[T3] Bad day at the (FI) office
Jim Adney
jadney at VWType3.org
Fri Apr 8 21:05:20 PDT 2016
Daniel,
I have a theory. While you've been having trouble with your '69 I've
also been having trouble with my '73. It finally occurred to me that
your problem might be similar to mine. Your symptoms are similar, but
I'm guessing that your FAR meter has been leading you to make the
wrong guess about the problem.
My '73 has developed a tendency to flood 2 cylinders when started up
from cold. In this condition it has almost no power and I'm sure that
leaves two spark plugs black. When I finally get all 4 cylinders
going, the idle speed comes up (naturally) and the full power
returns.
Here's what I've been doing to get the engine "unflooded."
With the engine running well above idle, I turn off the fuel pump
leaving my foot at the same place on the throttle. As the engine
continues to run, the fuel pressure drops and the engine first speeds
up, indicating that it likes things a little leaner. After a few
seconds of that, the rpms start to drop and, just as I think it's
going to die, it suddenly picks up again. This is the 2 flooded
cylinders kicking in. So 2 cylinders die just as the other 2 come to
life. This continues for a few seconds, the time depending on how
flooded they were. Then the rpms drop quickly and the engine will
die.
If I can turn the fuel pump back on just before everything dies,
sometimes I'll get 4 cylinders back. I repeat the whole process until
I get 4 cylinders. Sometimes it helps to crank with WOT and the pump
off, just to flush out those cylinders, then turn the pump back on
while cranking and see if you get 4 cylinders.
It's a pain, but, with practice, you can do this by reaching under
the dash and unplugging the FP relay. It takes practice to be able to
pull it off just far enough that one terminal is still connected, so
you can push it back on easily when you need to. But, I have a better
solution:
I've been working on a different form of fuel pump primer switch. The
first version, which you have, is just a push button which turns the
pump ON. This 2nd version has a 3 position toggle switch:
OFF/Normal/(Run.) (Run) is momentary; the equivalent of the
pushbutton. OFF turns the pump off. It's a LOT easier to reach under
the dash and flip this switch between OFF and Normal than it is to
plug and unplug the FP relay. I haven't really put this version out
for sale, but this sounds like a good time for a beta test.
I suspect that your AFR meter is lying to you because it's getting
treated in ways that it was never meant to operate under. It's
expecting hot combustion exhaust, but instead it's getting fairly
cool combustion exhaust mixed with fuel-air mixture that never
burned. Plus it's seeing gasses that are varying in temp so that it's
never steady state. It will be VERY interesting to know what it says
once you get things working right, but for now I think you have to
either turn it off or learn to ignore it.
I'll make up a 2nd version primer switch to send you, but that will
take me a few days. By that time I should have 1 more brain that
might work for you. I'm afraid only 1 of yours turned out to be
fixable given my incomplete knowledge of the schematic. You should go
thru the A & B brains you have there, because some of them that
you've rejected in the past are probably just fine. The switch and
brain can ship back to you next week. In the meantime, play with
leaner mixtures. Start with no resistor and the pressure sensor back
to /. Then if you get the bucking (missing), try the pump relay
trick. You might want to try your other CHTS, too, because if this
one goes open (R= infinity) that will push things very rich.
--
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Jim Adney, jadney at vwtype3.org
Madison, Wisconsin, USA
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