So the van's been running fine all winter, except for the gas mileage has gone to crap - from 7L to 10L / 100 km. On start-up, the exhaust smells very rich and gassy for at least 2-3 minutes. If I'm doing short trips around town the fuel gauge just plummets. Once it warms up the gassy exhaust smell goes away and highway mileage is reasonable. The problem reminds me of what EchoVictor experienced in the arctic - EF12 engines burn a lot of gas when cold. So from what I've read, the fuel injection system can run in two modes: 'open loop' when the oxygen sensor is cold<600F, where the ECU guesses how much to run the fuel injectors based on engine coolant temperature, throttle, and manifold pressure sensors, etc. 'closed loop' when the oxygen sensor is warm>600F, where the ECU tunes its open loop guess based on oxygen sensor feedback. I read that open loop is set to run rich for easy drivability, so there's no stalling. But the exhaust really stinks, it seems a little much! I've checked the engine coolant sensor, its resistance is roughly in the ballpark. Spark plugs aren't fouled and indicate the mixture is OK (after a run on the highway). Distributor cap, rotor, and wires are new. Fuel and air filters have <5000km on them. Things I'm going to try: Running a few more tanks with fuel injector cleaner Unplugging manifold air pressure sensor (with engine cold idling) to test if the ECU's "limp home" substitution value is better Disconnecting the vacuum line to the fuel pressure regulator and sniffing for gas leakage (with engine stopped but ignition on and fuel pressure up) Checking the voltage at the connectors for engine coolant / manifold pressure sensors Any suggestions?