problem with "cold" starting

[joeblow55]

Hello all...I am new here. I found this site when experiencing alternator problems about two months ago.
Here goes... 2008 Aveo...when cold, particularly in the morning, the car behaves strange on the initial start up. Example... Engine fires, and after releasing the key from the start position, engine runs rough for about 1-2 seconds then dies. Try again...same thing. Third (sometimes the fourth) try the engine will stay running, but run rough for about the first 5-7 seconds, then "smooth out" to normal. There is not a check engine light, so I am not sure there will be any codes set on the ECM. Strange thing is, this just suddenly started happening about 7-10 days ago, and it only does it when cold. I am wondering if there might be a bad sensor and the ECM "thinks" the engine is already warmed up. Once warm, the engine runs and drives fine. No drivability issues that I am aware of. Any suggestions ?

[petrified.rabbit]

it could be the temp sensor, i'm not sure if that would through a code or not, i think it should, but you might still be in the parameters.. but it could really be a few things. This is a tough one to diagnose online..

[joeblow55]

Yeah, I know. And it will be tougher to try and diagnose as the weather warms up. We had a few days in the mid 70's here already, and the car seemed ok on those days. I notice that it's worse the colder it is. That's why I think it might be a bogus temperature sensor. If I don't get it figured out now, it may have to wait until the fall when we have those crisp early autumn mornings... Anybody know what/how that inlet air temperature sensor is supposed to work ? I did have to remove the connector when removing the air inlet tube a couple months ago when replacing the alternator. If I knew the pin-out or color code of the wiring on that connector, concievably I could look at the voltage on those pins (when cold) to see if it is in the normal expected range. I am relatively new to these ECM / computer controlled cars. I am old school, Holley carburetor, rear wheel drive, and lots of cubic inches "old", so try to be patient and bear with me as I learn...thanks

[petrified.rabbit]

its the coolant sensor that sets the cold start, not the intake air temp..

[ontarian_frog]

its the coolant sensor that sets the cold start, not the intake air temp..

That is correct. A coolant sensor that reports higher than normal temperature will affect cold starting and warm up.

[joeblow55]

Coolant sensor...gotcha. Now, where's it at, and how do I tell if it's reporting the correct voltage for the ambient temperature ?
I am assuming that there's some kind of "look up chart" where there should be say...3 volts at 30 degrees, 5 volts at 50 degrees, 10 volts at....blah blah blah. You get what I mean. I would hope I could read it directly across a couple of pins on the sensor with a voltmeter. Or is it more complicated than that ?

[petrified.rabbit]

i think its just an open/closed. not 100% sure, but in the wiring diagram its just 2 wires, no reference.

[ontarian_frog]

i think its just an open/closed. not 100% sure, but in the wiring diagram its just 2 wires, no reference.

Nope. Straight from the manual:

The coolant temperature sensor (CTS) uses a thermistor to control the signal voltage to the engine control module (ECM) and controls the instrument panel temperature indicator.

Here is the chart of both the IAT and ECT sensors.

[petrified.rabbit]

thats good to know. i guess i need to read more of the manual.

[ontarian_frog]

The reason I know is I couldn't start the car one morning and the reason was the coolant sensor.

#24 on the next picture is where the sensor is: