Follow-up: RE: '86 4kq no wanna take gas, Capt'n

[Bernard Littau]

Hi Huw,

Thanks for the suggestions and the link! (And thank you everyone else who
sent suggestions; Huw's email spurred me to finally send a follow-up to the
list...)

I did check the fuel distributor -- was thinking maybe it was binding as it
got hot.  That was not it.  I finally sat down with the Bentley for the 4kq.
I always thought of the 5ktq as the more sophisticated car, but the 4kq has
a more modern FI system.  Originally, I had discounted the temperature
sensor influencing the DPR, because on the 5ktq it cannot influence the
mixture by enough to make the car spew black smoke from being overly rich.
Well, on the 4kq, the DPR appears to be used for both the fine mixture
adjustment dictated by the O2 sensor, and for the course enrichment dictated
by the temp sensor on the bottom of the water neck from the head to the top
of the rad.

It was this temp sensor that was bad.

On the 5ktq, the course mixture adjustment is done hydro-mechanically by the
warm up regulator.

Anyways, problem mostly solved with a new $32 temp sensor.  Now it appears
that someone must have tried to compensate for the failing temp sensor by
messing with the idle mixture, as the car now runs more poorly at idle.
This is where your web site will come in very handy :-)  Basic tune up time.

Now its a $532 Audi 4kq.  Not too bad!

You might appreciate my interim kludge fix while I waited for the new temp
sensor to come in.  The Bentley mentioned a 15K ohm test connector, so I got
3 4.7K ohm resistors and wired them in series, and used the resistors
instead of the temp sensor as a starting point to run the car.  It was still
way rich, so I added a fourth resistor.  Way worse; duh, it must have a
negative temperature coefficient -- no wonder it hardly runs at all when you
disconnect the temp sensor.  I went to two 4.7K ohm resistors, and that got
me to the parts store and work.  On the way home I went to one 4.7K ohm
resistor, and that was better but not perfect.  The next morning, I started
the car with the three resistors, and drove it out the driveway and up our
dirt road to the mailboxes.  There I grabbed the paper and went down to 1
resistor.  I drove that way to Starbucks (10 min), grabbed my latte, and
connected 2 of the 4.7K ohm resistors in parallel (2.35K ohm) and went to
the part store to pick up the new temp sensor, and on to work.  Car ran
great from start to finish.  Gee, I don't need a temp sensor, just a few
resistors and some test clips :-)

The new temp sensor must have even less than 2.35K ohm resistance once the
car is fully warm, as my real-time gas mileage on the instrument cluster
showed even better gas mileage than with the resistors in parallel.  In
fact, it gets better mileage with the new sensor than it ever did before,
which kind of confirms the sensor was bad for a long time.

Just as a side note to a long side note: the real-time gas mileage display
was showing 5.0 MPG during much of the bad periods when the sensor had
failed. (5.0 is its minimum reading, so it was likely lower)  The MPG
sometimes got to 6.0 or so.  Hardwired at 14.1K ohm, the MPG hardly got
above 8.0, even though the car ran tolerably.  I am amazed at how well the
car actually ran when it was using 3-4 times as much fuel as normal.

Best,

Bernard Littau
Woodinville, WA
'86 4kq
'88 5ktq


> > Well, the problem with the '86 4kq that the PO told me about
> has finally,
> > after 4 months or so, surfaced.
> >
> > The car starts and runs fine cold.  In fact, it runs fine until
> after about
> > 20 minutes of driving, way after the water/oil temps are up to normal
> > running temps.
> >
> > The unwillingness to take gas, and the stumble and black smoke
> phase seems
> > to last 10 minutes or so, then the car gets progressively
> better at taking
> > gas.  It always idles fine.
> >
> > All of the things I can think of would make lean, not rich
> (vacuum leaks,
> > for instance).  I'm thinking the black smoke is a rich indicator.
>
> i think you are right...
>
> > Any clues of what might be wrong, or hints at what/where to
> test would be
> > most appreciated.
>
> Fuel distributor... remove the boot and see if the air plate moves
> freely up and down.  Check the OXS output voltage while running, and DPR
> current... are they doing the right thing?
>
> (see http://www.humanspeakers.com/audi/timing2.htm )
>
> --
> Huw Powell
>
> http://www.humanspeakers.com/
>
> http://www.humanthoughts.org/