[T3] STUMPED!!???

[Dennis Stiefel]

It's been awhile since I've been stumped by one of my Squarebacks, but a
couple months ago I started having a problem that I couldn't find a cause
for.

Here's the problem I'd been having with my "daily driver" squareback:

It would run fine when cold, but once it was warmed up a bit it would
develop a really bad miss at low speed that would make it hard or impossible
to get started from a standstill. At best, the car couldn't get out of its
own way; at worst it was bad enough that sometimes it would kill the engine.
If I could pull over and let the engine cool off a bit, I could get rolling
again, but pulling over was not always an option.

This caught me a couple times in heavy traffic, stuck in the front of a long
line of waiting cars, when the light turned green. I'm afraid I didn't make
any friends that day.  

There was no hint of a problem at speed, so I couldn't tell whether I was in
trouble until I got nearly stopped.

I swapped coils and distributors, after checking the points, both FI and
ignition, and resetting them.

I checked the fuel pressure and made sure the pump was getting full voltage.
Ditto for the power to the FI brain.

I went thru all the FI tests with the Bosch D-jet system tester. 
Everything checked out okay: no bad parts, no bad connections.

I swapped every fuel injection component except for the injectors
themselves. I tried a different intake temp sensor and subbed a 120 Ohm
resistor for the warm cyl head temp sensor. (120 Ohms was what my sensor
measured when the engine was warm.) All with no effect.

All this was made more time consuming by the fact that I had to take it out
for a drive after every change, to get the engine hot enough to bring on the
symptoms. And once this happened, it meant that I was stuck somewhere away
from home until the engine cooled down enough to get me home. I eventually
started staying closer to home, driving around this neighborhood and
entertaining all the people out raking leaves.

After spending the better part of 2 days at this, I grabbed a different
distributor cap and rotor, because I hadn't changed those when I swapped
distributors. Suddenly it ran fine. Put the old cap back in and it still ran
fine. The old rotor LOOKs just fine, no tracks or cracks.

Drove it around on errands this morning and it ran fine. I'll put the old
rotor back in one of these days just to make sure, but I'm pretty convinced
that it must have been the problem.

Trouble is, I don't get how this could be.... I've seen bad rotors before.
They usually get a track or crack that lets the spark jump to the grounded
distributor shaft rather than going to the plugs. Once that happens, it
ALWAYS becomes a permanent short. Those kinds of things don't heal, and
insulators always get BETTER when they are warm.

The interesting fact here is that this rotor was a Taiwanese Bosch copy, not
genuine Bosch. I've never seen a Bosch rotor go bad, but the aftermarket
stuff is always suspect. In this case, I got caught because I tried to use
up a second rate part for myself because I wasn't willing to either throw it
away or pass it on to someone else.

I guess now I can throw it away. (After I reinstall it to verify that it's
the problem.)

And that's the rest of the story,

--
*******************************
Jim Adney, jadney at vwtype3.org
Madison, Wisconsin, USA
*******************************

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Sounds like the Taiwanese is made out of a material that expands and
contracts more with heat than the genuine one does so when it gets a crack
the crack expands more that it does with the other one if it got a crack.
Would be my guess at least.

Dennis Stiefel