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Old 05-06-2008, 07:26 PM   #1
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Fooling the Engine Computer to alter Mixture

I have two OBD II 6 cylinder Jeep vehicles (4.0 liter straight six pushrod engines). One is a 5 speed Wrangler, and the other is a 4spd auto Cherokee. Both weigh about 3300 pounds.

The Wrangler was getting 25mpg, but would at extended idle set a MIL for slow O2 response. The engine ran flawlessly and the vehicle passed emmissions easily in this condition.

Thinking I was doing the right thing, I replaced the O2 sensor and now am lucky to get 18-20mpg. The engine STILL runs well, and I STILL pass emmissions, but my fuel economy sucks. Obviously my old O2 sensor caused the engine to run leaner.

The dealer told me most owners get 15-17mpg, so my 20 is now considered "exceptional."

My Cherokee gets 16mpg, which everyone says is normal.

I disagree on both counts.

I am a VERY "easy" conservative driver and usually get MORE than the EPA rated mileage from a vehicle, yet these Jeeps are gas hogs. I rarely exceed 55mph, and do no stop-and-go city driving. Most of my driving is 2 or 4 lane country driving in flat Texas country. My other vehicles weight between 2900 pounds and 3500 pounds and driven the same way get between 24mpg and 29mpg.

The exhausts pipes on both Jeeps are sooty black (as are those on other 4.0 liter Jeep tailpipes I have inspected) which makes me think they are all running rich.

The O2 sensor is the primary sensor that sets the fuel mixture, since any variation from what it sees as stoichemetric mixture causes the mixture to be altered to produce the voltage corresponding to stoichemetric, or about 0.445 volts.

My proposal is to somehow alter the O2 output voltage by adding a small voltage (200-300 millivolts) in series with the O2 output voltage. This should fool the ECU into believeing the mixture is a bit rich, which would cause it to lean the fuel output in an effort to reduce the O2 output voltage to the desired 0.445 volts or so. This will result in an overall leaner air/fuel mixture ratio.

I find it compelling that these Jeeps all seem to get mediocre fuel economy and at the same time produce very sooty exhausts, while my other vehicles all have tan-to-brown tailpipes and get much more reasonable fuel economy.

My question is if this has been tried before, and if so has it been successful in altering the mixture enough to improve fuel economy?

I realize that a leaner mixture CAN cause problems at high power settings (higher combustion temperatures, detonation), but a cruising throttle in flat terrain, it would seem that running a bit leaner (at least where the exhaust pipe isn't excessively sooty) could save some fuel. A variable voltage supply could easily mounted in the passenger compartment to restore the factory settings should terrain or engine load require it.


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Old 05-07-2008, 07:11 AM   #2
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There is a device called a "fever buster" that you can buy.

I have had the same thoughts. Here is a thread that I started

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Old 05-07-2008, 07:25 AM   #3
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Folks who want to bypass emissions for performance purposes can buy o2 sensor simulators, perhaps such a device could help you. I don't know if that's what the above mentioned "fever buster" is.
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Old 05-07-2008, 01:41 PM   #4
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Yep- that's what the "fever buster" electrical box does
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