Discussion
I've seen this discussed before on here, but can't find the info at the mo'. Ringram probably knows the answer (as he says 'fuel trims are the work of the devil') how do you reset the long and short term fuel trims on a VZ CV8? I know it involves removing a fuse or two, but I don't know which ones - anyone? 
HSV_Rulz said:
I've seen this discussed before on here, but can't find the info at the mo'. Ringram probably knows the answer (as he says 'fuel trims are the work of the devil') how do you reset the long and short term fuel trims on a VZ CV8? I know it involves removing a fuse or two, but I don't know which ones - anyone?
can someone explain fuel trims to me please
My understanding is that the ECU , over time, monitors the usage of fuel and the way that the car is driven, it then modifies the fuelling of the engine with a calculated figure related to the above parameters. For example, if the car is driven by 'Miss Daisy' then it will apply a larger fuel trim which will result in a reduction of fuel applied and hence reduce BHP, drive it like 'Skaifey' and a much smaller trim is calculated (if any) and the tune will stay much closer to the initial map. I believe these 'Fuel Trims' can be disabled in your tune, but this prob requires an EFI-Live boffin. The fuel trims can be reset to nil by a remove power/fuse out procedure, but unless disabled will recalculate and reapply over time.
HSV_Rulz said:
My understanding is that the ECU , over time, monitors the usage of fuel and the way that the car is driven, it then modifies the fuelling of the engine with a calculated figure related to the above parameters. For example, if the car is driven by 'Miss Daisy' then it will apply a larger fuel trim which will result in a reduction of fuel applied and hence reduce BHP, drive it like 'Skaifey' and a much smaller trim is calculated (if any) and the tune will stay much closer to the initial map. I believe these 'Fuel Trims' can be disabled in your tune, but this prob requires an EFI-Live boffin. The fuel trims can be reset to nil by a remove power/fuse out procedure, but unless disabled will recalculate and reapply over time.
cool im guessing mine hasnt any then as the needle dosent stay on full for long
Fuel trims are designed to make sure the fueling in closed loop maintains stoichiometric air/fuel ratio. which is the most complete burn of the fuel and the most enviromentaly friendly (NOT the best for power)
As you drive the car the fuel is adjusted using the O2 sensors as the guide.
On standard cars the base map is a little rough (Monaro etc VXR8 are much better)and trims can move a little to high/low and this has a knock on effect when you put your foot down and the car over fuels (wastes fuel)
On re mapped cars this is fixed when the tune is done.
Turning off the trims is NOT a good idear if you cant monitor your PCM as triming allows for wear and tear (blocked filter etc).
When you add mods your car will need to trim more to maintain stoichiometric and on early cars headers, exhaust and CAI could push trims to 25% which is when the engine management light comes on.
As you drive the car the fuel is adjusted using the O2 sensors as the guide.
On standard cars the base map is a little rough (Monaro etc VXR8 are much better)and trims can move a little to high/low and this has a knock on effect when you put your foot down and the car over fuels (wastes fuel)
On re mapped cars this is fixed when the tune is done.
Turning off the trims is NOT a good idear if you cant monitor your PCM as triming allows for wear and tear (blocked filter etc).
When you add mods your car will need to trim more to maintain stoichiometric and on early cars headers, exhaust and CAI could push trims to 25% which is when the engine management light comes on.
Edited by wortec1 on Tuesday 28th April 16:55
Yep, short term trims are useful, they are constantly "reset" by the o2 sensors as they switch from rich to lean, its the evil long term trims which I think are bad. Like Paul says they are designed to correct base tune issues. They are used as a learnt "base" for fueling (built up over time from the short term values) and when in open loop (open throttle settings) can result in a very rich condition, which any dyno day person knows is the stock condition. Problem is they may get built up from some bad fuel or an exhaust leak and nothing to do with the tune.
Personally I like to disable the long term trims and leave the short terms running so under 95% of the time you have perfect fueling with no learnt "corrections". When in open loop the ecu would normally dump in extra fuel (from the long term value) it doesnt (because they are turned off) and runs off your hopefully now good tune.
While this can change over time as the car wears etc, its going to be very minor so probably only needs checking when you notice a power drop or perhaps every year or two. Supercharged blokes might like the extra protection that the trims give by running rich.
Ideally the long term trims should remain near zero on a tuned car, if they dont, either the tune is bad or it needs redoing.
If you want to feel the difference reset the ecu by pulling the fuse for a min or so. That will zero your trims. Which is the same as disabling long term trims in the car. Short term still work and the car will still drive fine. Long term trims will be relearnt as you drive starting from zero.
Personally I like to disable the long term trims and leave the short terms running so under 95% of the time you have perfect fueling with no learnt "corrections". When in open loop the ecu would normally dump in extra fuel (from the long term value) it doesnt (because they are turned off) and runs off your hopefully now good tune.
While this can change over time as the car wears etc, its going to be very minor so probably only needs checking when you notice a power drop or perhaps every year or two. Supercharged blokes might like the extra protection that the trims give by running rich.
Ideally the long term trims should remain near zero on a tuned car, if they dont, either the tune is bad or it needs redoing.
If you want to feel the difference reset the ecu by pulling the fuse for a min or so. That will zero your trims. Which is the same as disabling long term trims in the car. Short term still work and the car will still drive fine. Long term trims will be relearnt as you drive starting from zero.
Just to explain a little more, even if the tune is perfect under full throttle the long term trims in the appropriate cell will be added to the full throttle fueling irrespective of this perfection. Therefore they can and do create an overrich condition. If they are turned off this does not happen.
Naturally aspirated I see no problem turning them off. Supercharged with obscene temps and pressures you may not want to.
It is possible for them to be negative values too though
Naturally aspirated I see no problem turning them off. Supercharged with obscene temps and pressures you may not want to.
It is possible for them to be negative values too though
stigmundfreud said:
HSV_Rulz said:
ringram said:
If you want to feel the difference reset the ecu by pulling the fuse for a min or so.
Thanks for the explanation, Richard, which fuse is it though?
its the BCM fues, its a bastard to get out so take tweasers.
HSV_Rulz said:
Thanks for the explanation, Stig, which fuse is it though?
wolfracer said:
stigmundfreud said:
HSV_Rulz said:
ringram said:
If you want to feel the difference reset the ecu by pulling the fuse for a min or so.
Thanks for the explanation, Richard, which fuse is it though? 
d to get out so take tweasers.HSV_Rulz said:
Thanks for the explanation, Stig, which fuse is it though?
The other trick is to simply disconnect the battery for 10 to 15 minutes as that has the same effect. Just dont shut the bonnet with the doors locked
I'll go take a look, the older Ro's probably have a slightly different wiring loom. I will check now and get back to youGassing Station | HSV & Monaro | Top of Page | What's New | My Stuff