[5speed5]

Something I noticed with my new-to-me 2006 HHR. It has the 2.4L Ecotec engine and if I use EOC before the engine is warmed up, the temp gauge can go dead. This issue is well understood. When the computer detects the car in motion and the temperature is too low for extended periods, it turns off the coolant temp sensor input. (Not sure why, I would think a dash light would be a better warning)

For non-hypermilers over on the HHR forums, this is an indicator that the thermostat is stuck in the open position and the fix is to get a new thermostat. In fact, how I found out about it was that it happened once to me and I changed the thermostat. Then it happened to me again and I knew it couldn't be the thermostat again, so I figured it out. It was because I was EOC-ing and it happened while traffic was moving slowly down a long grade, so I just coasted (engine off) for a good 10 minutes while the car wasn't up to temp yet. The conditions would look the same as a stuck thermostat to the car's computer.

Anyway, the purpose of this is to let others with GM Ecotec 2.2L/2.4L engines and manual transmissions who can use EOC (e.g. Chevy Cobalt, HHR; Saturn Ion, L200; Pontiac G3, Grand Am) that if this happens, don't spend money trying to fix it. Just drive it a little ways to get the temp up and it should fix itself after a few minutes.