Hot start - Bypass Switch

Bernard Littau bernardl at
Wed Dec 4 16:07:22 EST 2002

> Well I did the shorting out of the cold start injector...
> The power starts when it is cranking come to find out.
> I ran a temperary jumper from the Blue/Whitestripe wire and touched it to
> ground when I was cranking.
> It sputered a little but it did start after a couple of seconds.
> So Do I do the switch or do I replace the injector.
> The check valve might be bad too.
> I checked the fuel pressure by cracking the fuel filter this morning cold
> and there was no presure.
> Of course it started right up.
> I drove it 15 mins and shut it of for an hour for an apt
> when I came out it cranked about 10 secs then protested..
> I have some cash outlayed for this and was wondering what I should buy...
> I read somewhere here that CS stands for CASH SUCKER
> it might be true

Cash Sucker might be a bit harsh :-)  These older cars need the odd part
here and there to stay happy, but the 5ktq as a whole is robust.  I think
you are seeing an accumulation of these odd parts all at once because small
problems were deferred.

It sounds like you are loosing pressure in the fuel system.  When you loose
pressure, it increases the likelihood of some kind of vapor-lock.  CIS is
especially susceptible to this because it has five direct, and long, hoses
between the metering head and the injectors.

You won't get vapor lock when the car is cold, because you don't have any
heat to cause the problem.

You also don't get vapor lock when the car is hot because you are starting
the car before enough heat is transferred to the gas to cause it to
vaporize, or before the pressure gets low enough to allow vaporization, or
some combination of the two.

When the car sits for a moment, and is "warm", you get the vapor lock.  The
long cranking is because it takes time to purge the vapor from the lines.

The primary thing to do is make sure the car does not loose pressure.  The
two end-points of the pressure part of CIS are the one-way check valve on
the fuel pump in the tank, and the pressure regulating spring and plunger on
the CIS metering head.  There is also the pressure accumulator, which is
under the RR part of the car, kind of under the gas tank.

The one way valve must not leak, and the plunger (o-rings) must not leak, or
you will loose pressure.  Now, the system is going to loose some pressure
over time, as none of the valves are "perfect".  The accumulator is unlikely
to cause you to loose pressure unless it is visibly leaking; it is there to
keep up the pressure for a long enough time to get past the period where
there is enough heat to cause vapor lock.

Another source of leaks is the injectors themselves.  They are only to open
and spray when the pressure is above a certain level.  Otherwise, they
should stay shut and hold pressure.

If your car has the original injectors, which is likely, then they probably
leak, and should be replaced.  A set of injectors is around $250, maybe
$300+ with the seats and rings and shrouds.

I had never considered that the cold start injector might be leaking, but
this, too, could be a cause of pressure loss.  Thank you to whoever
mentioned this recently, I learn something new every day on this list :-)

The check valve is not expensive, but it is a pain to install.  If you have
the money, and want to keep the car a while, you may want to replace the
fuel pump while you are in the tank.  Sometime the fuel pump comes with the
check valve, sometimes you have to buy the check valve separately.

I was able to find new o-rings that worked for the fuel pressure regulator
part of the Metering Head for pennies at a FLAPS.  I have heard there is a
kit of sorts one can buy that has these o-rings.

Of course, all the joints and fittings need to be clean and tight, and seal

I doubt that this is a problem, as it is too cold this time of year, but
when it gets hot enough outside, even if all the things above are perfect,
there may still be enough heat in the engine compartment to cause vapor
lock.  For this reason Audi put in the injector cooling fan.  If you put in
new injectors, you should then make sure the injector cooling fan and its
thermo-switch are working correctly, and that the plastic ducting is truly
directing the output of the fan to the injectors.

I would make sure all the joints are not leaking.  Then, pull the five
injectors and the cold start injector.  Use the fuel pump relay jumper to
run the fuel pump to make sure the system is well pressurized, and while the
fuel pump is running, trigger the cold start injector until fuel comes out,
then raise the vane in the air box until the five CIS injectors spray.  Turn
off the fuel pump, and wait and see if any of the injectors are leaking.


Bernard Littau
Woodinville, WA
'88 5ktq

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