It's not a diesel ( engine ) .   - see original post -  :-)
OK..
I read a few posts, and no one has asked which fuel system it has.
Being and 87 Jetta it could be CIS-E
or 'regular' EFI - Difignat with electnically controlled fuel injectors,
like vanagon waterboxers use.

no vanagons of any type have an in-tank fuel pump of course.
If the Jetta engine has Digifant , it uses the same fuel pump vanagon
waterboxers do.
That pump and system, with fuel pressure regulator on the fuel rail, run
around 30 - 34 psi....32ish, that rangne.
If you block off the output to a vanagon fuel pump ...so it pushes as hard
as it can ...it'll reach 100 psi, but they never run that way, and are made
for systems in the 30psi-ish range.

CIS on the other hand ..
Easily identified by braided fuel hoses going 'everywhere' ....about 6 or 7
separate ones.
That system .......in some cases those to have a smallish delvery pump
..that feeds fuel to the main pump.
And the pressure that system runs at is ....60ish or more....
Won't get into how that system works...but ....ingeniously.....'control
pressure' varies by a temperature-sensive fuel pressure regulator ..
and that pressure is used to control the fuel mixture ...by how much fuel
sprays .....and it sprays contiuously  not pulsed like EFI.

first is find determine which fuel system it is.
if it's Digifant EFI - fuel pressure is pretty easy to check.
simple system. - need adequate supply to pump, good pump and filter,
correctly working fuel pressure regulator ...and good lines/hoses ..that's
it.
problems are usually with pump, filter, and fuel pick-up in the tank being
restricted.
Rarely the fuel pressure regulator is faulty.
I just test for clogged tank by running the car off an other fuel source
like can of gas.

also ..
besides whatever tune-up or fuel injection issues there may be..
always consider clogged cat or muffler.
That can happen easily ....usually doesn't cause sputtering, but it shows up
as one customer used to put it............... 'delayed gratification.'

so which fuel system please  ?
Scott
www.turbovans.com

----- Original Message -----
From: "David Beierl" <dbeierl@ATTGLOBAL.NET>
To: <vanagon@GERRY.VANAGON.COM>
Sent: Wednesday, April 21, 2010 1:30 PM
Subject: Note Re: More Fuel Pump Weirdness - Has Mercury gone retrograde
again?

> At 04:28 PM 4/21/2010, David Beierl wrote:
>>The standard pump pulls fuel from the tank very nicely -- it's a
>>positive-displacement pump and would likely pull at 100 psi except
>>that gasoline vaporizes at much less pressure.
>
> That's the standard gasoline pump.  I know zero about the diesel setups.
>
> d