Q: Failing o2 Sensor - Horrible Fuel economy

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I have been getting horrible fuel economy in my 1991 BMW 318i. Since the engine is an Inline 4 with 1.8L displacement I do not see any reason for it to not perform efficiently on gas. The tank is about roughly 11 gallons and every full tank lasts me 150 Miles, when in fact people on BimmerForums are achieving 400 miles per tank. So I checked on all of the possible leads that would explain this issue but I can't figure it out. So I unplugged my o2 sensor and the exhaust became normal and the air/gasses out of the tip have less pressure when I hold my hand against the exhaust tip, but as I drove a few miles without the o2 sensor the check engine light came on. So I went home and plugged it back, fired it up and the exhaust gasses were much more aggressive when coming out, as if the engine has a dead cylinder although the rhythm is continous.

My car has 290000 miles.
My car has a manual transmission.

There are a number of things that can cause poor fuel mileage. A faulty camshaft position sensor is one of them. The camshaft position sensor is a magnetic sensor that monitors camshaft speed to regulate ignition timing and fuel injection timing. It gathers and sends information about the car’s camshaft speed (and as a result the position of each piston) to the car’s electronic control module. This information is received by the computer, which then uses this data to further calculate the time of ignition and the timing of fuel injection required by the engine. When this is not working properly, you may experience a drop in power, poor idling, poor fuel mileage and problems starting the vehicle.

A clogged or dirty EGR valve may also cause poor fuel mileage. The EGR valve is a small component designed to allow the flow of exhaust gases into the intake manifold in controlled amounts. Whenever you start the engine, the valve comes alive and waits in a closed position, blocking the flow of exhaust gases. Once the engine reaches operating temperature and speed increases, the valve, either through vacuum or electronic control will gradually open, allowing burned exhaust gases to enter and combine with the air-fuel mixture inside the combustion chamber. When the EGR valve is clogged or dirty, as they are prone to, it may not function properly causing the engine idle rough, stall at idle speeds, cause the car to get poor fuel mileage, or emit a strong smell of fuel due to excess hydrocarbons which also results in failing emissions tests.

A clogged or failing catalytic converter can cause the vehicle to have poor fuel economy. As you may already know, the catalytic converter converts toxic exhaust gases into less harmful pollutants that are suitable to be expelled into the atmosphere. When this is clogged, this causes these gases to be backed up into the exhaust system not allowing the engine to breathe properly. When this happens, the engine is forced to work much harder to expel these gases. This often results in the engine bogging down, hesitating, shaking and causing very poor fuel economy. I would recommend having a professional from YourMechanic come to your location to diagnose and inspect your vehicle.

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