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Old 06-19-2008, 08:10 AM
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hugh1850 hugh1850 is offline
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Location: Houston
 
Join Date: Jun 2003
Posts: 2,318
Mein Auto: M3
Quote:
Originally Posted by jimoreno View Post
That is not normal. My '86 535i is currently giving me 21.0 MPG as reported by the OBC since I'm too lazy to write down mileage and fuel amount when I fill up. You are running rich which could be due to a bad thermo-time switch (about $40), a leaking cold start valve (about $50) or a very badly tuned motor. If this is the case you WILL destroy your catalitic converter so fix ASAP! You mentioned that you just recently picked it up, have you done a tune-up on it? This means, at least for me, replacing all filters (fuel, air, oil), oil change, spark plugs (use the correct ones, DO NOT use platinums!) and also the ignition cap and rotor. Depending on mileage and condition I might even change the spark plug wires (DO NOT use Bosch here!). Also make sure that there are no vacuum leaks by looking at all the small hoses and the inlet elbow (they tend to crack with age).
After you have done all this, adjust the valves and see how it feels. If you still have bad economy it might even be bad injectors.
How does the engine feel when idling and pulling? If you are planning on keeping it, get a Bentley so you can fix it yourself.

Good Luck!
The OBC is notorious for reading high. If yours is reading 21 w/ mixed driving I'm willing to bet that your true mpg is closer to 17-18 range. I get around 15-16 mostly city driving as calculated by miles driven and gallons burned at fill up. My OBC says I get a little over 18.5 mpg.
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