sperkins

I usually turn off my kill switch anytime the car is going to sit for more than a few hours to save battery life (11lb Braille).
I have a my fuel trim set relatively safe in the tune. Found this today:

Anytime the battery goes dead in the car--or the voltage drops below 12v or the battery is dis-connected, the fuel trims are all re-set to "0" and the P/T fuel all has to be re-learned--the more mods the longer it takes--usually in stock trim about 50 miles of normal driving will make it learn-out.

So does this mean I am bypassing my tune and running the car in some predetermined stock safety fuel trim by disconnecting the battery power to the PCM?

meatbag

It just means that all the LTFT's have been reset. As long as your tune is good they shouldnt be more then +/-3% anyways so it wont matter.

wallyman424

Yeah youre only resetting the long term learning tables. No biggy unless your tune is way off.

brkntrxn

On that note, my car failed inspection after Nationals. Seems that racing the car for a week after disconnecting the battery never allows it to go through its "50 miles of normal driving" routine. LOL. It was my sign to take it off the street, turned in the tags yesterday and getting out the Sawzall tomorrow.

-Kevin

sperkins

I don't pretend to know much about tuning, but isn't the LTFT the only one that's stored in memory to control injector duration? Wouldn't it be a bad thing to constantly clear these values by flipping the kill switch off?

Rob's 73

Your tune is set. LTFTs are learned (from STFTs) and applied to the tune. STFTs are constantly adjusting the "tune" based off of what your narrow band O2 sensors for any given moment. Your running fuel mixture is your stored "tune", LTFTs, and STFTs.

You have nothing to worry about unless your tune is way off.

AU N EGL

Would the battery tender help?

larryfs

you failed inspection because of the "readiness monitors". you need to drive a few hundred miles before they are complete after the battery is disconnected.

A battery tender would solve that problem.

Timz06

For a track car, you would want your LTFT's to be zero at full throttle, that way your fuel ratio would be according to your tune, not trim readings from the O2 sensors.

I am not sure about the LS1, but in many engines they are not used when you go WOT, only for part throttle driving.

There is also a parameter where you can set the maxium LTFT percentage.

For my LT1, I had a false lean indication caused by an exhaust leak. This caused the LTFT to lean one bank to the max which was 20 %. I had many discussions and people told me that LTFT's did not apply at WOT, but looking at the injector band width, I had one bank at 100% and the other at 80%. I later found a program that blocked the LTFT's at WOT.

So in the worst case, these could hurt more than they help.

To pass emissions after you switch off the power, you need to fulfil the OBDII drive cycle. You can do it in 15 minutes here is what you do;

Cold Start. In order to be classified as a cold start the engine coolant temperature must be below 50°C (122°F) and within 6°C (11°F) of the ambient air temperature at startup. Do not leave the key on prior to the cold start or the heated oxygen sensor diagnostic may not run.

Idle. The engine must be run for two and a half minutes with the air conditioner on and rear defroster on. The more electrical load you can apply the better. This will test the O2 heater, Passive Air, Purge "No Flow", Misfire and if closed loop is achieved, Fuel Trim.

Accelerate. Turn off the air conditioner and all the other loads and apply half throttle until 88km/hr (55mph) is reached. During this time the Misfire, Fuel Trim, and Purge Flow diagnostics will be performed.

Hold Steady Speed. Hold a steady speed of 88km/hr (55mph) for 3 minutes. During this time the O2 response, air Intrusive, EGR, Purge, Misfire, and Fuel Trim diagnostics will be performed.

Decelerate. Let off the accelerator pedal. Do not shift, touch the brake or clutch. It is important to let the vehicle coast along gradually slowing down to 32km/hr (20 mph). During this time the EGR, Purge and Fuel Trim diagnostics will be performed.

Accelerate. Accelerate at 3/4 throttle until 88-96 km/hr (55-60mph). This will perform the same diagnostics as in step 3.

Hold Steady Speed. Hold a steady speed of 88km/hr (55mph) for five minutes. During this time, in addition to the diagnostics performed in step 4, the catalyst monitor diagnostics will be performed. If the catalyst is marginal or the battery has been disconnected, it may take 5 complete driving cycles to determine the state of the catalyst.

Decelerate. This will perform the same diagnostics as in step 5. Again, don't press the clutch or brakes or shift gears.

larryfs

Maybe this is why it takes a few hundred miles to complete all the readiness monitors. Typical driving does not comply with the rules below.

wallyman424

nope. As rob said you have your regular fuel trim tables, which are the ones you actually tune. Then you have your learning tables stft and ltft.

in newer vehicles there are two different modes of fueling, open and closed loops. Closed loop is typically lower rpm part throttle driving, where the computer uses the O2 sensors to lean out to stoiche. Open loop disregards the O2 sensors and relies solely on the fuel table and ltft table to fuel the car.

Falcon

So, what's the answer, is it a big deal to disconnect the battery or not?

How much effect will it have on performance, if any?

wallyman424

It wont effect anything. Its not a big deal at all, as long as your car has a solid tune on it.

If you have a marginal tune on it and the LTFTs are correcting for that, then it might cause a change in performance, but i doubt that's the case on a tuned car.

Timz06

I did some research and the computer will only use the LTFT if they are positive. If negative, it will zero them, so the worst thing that could happen is that you would add a bit of fuel.

Correct. If the LTFT are positive the PCM will add that amount of fuel at WOT.

You can force your LTFT negative by multiplying the Injector Flow Rate table by a number less than 100.
For instance it your LTFT is +10% then you should multiply the IFR table by 95 and do some test driving to revaluate your LTFT.

FWIW I have never seen any difference in performance with LTFT values between 5% and +5%. If your LTFT are within this range then I would leave them alone.

TRACKMAN2

thnk its other way around brian told be if iunplug my o2s it would go to cloosed loop i get you point though....he says kill switch woult hurt a racecar at all is all street driving stuff learned...

wallyman424

No, its definitely closed loop that uses the O2 sensor, think of a circuit. Closed loop = feedback circuit, open loop = nonfeedback.

0Bobby @ LG Motorsports

Race car?

narrow band o2?

Thats an oxymoron at its best!

take the sensors out, they can do more harm than good in race cars and high HP street cars.

TRACKMAN2

i was told that too but was suspicious....

TRACKMAN2

your right sorry sometimes i get loopy