Fuel pump stuff (everybody take a look)

[Roy Wendell]

On Monday, July 30, 2007, at 10:20PM, Cody Forbes <cody at 5000tq.com> wrote:

>I know theres a bunch of people out there on the list that have trouble for a bit with getting the fuel pump out of the 5k's and others that use the in tank tripod mount. I figured some well lit photos may help:
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>http://jcforbes.com/jcfpics/fuelpump
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>You'll also notice the right down where the bottom of the pump sits you can see some stuff. That stuff left me on the side of the road a week ago. Soooo many times people think they have bad pumps when it's just the crap in the bottom of the tank getting sucked up to the ultra-fine screen on the pump. It only took 15 minutes to get in the tank, suck the junk out of the bottom, clean the screen on the pump (sometimes I cut them off when I'm in the mood - not guna debate on the logic there hehe), then put it all back together.
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>Anyways you just need to push those clips back while moving the pump upwards on the barbs. One at a time works ok since the rubber bushing allow enough movement to move the pump up crookedly. Pretty simple when you look at it really, but the first few times I did it it was a PITA.
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>-Cody

I couldn't agree more. I went through a similar episode last year. Installed a brand new pump roadside after the previous one stopped dead with no warning. I decided to leave the stainless steel screen on the new pump because I suspected that the old one had died due to ingestion of rust and general tank crud. Big mistake. Even after doing my best to wipe up the crud in the bottom of the sump before installing the new pump, it only got me about ten miles down the road before it started screaming and the engine cut out. After sitting for a few minutes the car would start right back up, pump quiet, and run for about a minute before the noise came back and the engine would die again. In desparation I managed to get back home by starting the car, accelerating, shutting of the engine and coasting, followed by restarting to accelerate again. I managed to drive about forty miles by running the engine for about a minute at a time. Every time,  just after the restart, the pump would start making noise which would increase in volume right up to the point when it would scream like the tank was empty at which point I had no more power.

When I got back home, I pulled out the new pump and found the screen clogged with minute rust particles. So I cut out the screen knowing that the sump has integral nylon mesh filter panels that should keep the larger crud out of the pump. But then I got to wondering how stuff was getting into the sump area. After thinking about it and looking at how the pump fits in the sump, I realised that the rubber lip seal in the top of the sump had gotten old and hard and was allowing stuff to get past it around the periphery of the pump. So I replaced the seal as well as vacuumed out the inside of the tank as best I could. ****WARNING-although most shop vacs now keep the motor cooling air and vacuum air paths separate there is still the possibility of explosion due to fuel vapors passing through the motor. At a minimum you should do as I did and use the exhaust side of the shop vac to ventilate the tank for at least a day. The inside of the tank should be bone dry with no dark areas visible. If you blow yourself up don't send your lawyers my way. I'm just an idiot who can't afford to pay someone else to fix my cars nor can I afford a new one. Don't bother suing.****

Some thousands of miles later the pump works just fine and while not silent is pretty quiet. What's odd, or maybe not, is that it gets quieter when I go around a corner pretty hard and then comes back up to normal volume less than a minute later. It's almost like there is still enough junk in the bottom of the tank to be clogging the nylon mesh  filter panels in the sump and by cornering hard I'm sloshing the fuel around hard enough to temporarily dislodge it. The next type 44 I resurrect will have the fuel tank removed, the top cut off, sump removed, tank sealer applied, sump reinstalled, and the top of the tank riveted back on with aircraft tank sealer applied. Curse Audi for installing a sump with filter that is much bigger than than the access whole in tank. Curse me for continuing to buy these cars even after they've sat unused for years. Curse Audi again for not galvanizing the inside of their tanks when they go through the trouble of galvanizing the chassis.

In summary, if your pump makes noise it's probably not a bad pump. Depending on the model it's either the intake sock or the fuel sump filters that are the problem. This explains why so may people with noisy pumps report that the new replacement is as loud or nearly as loud as the old one. What makes the noise is cavitation of the pump element caused by a drop in pressure at the intake due to a restiction. Remember that pumps don't suck, they blow. The only thing that gets fuel to the intake side of the pump is gravity and atmospheric pressure. It doesn't take much of a restriction to counteract those forces and cause a pressure drop at the pump intake. Something tells me it's not only the 4k crowd who are going to get good at cutting apart fuel tanks.

Roy