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cortfitzer 12-05-2012 11:46 PM

90 735i Bad Mileage
So I just bought a 1990 735i. I already knew it didn't get good mileage, thats not why I bought it. Most people said it gets between 15 and 20 mpg. So I bought it. But heres the thing. Its getting 10 MPG! Thats horrible. Heres the things i have tried. Tire pressure, fuel injector cleaner, new air filter, new oil and oil filter,new tires, and even better driving habits.

The only thing I can think of now is the spark plugs. :dunno: Ideas?

02540ico 12-10-2012 06:35 AM

Replace spark plugs, dist cap & rotor. Test plug wires, make sure they're good. Make sure brakes/parking brake are not sticking. Make sure the torque converter is locking/unlocking properly.
Remove the 4 dead hookers from the trunk (should fit 4, the 540's fit 3) :rofl:

BigBoy740il 01-12-2013 03:59 PM

You should also look at replacing the O2 sensors. How many miles? They last about 60k miles. Lastly, check for vacuum leaks. The additional unmetered air means more fuel.

cortfitzer 01-12-2013 06:58 PM

I just recently replaced a bunch of vacuum hoses, so I know theres no leak. However, it has 154,000 miles and the Oxygen Sensor has never been replaced.

BeaterBimmerGuy 02-02-2013 02:05 AM

At 10 miles per gallon its got to be the O2 sensor. I got a real true Bosch one with low miles. Put on a 88 735i and the trans went out not long after. Took the car apart and crushed what was left. IN the link below in the 6th picture down just above the left blue speaker is the white wire of the Os sensor. 25.00 plus shipping got to be like 7 bucks shipping. If your interested email me and I will take pictures.

Post is old... But you never know..

waehrik 03-22-2013 08:26 PM

O2s are a royal PITA on these cars as you basically have to drop the entire exhaust system to get to it. There's only one O2, the connector is below the intake manifold (easy to get to) but the O2 itself is on the top of the exhaust and the body of the car interferes with access and removal. Thankfully it's easy to drop the exhaust if you have a lift. If not, it may be a better idea to pay an exhaust shop a few dollars to have them do it than curse on jack stands all afternoon. I had mine swapped in 1/2hr, but I have access to a lift.

I paid only $32 for a new O2 including the factory plug and wiring (no splicing needed). I bought it from Rockauto, MFG: APWI, P/N: AP4404. It's working great. I believe the sensor was stamped Made in Japan but my memory is a little hazy.

BeaterBimmerGuy 03-22-2013 08:38 PM

O2 sensors are different from model to model generally. Even tho aftermarket chepos work they are not quite right. I bought a E34 to flip since I posted my previous answer. AFter driving around the car for a few weeks I noticed the MPG was off but not bad O2 sensor off. I tested the O2 sensor and it tested fine got the .6 volts and all but I changed it anyways with the one I offered above and the MPG went up a little over 2 mpg. I read a long time ago how somewhat generic sensors are not quite right in most cases and how they affected the car. Its not a night and day change but there is a change. I also think not positive but think its a bit more powerful.

BMWFatherFigure 04-19-2013 12:52 PM

Go through the vaccuum hoses carefully. Have the injectors rig cleaned and check all fuel lines for leaks.

waehrik 04-19-2013 02:48 PM

For comparison's sake mine gets 16 around town and 21 highway, calculated at the pump. The OBC is surprisingly accurate though.

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