Cadillac: 1990 deville poor mileage, air fuel ratio, poor mileage

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I have change the plugs,wires,distributor,fuel filter,o2 sensor, converter and finally the ECM. The timing has been checked and is at approx. 0 degrees, I changed it to 10 and the engine runs poor (knocks). At this point I'm only getting 9 - 10 MPG.---No Codes shown.

What am I missing?
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How are you calculating your fuel mileage ?.

The old fashioned way - miles driven / Fuel fill gallons. (the DIC is telling me that I should be getting 19-20mpg).

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That DIC ios totally useless as I have yet to see one that was accurate since they first installed them in 1980.


Pull a spark plug and see if it is carbon fouled on the electrode. If it is this would indicate a rich air/fuel ratio.

Some people claim that replacing the qxygen sensors has improved their fuel mileage but after working on Cadillac's for 25 yaers I am skeptical.

Make sure that you are setting your timing EXACTLY as indicated.

You could perform an exhaust BACK PRESSURE TEST to see if you are starting to get a catalytic converter restriction if you have high mileage on the vehicle.


Another item everybody is guilty of is not making sure that their tire pressures are at factory specifications.

Reply:
The tire pressure is correct. The Catalytic covverter has already been replaced.... no change. I also forgot to mention that I replaced all of the vacumn lines.
What next..?
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Perform this test procedure:

Test for codes:

1. Ignition ON, Engine oFF.

2. Simultaneously push and hold for 2 seconds the OFF and WARMER buttons on the display panel.

3. Write down ALL the letters and numbers that appear and how m,any times they appear until the display stops and indicates .7.0.

4. Turn ignition OFF.

5. let me know what codes you get.

Reply:
No codes I show 7.0...

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The only other option is to have a scanner put on the vehicles computer to see what all the sensors are doing.

Are you calculating in-town and highway driving together or just highway driving ?.

Reply: Most of the driving is on the highway / rural roads. about 10-15% city.
Would the coolant sensor create an issue... that's the only part I did not replace?
Thanks.....  

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If the coolant sensor was out of calibration you would get a CHECK ENGINE light illumination on the dash and a code 14 or 15 would set.

I have heard tell that some people have randomly replaced the coolant sensor and or the oxygen sensors and got better mileage, But I am very skeptical.