Could it be an O2 sensor related problem?

The ECU runs "open loop" until the motor is warmed
up, then it goes into closed loop and starts looking
for input from the O2 sensor.
If the O2 sensor or related wiring is screwed up it
might loose it's mind when warms up enough to
go into closed loop.
If you've just swapped engines, I might be inclined
to look for a problem with the O2 sensor wiring.
Did you hook the O2 sensor ground back up
when the new motor went in?

Steve
'86 Westy

At 08:46 AM 4/25/2003, you wrote:
>1986 2.1 2WD
>
>I just swapped my engine for my spare engine (old engine had
>blown head gasket).  Starts great, idles great.  Only one
>problem.  Once the engine is up to operating temperature, it
>stumbles and hesitates.  It behaves like it has water in the
>fuel.  To not overlook the obvious, I added methyl hydrate
>to the gas, but it's still behaving the same way.
>
>Basically, you step on the gas, and it just feels like the
>engine doesn't catch.  Then suddenly, it will catch for a
>few seconds, then hesitate again.  The engine sort of
>"stutters".  Idles fine.
>
>The thing that bugs me is that it runs GREAT when cold.
>When I swapped the engines, I replaced the Temp II sender,
>thermostat, upper and lower thermostat housings, coolant
>tee, rear main seal, clutch.  I have tried un plugging the
>ISV to no avail.  Checked to make sure I connect the engine
>ground strap.  Turn off the van, re-start, no help.  Nuttin.
>
>Any thoughts?
>Keith Adams | calgary.ab.ca
>http://www.clubveedub.com
>86 Westy | 59 Euro Beetle | 66 Beetle