go to the gas tank and the "outlet" go to the engine? Could I have the hoses hooked up backwards?

Any ideas, gents??

Thanks for the help!

 

[vagt6]

With help from the great Dave Dubois, the SU guru, I fixed the problem.

This SU fuel pump must be installed with the outlet line oriented on top (I didn't do this), AND, I had the outlet and inlet lines reversed.

So, I re-oriented the pump and reverse the lines. FIXED!

I must say, if you ever need help with anything that has "SU" stamped on it, rebuilding, fixing, etc., call Dave Dubois. Not only is he very knowledgeable and helpful, but he's an honest businessman.

Never met the guy except on LBC websites. Free expert advice, just for the asking. The kindness of strangers, it's what makes the hobby feasible for amateur mechanics like me. Couldn't do it without 'em.

Thanks, Mr. Dubois. :thumbsup:

 

[dklawson]

I have exchanged several emails with Dave over the years. I could not agree with you more. He is a wealth of knowledge and very willing to share what he knows.

 

[jlaird]

Lots and lots of business men in the community that are like that. If I listed them all I would miss someone important.

 

[vagt6]

Sadly, I still have the problem!

Fuel pump acted okay for a bit, then went back to continuous running.

Question: Where does the "inlet" line on the fuel pump go: to the gas tank, or to the engine end???

 

[dklawson]

The hose from the tank goes to the pump inlet.

If you can clamp off the fuel hose coming out of the tank, you may benefit from bench testing the pump. You can use a battery charger to power it and a "safe" substitute for fuel would be mineral spirits, diesel, or kerosene. You want something that won't accidentally burst into flames.

 

[Billm]

As Doug said a bench test is the best way to test the pump.
one test that I have done is to see what the pump pressure is; I hook about 15 feet of clear plastic (Tygon) tubing to the output of the pump and run it up the side of my house, then I power up the pump. 3' of rise is 1 PSI, 6' is 2 PSI, etc.
BillM

 

[vagt6]

While bench testing the pump, I can feel suction at the inlet and air blowing out of the outlet. However, when I plug the outlet turret with my finger, the pump keeps running. Blocking the inlet turret will cause the ticking to slow way down, but not stop completely.

Could the brand new Moss pump have an air leak? What am I missing?

It doesn't make sense that a fuel pump would run all the time, whenver the ignition key is turned on. Is that normal for an SU AUF 214?

Other ideas???

 

[vagt6]

As Doug said a bench test is the best way to test the pump.
one test that I have done is to see what the pump pressure is; I hook about 15 feet of clear plastic (Tygon) tubing to the output of the pump and run it up the side of my house, then I power up the pump. 3' of rise is 1 PSI, 6' is 2 PSI, etc.
BillM

Bill, the pump is definitely blowing air, seems fairly robust. I'd very roughly estimate that it's at least 2-3 psi.

The problem is the continuous running. Could that be right, to run all the time?

 

[dklawson]

Repeat your bench test with liquid (mineral spirits, etc. as mentioned above). Air is compressible and the way the pump toggles, it will not stop until something incompressible (like fuel) prevents the pump's moving parts from opening the points.

 

[vagt6]

Many thanks, Doug. I'll do the flow test and see what happens.

Back in a few minutes . . .

 

[DrEntropy]

Plug the outlet side of the pump <span style="font-style: italic">after</span> you have <span style="font-style: italic">fluid flow</span> and see if it slows/stops the ticking?

 

[JPSmit]

As Doug said a bench test is the best way to test the pump.
one test that I have done is to see what the pump pressure is; I hook about 15 feet of clear plastic (Tygon) tubing to the output of the pump and run it up the side of my house, then I power up the pump. 3' of rise is 1 PSI, 6' is 2 PSI, etc.
BillM

Bill, the pump is definitely blowing air, seems fairly robust. I'd very roughly estimate that it's at least 2-3 psi.

The problem is the continuous running. Could that be right, to run all the time?

yes, I'm sure it should be continuous. in the same way that the mechanical runs continuous when the engine is running - only difference is with electricity, you can run the pump without running the engine - either way the float prevents too much gas getting in.

 

[vagt6]

Here's what Dave Dubois said about the pump running continuously: it doesn't. It's supposed to stop clicking when the float chambers are full. Makes sense.

<span style="font-style: italic">Mark - When the engine is not running, the pump will pump fuel into the float chambers of the carburetors and once they are full, the pump will stop clicking. A pump with new check valves that are sealing almost perfectly will not click again (theoretically), as long as the engine is not running - in the real world, the pump may click once ever 15 seconds or longer. When the engine is running, the pump will continue to click as it replenishes the fuel used from the float cambers. The rate of clicking will be determined by how fast fuel is being used from the float chambers.
Cheers,</span>

And, the problem seems to be fixed. It appears that air was sucking into the fuel hose where it connects to the hard lines on the outlet, the hose clamp was too loose. This was causing the pump to run-on. When I turned on the ignition I noted fuel dripping under the car! So, I snugged up the offending fuel hose clamp and the leak stopped. The pump also stopped running/clicking constantly.

Went for a test drive, seems fine. And, the pump does NOT click all the time.

Thanks to everyone who offered advice here, couldn't drive this car without you.

Cheers to all . . . :thumbsup:

 

[Billm]

Congrats! Nice to get these things fixed!
BillM

 

[dklawson]

I'm glad you got it resolved. When you let Dave know, please tell him I said hello.

 

[vagt6]

Will do, Doug. Thanks again for all your help. :thumbsup:

 

[DrEntropy]

And, the problem seems to be fixed. It appears that air was sucking into the fuel hose where it connects to the hard lines on the outlet, the hose clamp was too loose. This was causing the pump to run-on. When I turned on the ignition I noted fuel dripping under the car! So, I snugged up the offending fuel hose clamp and the leak stopped. The pump also stopped running/clicking constantly.

This is where it was going when I posted the suggestion about blocking the line at the output end after fuel was present. If the output is blocked off and the pump keeps running there's a good chance it's sucking air at the inlet side or the flapper valves in the pump are not seating. Most times it's sucking air. And it will not always be evident by fuel leaking, either. A deteriorated or cracked hose can "valve", allowing air IN but not allow fuel back out.
Good ya found it. That stuff can make ya crazy. :wink:

 

[vagt6]

Good point, Doc. I installed new fuel hoses when I installed the new pump, too, to be safe.

Strange about the fuel hose sizes: the correct size fuel hose is 5/16", which fits the fuel pump turrets fine but seems a bit large for the hard fuel lines which are 1/4". You really have to snug up the hose clamps tightly to the hard fuel lines with the 5/16" hose to get a good, airtight seal that won't leak.

And, you have to be very careful NOT to tighten the hose clamps too tightly on the plastic SU fuel pump turrets, it will crack the turret (I prefer the metal turrets on my old SU pump - the replacement had plastic ones). A hair-line crack that let in air in one of those fuel pump turrets would be difficult to detect. Now THAT would drive you crazy.

A learning experience, and a good one thanks to Dave Dubois and the helpful folks above.

Jack of all trades, and master of none. :crazy: