Fuel pump question

johnpstoll · Post by **johnpstoll** » Thu Feb 16, 2017 11:36 am

I have a question about the fuel pump. I installed a rebuilt pump on my M38. I checked the psi and all I could get was 1 psi. I took it apart and checked everything out, visually it all looked fine. I tried a different diaphragm and the same thing. The vacuum side was at 5 psi. I bought a rebuild kit for the fuel pump side. While I was putting it back together I noticed that the new spring was noticeably stronger then the one I had removed. Now I get 5 psi out of the pump. Is the spring tension crucial to the pump pressure? Or did I answer my own question?

THanks

Post by **wesk** » Thu Feb 16, 2017 1:57 pm

If I assume you are referring to spring N in the illustration:

That is a good probability. The diaphragm is pulled down by the cam to suck fuel in from the tank but the cam does not push the diaphragm back up. The spring does.

johnpstoll · Post by **johnpstoll** » Thu Feb 16, 2017 2:28 pm

Yes, it was the N spring. I was curious what controlled the pressure. I almost put the old spring back in. I think I'll just get rid of it so I don't unknowingly use it in the future.

Thanks WES

4x4M38 · Post by **4x4M38** » Thu Feb 16, 2017 2:50 pm

Wes,
Isn't there a preload or something you have to do with the diaphragm
when rebuilding the pump? Maybe whoever rebuilt his didn't do that and
he did when he redid it.

Does that sound right? I have to rebuild mine and remember something
about preloading ? the diaphragm. I'm sure that's not the right term.

Thanks,

Post by **wesk** » Thu Feb 16, 2017 10:51 pm

Yes it is an assembly step that requires the fuel diaphragm be fully flexed. We all should have the same book open during a discussion like this. The details are in TM 9-1828A page 119. Read Para's b & c carefully.

johnpstoll · Post by **johnpstoll** » Fri Feb 17, 2017 7:28 am

I didn't have the tool shown in the TM, so I used the primer handle to flex the diaphragm and then tightened down the screws. It seemed to work. SO far so good.

4x4M38 · Post by **4x4M38** » Fri Feb 17, 2017 8:07 am

Thanks Wes!

That was it.

RonD2 · Post by **RonD2** » Tue Jun 19, 2018 8:36 pm

I've read the TM 9-1828A page 119 carefully but it's not clear to me. There are 10 screws around the perimeter of the top cover on the pump, and two screws by their lonesome in the middle of the cover. Are these the 2 screws that must be loose to pre-load the pump?

And 2nd question: can pre-loading be done with the pump installed?

I appreciate the advice!

Post by **wesk** » Wed Jun 20, 2018 12:38 am

I've read the TM 9-1828A page 119 carefully but it's not clear to me. There are 10 screws around the perimeter of the top cover on the pump, and two screws by their lonesome in the middle of the cover. Are these the 2 screws that must be loose to pre-load the pump?

No, the 2 screws are used to secure the top plate on the cover assy and have nothing to do with the pre-loading of the main fuel diaphragm.

The ten screws are used during the pre-loading of the main fuel diaphragm.

Larger illustration here: http://willysmjeeps.com/v2/modules/gall ... agm_sm.jpg

can pre-loading be done with the pump installed?

No. See illustration below for the reasons.

RonD2 · Post by **RonD2** » Wed Jun 20, 2018 7:26 am

That clears it up perfectly. Thanks Wes!

Jason86 · Post by **Jason86** » Thu Jun 21, 2018 10:59 pm

Then and Now Automotive was asking on his Facebook page if there were any videos that people wanted him to make, so I asked him to make one showing how they preloaded the pumps when they did a rebuild so he made a video posted it to their Facebook page and onto YouTube a month ago or so. It's not the way the book describes doing it with the special rod but it's apparently how they do it.
Not sure how you feel about links Wes but I will post the link just to get the message across and then you can go from their and edit or delete as you need or redirect people in that direction. I'm not sure how correct this method is but it's what they recommend. The Google link to YouTube is: https://youtu.be/v0yLWQvjrPg

Or you can do a search on YouTube to find the video type in "Preloading the diaphragms" should be first video.

Post by **wesk** » Thu Jun 21, 2018 11:27 pm

The video is adequate & useful as long as you have your manual open to the right page and make sure you make all the steps in the manual.

His video does quite well with the fuel side diaphragm but he was a wee bit to brief on the Vac side diaphragm and did not cover all the steps.

RonD2 · Post by **RonD2** » Sun Jun 24, 2018 2:33 pm

Followed the directions and believe I successfully pre-loaded the pump (9810 on the flange, has priming lever, correct spacer). It puts out plenty of fuel but my pressure gauge only indicates a tad over 3 PSI when cranking the engine, maybe 3.1 or 3.2 PSI. I know the TM spec says 4.5 to 5 PSI with the engine running at 600 RPM. I can't start my engine at the moment due to a distributor issue. Never had this pump on a running engine before, it's a rebuilt unit from one of our reputable suppliers. I have no reason to suspect my gauge isn't accurate but have no way of knowing for certain. Am not inclined to buy another gauge just for that.

Searched and researched this question with no joy. Nothing in the TM I could find. Is 3 PSI out of a cranking fuel pump ok? Or will I just have to start the engine to know for sure?

Post by **wesk** » Sun Jun 24, 2018 8:41 pm

There is nothing published except the required minimum which is obviously for an engine at 600 RPM running.

Don't mis-understand the purpose of pre-loading the diaphragm. It is not done to help make more pressure. It is done to ensure the diaphragm is seated evenly the full circumference of the cover and is pre-stretched to ensure it does not develop too much pressure.

RonD2 · Post by **RonD2** » Sun Jun 24, 2018 8:56 pm

Hi Wes. Thanks! Yes, I understood the need for pre-loading as you described. I'll measure it when I have the engine running.