Q: Help to diagnose low MPG and high oil consumption.

asked by on June 03, 2017

My 1999 10th Anniversary MX-5 has about 256K on it. Now, it gets about 18 MPG on highway, but used to be close to 30 MPG. It consumes about 3 quarts of oil per 1000 miles and there are no obvious oil leaks. I did replace the valve cover gasket. There are neither black smoke nor soot deposits around the exhaust. No codes – everything is clear. Sometimes, I can hear misfires when the engine is cold and the low-end torque feels lower than what it used to be. Otherwise, the car drives fine. The following has been done so far: 1) Replaced spark plugs and wires. 2) Compression for all cylinders was within 205-210 range which a believe is good. 3) Replaced oxygen sensor 4) Replaced fuel pump 5) Replaced camshaft position sensor 6) Cleaned throttle body 7) Replaced MAF I am thinking of testing fuel pressure, injectors and vacuum, but something tells that the engine probably needs a serious overhaul. Yet, I find it hard to reconcile the need to overhaul with good compression numbers Any ideas w

My car has 256000 miles.
My car has a manual transmission.

A few different things may contribute to oil consumption and poor fuel economy. Typically, high oil consumption is related to worn piston rings or worn out valve guide seals. In your case, it is unlikely this is related to worn piston rings as indicated by the proper compression you have already tested for.

A clogged or failing catalytic converter can cause the vehicle to have poor fuel economy. As you 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. 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 to 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. I would recommend checking the above to determine what may be causing your poor fuel mileage.

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