92 Deville fuel mysteriously disappearing - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


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Old 08-13-2014, 09:25 AM   #1
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Default 92 Deville fuel mysteriously disappearing

I have a 1992 Cadillac DeVille. Gasoline is disappearing but I have absolutely no clue where.
The trip computer always show 20-25 % better mileage than Iím getting unless Iím going on the highway when the computer is almost exact. So for instance on the highway I might be getting 26 mpg according to the trip computer and when I fill the car up with gas I see that thatí* exactly right. Then during normal combined driving I always lack about 20-25 % of the gas. So I might fill it up with 5 gallons and then when the trip computer says ití* consumed 5 gallons, then 6-7 gallons are missing. So the computer might say I am getting 16 mpg when according to my own measurements at the pump I'm getting maybe 12-13 mpg. Where is the gas going?!

1. Car buckles when coming to a red light.
2. High fuel consumption is not indicated on the trip computer
3. A little rough idling
4. No gas smell from car
5. Recently replaced fuel pump

Iím at a loss. What could this be? Anyone recognize this symptom? Gas disappearing and not showing up on the trip computer? I figure that if the gas went into the engine the engine would compensate with more air to compensate for the extra fuel in order to maintain the proper ratio between fuel and air and if this happens the computer should know that more air is getting into the engine and from that adjust the calculations of the fuel computer and the extra gas going into the engine should show up on the fuel computer. What do you all think?

All answers appreciated
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Old 08-13-2014, 03:02 PM   #2
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The trip computer isn't very accurate in that car and with it having a drivability issue it could be making it worse. The 4.9 had a ton of issues with the secondary ignition system. I would replace the plugs, cap and rotor and take a good look at the coil for clouding and possibly replace it as well. I bet that will take care of the bucking issue.
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Old 08-14-2014, 02:35 AM   #3
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I have changed plugs and wires. I have looked at the rotor and cap and they look new. What is clouding of the coil and how can I check for it?
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Old 12-20-2014, 08:38 PM   #4
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On my 85, the trip computer display uses the fuel injector pulse width to calculate fuel consumption dynamically. I am pretty sure that your 92 works the same way. A leaky injector could account for the run-on and stumble, and also account for the miscalculations in the computer.

edit--whoops old thread

Last edited by ehall; 12-20-2014 at 08:40 PM.
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Old 12-21-2014, 04:33 AM   #5
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I replaced the fuel pump. It took care of the bucking and all of that. The high consumption I don't know what'* wrong but if I pop the pcv valve off of the engine the consumption comes back to normal. So I'm just running the car without the pcv (yes I did try buying a new one didnt change a thing).
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Old 12-21-2014, 11:48 AM   #6
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did you replace the vacuum line or see if something is causing a blockage there, has to have good vacuum to operate properly
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