2004 chevy trailblazer_lt
2002 Trailblazer LTZ I-6 4WD 143k miles. When starting after it has been sitting for a while (like overnight) the Service Engine Soon and Reduced Engine Power lights come on. While idling with the REP light on the RPM will vary back and forth from 600 rpm (idles very roughly at this point, almost to the point of stalling, and spits water from the exhaust pipe) to 1200 rpm (idles fine). If I let the truck idle for a while (anywhere from 10-20 minutes), shut it off and restart it, the REP light clears and the truck idles and drives normally, however the SES light is still on. However, after the truck has been driven some distance, when turned off and restarted, the SES light will go out. It seems that the colder the outside temp the longer it has to run to clear the REP light. When I am able to drive the truck, with the SES light on and the REP light off, the OBD2 reads P1515 and P1516 (thanks to my local Auto Zone) which are both throttle position sensor codes. The work done so far is cleaning the throttle body, replacing the throttle body/TPS sensor (which is all one unit), and replacing the accelerator position sensor. Still having the same problem. I should add that I had this problem last winter and didn't get around to fixing it. When the weather got warmer the problem went away and I was hoping that was the last of it. But now that the nights are getting colder the problem is back. Any ideas?