[T3] STUMPED!!???

[Jim Adney]

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,

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