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92 deville poor mpg o2 sensor E44

7995 Views 19 Replies 6 Participants Last post by  Ranger
Hi guys,

i have a 1992 sedan deville 4.9L with about 250,000 miles on the clock. i have nearly finished restoring it as it was due for the scrap heap. Anyway the exhaust had like 9 holes running down it so i replaced the entire exhaust including catalytic converter and silincer and the O2 sensor as well.

Now it runs like a dream but the MPG is down to 10MPG from 15MPG that is just crazy. the DTC that comes up is E44 O2 sensor running lean. i do maninly highway miles and if you turn on cruise control the mpg meter goes slight erractic then the code comes up.

just so you know i have changed the following in the last 2 years:

spark plugs, ISC motor, genrator, Vbelt, idler, battery, HT leads, airfilter, water pump all are AC Delco

many thanks

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250,000 miles on the clock. I think all engines wear out eventually and lose some efficiency. It probably has some leaky injectors too. I would check the vacuum to see how normal it is running.
I know it doesn't sound logical, but the PCM confirms it every time. Richened fuel trim on the leaking bank means the 02 sensor is somehow reading oxygen rich exhaust...

See this link:

There shouldn't be a terrible amount of pressure in the exhaust system, I would think no more than a few psi. Otherwise, it'd be blowing donut and manifold gaskets all the time. Exhaust is made to flow, not restrict. Flow is inversely proportional to pressure :)
I wise man once said "you can't believe everything you read". I've always been highly skeptical of everything i read. When I read that link it seems fairly logical yet highly theoretical; almost like something from Ripley's. I'm not raggin' on ya Drew because you are usually full of good info.

My Deville has had an exhaust leak for 4 years. It never affected the fuel mileage. 2 months ago I started getting a code that my O2 sensor isn't ready and now my fuel mileage has gone down quite a bit. I think the O2 sensor is just failing from 195,000 miles. An exhaust leak is a major nuisance but I doubt it will really ruin your engine. These 4.9 manifolds are hard to work on because the bolts or studs are so wimpy that they snap off easily. I don't have access to a lift to fix it properly (yet) so I've had to live with it.
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