[s-cars] cold start issue or external temp sensor malfunction?

[Vincent]

IIRC, the two air temp sensors are:
- one in the bumper, behind the lower grille, used by the external temp
display (the flaky one in your case),
- one at the fresh air intake for HVAC, used by the HVAC head unit,
none of them linked to the ECU.

The ECU use an mass air flow (MAF) sensor which takes into account the air
temperature by measuring the mass of air entering the engine instead of the
volume (in fact, air temperature is not really relevant for the engine,
except for its impact on air density, therefore the MAF sensor instead of
air flow sensor).

As for temperature, the ECU adapts to _engine_ temperature using the coolant
temperature sensor, which is not related to the external air temp sensor.
IIRC again, there is two coolant temperature sensors:
- one on the coolant metal hose on top of the engine, used by the coolant
temp gauge, known to fail but not linked to the ECU.
- one on a T fitting at the back of the engine (blue plug), used by the HVAC
and the engine ECU, fairly reliable except when T fitting crack open (and
you would have notice the big pond of coolant below your car).
So cold start problem related to a temperature sensor is unlikely
considering the ECU relies on a fairly reliable sensor.

Since both problems happened at the same time, I would rather suspect a
connector problem, connector shared by the external temp sensor and some
sensor used by the ECU... or it may be pure coincidence, there's many
component in old cars which don't like cold.

HTH




Vincent

-----Original Message-----
From: s-car-list-bounces at audifans.com
[mailto:s-car-list-bounces at audifans.com] On Behalf Of chris chambers
Sent: 24 January 2011 11:22
To: Scar
Subject: [s-cars] cold start issue or external temp sensor malfunction?

Gentz,

One of (I am told there are two) external temp sensors caused very
intermiitent 
problems last summer indicating
in the display that it was -49 degrees outside instead of the warm +84 it
was. 
Everytime it acted up I was wearing 

a suit and unable to diagnosis.

It hasn't acted up for MONTHS, well it acted up this morning indicated -49 
degrees instead of the balmy +16 that it was.
Car did not want to start, it tried but didn't .....never had this problem 
before.  As I sat there freazing my arse off pondering
what to check I glanced down and the temp display now read +16, tried
starting 
again and Vrooom it started, however it
seemed to miss on one cylinder for ~30 seconds and then ran and idled fine.

I have started the car three more times this morning since then and have no 
issue.

Does sthe external temp sensors provide data for the ECU? Could the flaky 
external temp sensor cause the fuel/air mix
to be changed?

I welcome input

Thanks
Chris



      
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