Common Forum acronyms...

IIRC - If I Remember Correctly
HTH - Hope that Helps/Happy to Help
AFAIK - As Far As I Know
IMHO - In My Humble Opinion
RTFM - Read the Manual
OP - Original Poster (Thread starter)
LOL - Laugh Ou Loud
PMSL - Piss Myself Laughing
WRT - With regard to
LMAO - Laugh My Arse Off
ROFL - Roll On Floor Laughing
FUBAR/SNAFU/FUBB/TARFU - all basically 'Its broken' or 'Its not looking good'

Car specific:
HBV - Heater Bypass Valve
ICV - Idle Control Valve
EGR - Exhaust Gas Recirc
SAI - Secondary Air Injection
EML - Engine Management Light
ECU - Electronic Control Unit
DBW - Drive by Wire (no throttle cable)
CPP - Coil Per Plug
MID - Multi Info Display
CID - Colour Info Display
TID - Triple Info Display
GID - Graphic Info Display