o2 Sensor Question

[ddrallstar69]

Ok here's the story. I put long tube headers on my car, and ran straight pipes into a roushe side exhaust kit. I removed the CATS and the rear o2 sensors are unplugged but the fronts are plugged into the headers. The check engine light is on of course but I also noticed I get horrible gas miliage and almost no boost during acceleration(Minivans beat me off of red lights and i have my foot halfway to the floor). I had it plugged into a computer and the guy told me it said its basically "starving for fuel". I was told its rear o2 sensor not being plugged in that is causing this. I was wondering if indeed this is the problem and if it is, I am planning on getting the MIL eliminators from Latemodelrestoration.com hoping thats what will solve the problem. ANY help on this would be greatly appreciated.

http://www.latemodelrestoration.com...g-46L-V8-38l-V6-Mil-Eliminators-O2-Simulators

 

[Bullitt347]

What were the actual codes that were in the computer? Getting horrible gas mileage is not usually associated with a engine that is "starving for fuel". Usually the engine is running to rich to be getting horrible gas mileage. Loss of the rear o2 sensors will cause the engine management system to default to a rich setting. The factory ECU will NEVER default to a lean condition. If the engine is rich, you just burn more fuel and get crappy gas mileage but you do not usually hurt anything, except maybe the catalytic converter it if runs long enough like that. Running lean for even a short time will have catastrophic consequences in a very short time. I do not know what scan tool was used or who "this guy" was, but I bet he really did not know what he was looking at or what the information actually meant. Or he totally knew what he was doing and you did not understand what he was saying. Either way, you should at a MINIMUM get the MIL eliminators as that will make the ECU and the engine happy, and hopefully yourself as well.

 

[ddrallstar69]

Ill ask him again what the code was. I do remember now he said the engine was running lean. The scan tool was a MAC brand auto scanner. What are the catastrophic consequences that would happen?

 

[Bullitt347]

Ill ask him again what the code was. I do remember now he said the engine was running lean. The scan tool was a MAC brand auto scanner. What are the catastrophic consequences that would happen?

Running an engine lean can cause anything from a blown head gasket to burning a hole in a piston and everything else in between. I have seen spark plugs melt or shatter, cylinder heads get burnt through to cracking a block because of the detonation from running too lean. Some of these are extreme examples, but I have seen head gaskets and burnt pistons on moderately lean running conditions over a long enough time. A lean mixture will make more power than a rich mixture, but you run the risk of damaging parts. If it is rich, you really do not hurt anything but gas mileage. I think MAC tools uses OTC hardware. Called a Genesis scan tool if memory is correct.

 

[ddrallstar69]

ok the Codes that were messed up were my long and short term fuel trims. Don't know what that means though.

 

[Mustang5L5]

Well, o2 sensor aside, running straight pipes tends to shift the powerband higher into the rpm range. As a result, you actually lose a lot of low end torque due to it now coming on in the higher rpms. Even deleting the cats alone can create this effect. But delete cats and mufflers on a stock car and you can lose a lot of power.

Not ideal for a street car which is why most street cars don't run that sort of setup.

My advice is to rethink your exhaust setup. Definitely get the o2 sensors back in place or go get an expensive custom tune done to turn off the o2 sensors and make the car drivable.



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[Mustang5L5]

Double post



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[jrod2423]

yeah in agreance with Mustang5L5 u have to have something in your straight pipe to give a little back pressure to the motor, cuz even though the motor has to work against the back pressure it keeps a constant temp on the valves and keeps them from warping, like when i did mine i cut out the muffler but kept the cats and i've seen both gas mileage go up and power. and on the o2 sensor side if you don't have that hooked up and it isn't sending data to the computer, than the computer will mod the air to fuel to fix the problem or until the o2 sensor responds. so basically a quick fix would be hook up the o2 sensor and get something for back pressure in the system because the 3.8 isn't strong enough to go true straight pipe and not ruin something in the long run.

 

[Bullitt347]

ok the Codes that were messed up were my long and short term fuel trims. Don't know what that means though.

Long term fuel trim (LTFT) and short term fuel trim (STFT) is the ECU's way of keeping track if the engine is running rich or lean. The ECU can compensate for a lean condition (a + or positive % on either LTFT or STFT) or compensate for a rich condition (a - or negative % on LTFT or STFT). A + or positive % is when the ECU is adding fuel, and conversely, a - or negative % is when the ECU is subtracting fuel. The ECU can compensate up to 20-25% rich or lean. If the ECU sees that more of a correction than that is needed, then the CEL (check engine light) comes on. So the ECU was trying to add more fuel (in the form of injector pulse width) to compensate for a "lean" condition. Since it could not add enough fuel to make the ECU "happy" the CEL came on. Also STFT is the correction that is happening right now, while LTFT is the overall correction over a period of time. Usually a + STFT and + LTFT codes are related to a vacuum leak or un-metered air that has entered the inlet manifold. To have to correct beyond 20% for a lean condition, requires a pretty good sized vacuum or air leak. It is possible also for the fuel injectors to be plugged up to the point that they simply can not deliver enough fuel. This is the "short" explanation for your issue as described by you.

 

[ddrallstar69]

Ok, I installed the MIL eliminators and hooked everything back up, reset the computer by disconnecting the battery for 15 mins, drove around without the check engine light for a lil while. It than came back on and I stopped at O'Reilys to hook it up to the computer. This is what it displayed.

 

[Bullitt347]

Bank 1 is passenger side, so whatever problem there is will be on that side. The fact that an MIl eliminator would give a "stuck lean" code is odd. You could switch the eliminators from side to side and see if the code switches sides, and then you would know if it was the MIL eliminator that was bad or not.