Hi there, I am new here so I am not sure if this has been discussed yet, but I did look around and didn't see it. On 02/07/07 our check engine light came on and there was a lot of what I describe as valve noise coming from the top end. We took it the dealer immediately and the next day they called and said the PCM needed to be flashed. They did the work and I asked for a copy of the Technical Service Bulletin. Here it is...
SERVICE BULLETIN NUMBER: 18-020-06 REV. A
GROUP: VEHICLE PERFORMANCE
DATE: SEPTEMBER 16, 2006
FLASH: Warm Weather Spark Knock and/or DTC P2173 **and/or P0452**
This bulletin involves selectively erasing and reprogramming the Powertrain Control Module (PCM) ** and if necessary, the Transmission Control Module (TCM)** with new software and performing a memory reset to clear the adaptive memory of the PCM.
2007 (PM) Caliber
**2007 (MK49) Compass**
Note: This bulletin applies to models equipped with a 2.0l or 2.4l gasoline engine and CVT transmission (sales code ECN or ED3 with DAV) built on or after May 11, 2006 (MDH 0511XX) through and including August 16, 2006 (MDH 0816XX)
Some cutomers may experience Spark Knock in warm ambient conditions (16 C (60 F) or higher).
The technician may find one or more of the following DTC's
P2173 - High Airflow/Vacuum Leak Detected (Slow Accumualtion) may be found in the PCM. This DTC may occur when a vehicle is driven from a higher altitude to a lower altitude (1372 meter (4500 foot) change).
**P0452 - Evap Pressure Switch Stuck Closed**
It goes on to describe the repair procedure.
Anyway, they flashed the PCM and told me that essentially the engines timing was off. This may be why experienced only 18 - 20 MPG. However, that may be due to my extremely heavy right foot.
Hope this helps someone.