MIL is on

The car is a 2003 VW Golf GTI 1.8T. Recently the MIL light came on and
it was diagnosed with a defective secondary air pump and thermostat. I
had them replace the secondary air pump only. The thermostat was not
replaced because the engine is not overheating and the temperature
gauge reads normally at 90 degrees Celsius.

The question is would a malfunction thermostat causes the MIL light to
come on?

On Sun, 08 Mar 2009 17:44:11 -0400, Dakkel Lateg >
wrote:

>The car is a 2003 VW Golf GTI 1.8T. Recently the MIL light came on and
>it was diagnosed with a defective secondary air pump and thermostat. I
>had them replace the secondary air pump only. The thermostat was not
>replaced because the engine is not overheating and the temperature
>gauge reads normally at 90 degrees Celsius.
>
>The question is would a malfunction thermostat causes the MIL light to
>come on?
Are you sure it was not a temperature sensor? Does the water temp
idiot gauge indicate proper temp? On my 2003 I have to use Vagcom to
clear out some error codes before warning lights will go away. If your
water temp sensor is not generating the correct values to the engine
management system your car is not going to run properly. If your
thermostat is stuck open your car may never warm up which will cause
issues.

Thermostats fail all the time on these. The code probably was cooling system
performance/malfunction.
the engine takes too long to warm up, increasing emissions.


"Dakkel Lateg" > wrote in message
news
> The car is a 2003 VW Golf GTI 1.8T. Recently the MIL light came on and
> it was diagnosed with a defective secondary air pump and thermostat. I
> had them replace the secondary air pump only. The thermostat was not
> replaced because the engine is not overheating and the temperature
> gauge reads normally at 90 degrees Celsius.
>
> The question is would a malfunction thermostat causes the MIL light to
> come on?

On Mon, 09 Mar 2009 09:21:52 -0400, Jim Behning
> wrote:

>On Sun, 08 Mar 2009 17:44:11 -0400, Dakkel Lateg >
>wrote:
>
>>The car is a 2003 VW Golf GTI 1.8T. Recently the MIL light came on and
>>it was diagnosed with a defective secondary air pump and thermostat. I
>>had them replace the secondary air pump only. The thermostat was not
>>replaced because the engine is not overheating and the temperature
>>gauge reads normally at 90 degrees Celsius.
>>
>>The question is would a malfunction thermostat causes the MIL light to
>>come on?
>Are you sure it was not a temperature sensor?

The temperature sensor was replaced in April 2006 and approximately
52000 kilometers ago.

> Does the water temp idiot gauge indicate proper temp?

It indicates 90 degrees Celsius, that is the needle is vertical. It
takes approximately 6 minutes for the needle to go vertical.

On Mon, 09 Mar 2009 22:35:28 -0400, Dakkel Lateg >
wrote:

>On Mon, 09 Mar 2009 09:21:52 -0400, Jim Behning
> wrote:
>
>>On Sun, 08 Mar 2009 17:44:11 -0400, Dakkel Lateg >
>>wrote:
>>
>>>The car is a 2003 VW Golf GTI 1.8T. Recently the MIL light came on and
>>>it was diagnosed with a defective secondary air pump and thermostat. I
>>>had them replace the secondary air pump only. The thermostat was not
>>>replaced because the engine is not overheating and the temperature
>>>gauge reads normally at 90 degrees Celsius.
>>>
>>>The question is would a malfunction thermostat causes the MIL light to
>>>come on?
>>Are you sure it was not a temperature sensor?
>
>The temperature sensor was replaced in April 2006 and approximately
>52000 kilometers ago.
>

So?

>> Does the water temp idiot gauge indicate proper temp?
>
>It indicates 90 degrees Celsius, that is the needle is vertical. It
>takes approximately 6 minutes for the needle to go vertical.

I'd suggest Jim's advice...

On Mon, 09 Mar 2009 22:35:28 -0400, Dakkel Lateg >
wrote:

>On Mon, 09 Mar 2009 09:21:52 -0400, Jim Behning
> wrote:
>
>>On Sun, 08 Mar 2009 17:44:11 -0400, Dakkel Lateg >
>>wrote:
>>
>>>The car is a 2003 VW Golf GTI 1.8T. Recently the MIL light came on and
>>>it was diagnosed with a defective secondary air pump and thermostat. I
>>>had them replace the secondary air pump only. The thermostat was not
>>>replaced because the engine is not overheating and the temperature
>>>gauge reads normally at 90 degrees Celsius.
>>>
>>>The question is would a malfunction thermostat causes the MIL light to
>>>come on?
>>Are you sure it was not a temperature sensor?
>
>The temperature sensor was replaced in April 2006 and approximately
>52000 kilometers ago.
>
>> Does the water temp idiot gauge indicate proper temp?
>
>It indicates 90 degrees Celsius, that is the needle is vertical. It
>takes approximately 6 minutes for the needle to go vertical.

So the answer is no to the temperature sensor. Have you checked out
the sensor to see if it meets book values? The Bentley repair manual
show a procedure to check that also requires a Vagcom/VCDS. So your
first sensor may have lasted 3-4 years depending on when your car was
made. Maybe second sensor did not last as long. Hard to know without
going through the book testing procedure.

Note that VW has a water temp idiot gauge. It will indicate 90 even if
the water is hotter or colder within a range. It reduces the amount of
complaints service techs receive of "my car is running hot" or "my car
is running cold." You need a diagnostic tool like Vagcom to read what
the temp really is.

>>>>The question is would a malfunction thermostat causes the MIL light to
>>>>come on?

Yes I think I have seen a light come on due to a faulty thermostat.

See if these codes have popped up.
16512
P0128
Coolant Thermostat/Valve Temperature below control range


--
later,
(One out of many daves)

On Tue, 10 Mar 2009 18:59:30 -0500, "dave AKA vwdoc1"
> wrote:

>
>>>>>The question is would a malfunction thermostat causes the MIL light to
>>>>>come on?
>
>Yes I think I have seen a light come on due to a faulty thermostat.
>
>See if these codes have popped up.
>16512
>P0128
>Coolant Thermostat/Valve Temperature below control range

The code is P2181.

What effect does a "temperature below control range" have on the
performance of the engine?

On Sat, 14 Mar 2009 14:22:17 -0400, Dakkel Lateg >
wrote:

>On Tue, 10 Mar 2009 18:59:30 -0500, "dave AKA vwdoc1"
> wrote:
>
>>
>>>>>>The question is would a malfunction thermostat causes the MIL light to
>>>>>>come on?
>>
>>Yes I think I have seen a light come on due to a faulty thermostat.
>>
>>See if these codes have popped up.
>>16512
>>P0128
>>Coolant Thermostat/Valve Temperature below control range
>
>The code is P2181.
>
>What effect does a "temperature below control range" have on the
>performance of the engine?
First guess is the engine thinks it is cold. It runs a richer mixture.
Catalytic onverter eventually burns up because of excessively rich
mixture.

http://tech.bentleypublishers.com/th...readID=237 37
go there and read......quite a few problems can cause this light to go on.

I have heard that the following could cause this:
CTS (need a green one and test it with the vag-com tool while the engine
heats up)
Thermostat
Water pump
Coolant mixture might need more water than coolant
fans

Something might telling the ECM that the temperature is not getting hot
enough quickly enough or maybe too hot.

"Jim Behning" > wrote in message
...
> On Sat, 14 Mar 2009 14:22:17 -0400, Dakkel Lateg >
> wrote:
>
>>On Tue, 10 Mar 2009 18:59:30 -0500, "dave AKA vwdoc1"
> wrote:
>>
>>>
>>>>>>>The question is would a malfunction thermostat causes the MIL light
>>>>>>>to
>>>>>>>come on?
>>>
>>>Yes I think I have seen a light come on due to a faulty thermostat.
>>>
>>>See if these codes have popped up.
>>>16512
>>>P0128
>>>Coolant Thermostat/Valve Temperature below control range
>>
>>The code is P2181.
>>
>>What effect does a "temperature below control range" have on the
>>performance of the engine?
> First guess is the engine thinks it is cold. It runs a richer mixture.
> Catalytic onverter eventually burns up because of excessively rich
> mixture.