Hard to start engine when ambient temp. below zero

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Outside Temperature Sensor - Chevy HHR Network

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Jul 9, 2010 - I had the outside temp sensor go on my 99 Buick. I don't know where it islocated on the HHR, but it was just a plug in sensor. Take out the old, stick in the new.... 2008 HHR LS 2.2 L Auto Built: 12-19-2007. Bought: 4-26-2009 Location of the ambient engine temperature sensor.... Help ...
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run the fault codes and look for a problem ambient temp sensor

You can check the fuel pressure regulator ,

1. Inspect for a fuel leak from the fuel pressure regulator vacuum port.

Is the fuel pressure regulator leaking fuel ?
This next test needs a scan tool with data reading capability's ,not a code reader ! Are you familiar with OBD 2 ? All the sensors , coolant , O2S , TPS , MAF , Intake air temp. etc. Sensor/System
Monitor the engine coolant temperature (ECT) sensor with the scan tool. Compare the engine coolant temperature with the ambient air temperature on a cold engine. If the coolant temperature reading is more than 5 degrees more or less than the ambient air temperature on a cold engine, check for high resistance in the coolant sensor circuit.
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Inspect the PCM grounds for being clean, tight, and in their proper location. Refer to Power and Grounding Component Views in Wiring Systems.

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Inspect vacuum hoses for splits, kinks, and proper connections, as shown on Vehicle Emission Control Information label. Inspect thoroughly for any type of leak or restriction.

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Inspect the air intake ducts for being collapsed, damaged areas, looseness, improper installation, or leaking, especially between the mass air flow (MAF) sensor and the throttle body.

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Inspect for air leaks at throttle body mounting area, mass air flow (MAF) sensor and intake manifold sealing surfaces.

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Inspect the wiring harness for poor connections, pinches, cuts, or other damage.

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Inspect for loose, damaged, or missing sensors and/or components.

Really need to check sensor PID'S with a scan tool, An maybe a scope too ! PID'S are parameter identification . All the sensors operate within a set voltage parameter , like the coolant temp. sensor an if it's off it can affect fuel delivery !
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I am assuming that you know this since the temp gauge is not reading at normal temperature. I say this because I have had people in the past tell me that the engine wasn't warming up only to find out that the real problem was the heater wasn't working. They are two completely different issues. That said, the primary reason that an engine won't come up to operating temp is that the thermostat is either

1. Stuck wide open
2. Not installed.

temp sensor failed. could be the air intake sensor... or the ecu temp sensor falling... easy to find out if intake sensor failing. before starting on a cold morning. or under 10 degrees.. unhook the air pipe and blow with your breath over the sensor put back and start engine. if engine starts fine then that's your problem.

run fault codes to check for ambient temp sensor problems . check for fuel pressure in the manifold

DTC P0128: Engine Coolant Temperature (ECT) Below Thermostat Regulating Temperature
Do you have any issues with heat in the vehicle? If so the thermostat may need replaced If not the engine coolant temperature sensor is most likely reading incorrectly due to a bad sensor or connection to the sensor.

I think you should wait until you know the ambient temp will not go below about 50. If you've ever seen a 20w50 at or near freezing temperatures, it is as thick as honey. This is hard on oil pumps and it take a few second longer for the pump to start building pressure. I think a better choice might be to put in an additive like Lucas or STP. These additives tend to cling to bearings and reduce wear at engine start-up.

Check to make sure your thermostat works right by monitoring the warming up of your engine. With a completely cold engine, start and run the engine for a few minutes, then put your hand on the top radiator hose to monitor if and when it gets warm as the thermostat opens more and more and note the gauge reading( if it works) the top radiator hose should be unbearably hot within 5-8 minutes. If the therm. opens right it should close right aswell, staying open a little while eventhough the radiator may have cooled may be what you are experiencing causing the rapid cooling and is nothing to worry about. The Hose should not get warm very soon after starting the engine. Regarding you gauge, lets take one step at a time, verify proper op. or the therm.

The ambient temperature sensor going bad causes this exact problem. I've replaced many of them because when they show outside temp as being below 60 degrees, the A/C control module will shut the compressor off.