I think OP mentioned the problem happened when damp outside, so I would
suspect plug wires, especially coil wire, or hall sender plug.

-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of
David Beierl
Sent: Saturday, February 28, 2009 7:48 AM
To: vanagon@GERRY.VANAGON.COM
Subject: Re: Engine stutters, tach nosedives. Then, just as suddenly, it's
normal again.

At 08:38 PM 2/27/2009, Greg Potts wrote:
>I suspect the coil or the hall sensor; though Mark Dearing at Salem
>Import suggested it could also be the ECU.
>So what do you'all think? Was it just a loose plug wire, or are there
>deeper issues lurking?

Deeper issues.  The tach is driven off the coil primary negative
side, which the ECU ignition drivers pull down toward ground and
release in order to create the spark.  It is my belief that an open
circuit in the coil secondary -- from unplugged distributor wire --
would not alter the primary characteristics enough to prevent the
tach indicating, though I could be wrong.  I think you're looking at
a short in the Hall-effect sender plug at the distributor, which is a
known soft point, or ECU internals or wiring between the two.  If
it's the ECU I'd be inclined to suspect bad solder, since I always do
whenever anything electronic stops working -- even when it's not in a
high-vibration environment.

And of course it could always be that the coil terminal from the ECU
is just resting against its lug instead of securely on it.  That
happened to me once, tachometer drop and all.

  --
David Beierl - Providence RI USA -- http://pws.prserv.net/synergy/Vanagon/
'89 Po' White Star "Scamp"