Hot start problem 88 5kt automatic

[Bernard Littau]

I agree with Joshua, check for spark.

On my 5ktq, I had these same sorts of symptoms, and it turned out to be the
distributor Hall Sensor was failing.  Sadly, I needed to purchase a new $200
distributor to fix, no board kit for the 5ktq and was unable to cross ref
the chip and go that route in a timely manner.

The Hall Sensor doesn't really stop "functioning", but the waveform gets
worse with increased temperature, to the point the ECU no longer can
recognize the signal.  I put my old distributor and a new one on an
ocilloscope and could clearly see the deformed wave from the Hall Sensor on
the old one even when cold.

Could also be the flywheel Hall Sensors, though I suspect they are not as
prone to temperature fluctuations as the distributor Hall Sensor.

If spark is absent when hot, one other test might be to spray the Hall
Sensor board in the distributor with some electronic component "freeze"
spray, or some of that "air" used for cleaning PCs, and then checking for
start/spark.

Best,

Bernard Littau
Woodivnille, WA
'88 5ktq


On 2/2/06, Joshua van Tol <josh at spiny.com> wrote:
>
> Has your friend checked for spark while this is happening? That would
> be the very first thing to do before swapping a bunch of parts. None
> of the things your friend has replaced are likely to fix a hot start
> problem.
>
> On Feb 2, 2006, at 6:52 PM, Doyt W. Echelberger wrote:
>
> > Friend's 1988 5000 turbo (fwd automatic) has developed a tendency
> > to not restart after being shut down hot. In desperation he asked
> > for suggestions yesterday.
> >
> > He drives for 30 minutes, parks in driveway, turns off engine, trys
> > restart and all it does is crank. Won't catch.
> >
> > MUST cool down before it will restart when this happens.....20-30
> > minutes. And it doesn't happen all the time. No apparent pattern.
> >
> > Runs_ great _when started cold and stays that way as long as it is
> > driven without being turned off.....sometimes. He already replaced
> > the oxygen sensor, the plugs, the wires, the cap and rotor.
> >
> > Good-running spare parts type 44 of same year is sitting there
> > waiting to swap anything and everything to see what will make the
> > problem go away.
> >
> > Here is what I found in the archives:
> >
> > This problem is rampant among UrQ's and 5000 models. Apparently is
> > a vapor-lock thing. It is the reason Audi put the jury-rig blower
> > in there to cool down the injectors and reduce under-hood temps.
> > Also one of the reasons for coming out with the next generation of
> > inline 5's.
> >
> > This vapor-lock explanation makes sense to me because we have been
> > having unusually warm winter weather (50 degrees F) here in Ohio,
> > and the gas station fuel mix is still being supplied to our area is
> > set for normal winters.
> >
> > Also could be aging of a relay that controls the fuel pump or the
> > one that causes hot-start pulsing of the cold-start valve. Or maybe
> > a poor ground for any of those relays. Or maybe a temperature
> > sensor is telling the ECU that the engine is cold. and needs the
> > cold-start valve open, and the mixture is so rich it won't run. Or
> > maybe the RPM sensor resistance is out of its range and needs
> > replacing.
> >
> > Anyhow, before I get back to my friend, are there any other ideas?
> >
> > Doyt Echelberger
> > 87 5ktq that runs just fine (!) (today)
> >
> >
> >
> >
> >
> >
> >
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>
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