Fuel pump woes

[Norman Connacher]

After rebuilding the fuel pump on my '58 356A I had a problem getting it to pump, but then after setting the prestroke to 35mm it seemed to be fine. I then pulled the engine to replace leaking pushrod tubes, put it back in without changing anything in the fuel system and it would not pump. Since then I have had the pump in and out numerous times to try different things. One thing I tried was bench testing it and I found it would not suck when the supply was about 2 ft below pump level but when I moved it up to about 1 ft it did fine. Pumping it by hand I produced about 4 ft of head (about 1.3 psi) before I ran out of height on my setup. I don't particularly want to give up and go to an electric pump but I am stumped.

Norm Connacher

[Alan Hall]

If it would not pump as soon as you re-installed the engine, it could be something simple like fuel cock closed or line from tank to pump not full and pump won't prime.

[John Brooks]

There is a small brown micartia six sided disc (photo. Lower right) that is missing or leaking. It is the suction side check valve.

[Dick Weiss]

When assembling the top 1/2 of the pump[, you must pre-compress (pull down) the diapham to the flat surface of the lower body 1/2 before snugging-up the 6-crews.
Pump output should be around 2.75 PSI, and the pushrod stroke spec is 29-to-34mm--gasket(s) are used to set the stroke.

[Bruce Smith]

Norm -

If all parts seem good and you've assembled it correctly (as per Dick's suggestion), it's probably a worn valve seat causing a leak-back. These were part of the alloy casting and a weakness in the design. You'll probably need to repair it with a valve set insert. We make them from brass, PM me if you want to know more.

- Bruce

[Norman Connacher]

I am going to remove and disassemble it once again so I can check some of the things suggested. I will also make sure the fuel line is full although when I bench tested it it pulled fuel up an empty line about 1 ft long. I have tried to run the engine a couple of times with the tank valve closed - didn't work too well. The times I have put it back together I did prestroke the pump lever by the 35mm spec value and the pump rod in the engine strokes over a range of 30.3 to 34.6mm. I have not carefully inspected the valve seats but will do that when I take it apart this time. Thanks everyone for your help.

Norm Connacher

[Bruce Smith]

I wrote a couple of articles recently for the magazine on fuel pumps. Some of the info. might be helpful. Here are the links to electronic versions:

http://www.sparkingplugs.com/resources/ ... p+39-2.pdf
http://www.sparkingplugs.com/resources/ ... +pumps.pdf
http://www.sparkingplugs.com/3.html

Cheers,
Bruce

[Jim Alton]

The shop manual says to use special tool VW 328b to reassemble the fuel pump. I thought i had a picture in a manual somewhere (I can't find it) but you can whittle down a piece of wood to stick in side the pump to hold the lever 35 mm (~ 1.4") from the base of the pump. You can put the pump in a vice.

Google VW 328b and you can find some nice adjustable tools which use a bolt to push the lever.

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I did have a problem similar to yours after reinstalling the fuel tank and doing other work on the fuel system. In that case, there was a lack of vacuum because the compression fitting where the fuel line entered the fuel pump wasn't 100% sealed.

You might want to check that.

[Norman Connacher]

I did use a small block of wood 35mm long to preset the diaphragm but checking the fitting is a good idea. That would explain why it will run with a can of gas in the engine compartment connected to the inlet with a hose.

[Bruce Smith]

Here's the Workshop instructions for stroke adjustment and an easy how-to from our web site:

The Workshop Manual's Description of Stroke Adjustment:

1. Place intermediate flange, actuating rod, and a gasket, which should be in perfect condition, on crankcase. The oil hole in the intermediate flange must face upwards. The convex end of the actuating rod must face toward the cam of the distributor drive shaft.
2. Attach gauge VW 328a to the flange and tighten it to the same torque as for the fuel pump in order to compress the gaskets in their usual thickness. The actuating rod stroke of about 0.16" (4mm) is determined by the cam on the distributor drive shaft. The stroke should move to within a range of 2" (5mm) which is marked on the gauge. The marks correspond to a length of 1.14" (29mm) and 1.34" (34mm) measured from the fuel pump contact flange (including gaskets) to the projecting actuating rod end. Crank the engine to check the pump stroke. The specified stroke can be adjusted by fitting an appropriate number of gaskets to the intermediate flange. Do not fit less gaskets than required, as this would have a detrimental effect on the diaphragm and the drive mechanism.

A Practical Translation (i.e. an easy How-To):

The steps described in the manual involve the use of a special VW 328a tool, which is not too common anymore. But the stroke adjustment can be quite easily carried out without this tool. The description above explains that the 4mm actuating stroke needs to be within a 5mm range of 29-34mm, as measured from the flange surface (including compressed gaskets) to the rod end. The location of this surface can be determined by using washers and nuts to hold and compress the flange and gaskets in place. Measure the stroke from this location to the rod end through two complete engine rotations. Add (or subtract) gaskets as needed. It will be safest to keep this stroke length below 33mm through the addition of gaskets. This resulting working stroke should be less than the actuating stroke, where 2-3mm is sufficient. It is more likely than not that the addition of one gasket (for a total of three) will provide the necessary safe separation for a working stroke within this range.

You can find more here: http://www.sparkingplugs.com/11.html

- Bruce

[Norman Connacher]

Hi all. Just to put an ending on this subject, it turns out there was no fuel pump problem after all. The problem was a rather embarrassing pinched fuel line. I should have caught it earlier but at least I learned a lot about fuel pumps. Thanks everyone for your help.

Norm Connacher

[Jeffrey Leeds]

It's always good to find out what the resolution of the problem is. Thanks for sharing the solution, Norm.

Jeff