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92 deville poor mpg o2 sensor E44

7998 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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You state in the original post that all symptoms, including the O2 lean code, occur when turning on the cruise control. Check the cruise control servo diaphragm and it's vacuum lines for leaks and proper connection.
The modulator controls shift firmness/softness, and nothing else.
The OP stated that he already replaced the exhaust due to leaks.
The exhaust is under pressure. If there is a leak air cannot enter, much like a leak in a garden hose. Unmetered air can only enter via an intake (vacuum) leak.
Er, it's my understanding that fresh air will enter the exhaust at a leak. In fact, I've witnessed this on a number of occasions, mainly through exhaust manifold leaks at the mating surface to the head. The bank with the exhaust leak always has a rich biased fuel trim problem.

Due to the velocity of exhaust gases, fresh air is drawn in through the leak. Do you know how a siphon sandblaster works? Sort of like that.
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 :)
The link was merely to support my personal experiences rather than to act as sole proof. If you are curious enough, you can easily replicate it yourself. Induce an exhaust leak ahead of the O2 sensor and view the fuel integrator or block learn values through the onboard diagnostics. You WILL see them increase on the leaking bank only.

As far as ruining your engine, I agree, won't happen. Throwing fuel trims out of whack on fuel injected engines is a definite truth, however.

As far as 4.X manifold studs, your comment is surprising. These engines are some of the few in which I've never broken a stud. I'll even go as far to say as they come out really easy! I find the problem is always the y-pipe-to-manifold bolts :/
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