Is this a proper diagnosis?

[boiseDSM]

ISSUE:
So, I've had this incredibly difficult starting issue on a basically stock 90 tsi. The colder it is, the worse off the starting. It takes awhile to crank and eventually will catch (barely) but then dies. Another crank and it catches a bit more. Repeat 10 or 12 times until it stays idling. Once warm though, it starts right up. The starting difficulty seems to be a direct function to how long the car is sitting cooling off.

I've noticed that I get a CEL while driving after about 30 seconds of the engine running. I also found out that the CEL also turns on an after 30 seconds with the key turned to the acc. position but not started.

DIAGNOSIS:
I'm betting it is a coolant temperature sensor. Seems like the only way a CEL could get thrown with the ECU on but the engine not running is if there is a dead sensor. This would then explain why it runs bad when cold and hard to start. Am I thinking along the right lines here??? I haven't had the time to really take a close look. Just wanted to know what people with more DSM knowledge than I have might think about it.


Thanks guys

 

[Quasimondo]

Hard starts when cold might be a an indication of a bad coolant temperature sensor, but unless you check the trouble code you won't know for sure.

If you don't have a logger, here's a quick way to test for diagnostic trouble codes. Go to autozone and pick up a voltmeter. Next to the fusebox in the is the diagnostic port for the ecu. With the voltmeter you picked up from autozone, connect the probes to pins 1 and 12 of that diagnostic port as indicated in this picture:

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Pin 1 is the upper left pin, Pin 12 is the lower right pin

Next, turn the key to the ON position, but don't start the engine. When you turn the key to the ON position, take note of the number of short and long sweeps of the voltmeter needle. Those sweeps indicate the trouble code. The first sequence of long sweeps is the first digit. The next sequence of short sweeps is the second digit.

Here is the complete list of trouble codes:
11 - O2 sensor
12 - Mass Airflow Sensor
13 - Intake Air Temperature (located in MAS)
14 - Throttle Position Sensor
15 - Idle Speed Control position sensor (1.8 only)

21 - Engine Coolant Temp sensor
22 - Crankshaft Position Sensor
23 - Camshaft Position Sensor
24 - Vehicle Speed Sensor
25 - Barometeric Pressure Sensor (Located in MAS)

31 - Knock sensor
36 - Ignition Timing Adjustment Signal

41 - Injector
42 - Fuel Pump
43 - EGR (California only)
44 - Ignition Coil/Power Transistor

 

[boiseDSM]

Wow! Way more knowledge than I hoped for. This is exactly what I needed. I will check it ASAP and let y'all know.

Thanks again

 

[femmeDSM]

Great post Mondo....But to address the original poster: I would be willing to bet cash that your temp sensor is indeed the culprit. Every symptom you listed perfectly describes a bad coolant temp sensor. I mean that is a "By The Book" explanation of what happens when that sensor dies.

Before you pull the sensor, check the wires that go to it. These can become old and brittle and will break off. No connection = no signal to the ECU = effectively "no temp sensor".

Good luck

 

[andymoraitis]

I have to agree completely with Armando and Claire on this one. A bad sensor will create all of the symptoms you're experiencing.

Nice post Mondo and it sure is nice to see you back Claire Bear.

 

[boiseDSM]

FOLLOW UP:

Yup, you guys were correct. I went down and bought a new sensor and it fired up on the first crank. My whole apartment complex thanks you!

 

[BigPerm]

FOLLOW UP:

My whole apartment complex thanks you!

Good info here, I just picked up a project in non running condition, I dont know how long it has been sitting, but I finally got an ecu in it today and it displayed the exact symptoms. I am hoping this fix will work for me, but I guess the first thing to do is check the ecu. How much does the sensor run?

edit: 91 tsi btw.

 

[BigPerm]

Ok so I followed the instructions found here:
http://www.dsm.org/archives/1998/09/19980907.txt/23.html

I dont have access to a voltmeter so I thought I would try this, and what I got was a constant short blink which according to the info in the link, there are no codes. My car has been sitting for a while apparently, the ecu was fried when I picked it up. I put it the new (rebuilt) ecu today and started it up for the first time in god only knows how long, and it ran just like the original poster described his car to be running. My question is, do I need to let the car run for a while to let the ecu determine if there is a bad sensor, or should it pretty much do this right off the bat?
I'm hoping that I just need to let it run for a bit and the conditions will improve.

 

[Dream On]

It may take awhile since the ecu has been cleared of any codes. After driving for a day or two, I'm sure if it is the problem, then the code will show up. It takes the ECU some time to learn and adjust to the inputs of the sensors.