Background:
I got a new cam installed recently. Since then, my fuel economy has been abysmal and Ive been plagued by reverse surging and slight idle stalling when coming to a dead stop hard. It runs rich on startup and consumes 4.7L/HR at idle and in gear, 4.5L/HR in PARK.
Last tank lasted 240kms, or 140 miles.
Diagnosis:
I concluded the problem must be proportional fuelling. The cam is closest to an F13 and I run LTs. I knew the O2s would cause issues being farther away from the manifold but not this much. Even cruising, they looked sick.
You can't trust O2s with large cams, especially with any significant overlap beyond stock. I have 8* positive overlap at 0.050.
Solution:
Part 1: Engine re-cal for open loop and O2s at idle and off-idle
Disable closed loop at idle and part throttle by setting my PE TPS vs RPM to 0 from 0RPM to 1200RPM inclusive
Set PE enrichment to 1.0 from 0RPM to 1200RPM inclusive
Raise all O2 rich/lean vs. airflow switchpoints by 40%. I set mine all to 541, as I wont be using Airflow modes 0-6 since I will be in open loop.
Set Closed loop proportional O2 error to:
0.06250 0.06250 0.06250 0.06250 0.06250 0.06250 0.06250 0.15479 0.18750 0.21484 0.24023 0.27002 0.27979
My stock settings were all 0.175, which I reckon caused too much gain. The above settings are stock for Closed loop proportional idle O2 error, so I just set them to be the same.
Save and Flash into the car.
Part 2: Dialling in idle AFR
Now for some live tuning. You don't need to use a wideband either.
Connect the scanner and startup the car. Turn on the scanner, opening up the VCM bi-directional controls. Remember that right now you are running in open loop.
Disable Adaptive spark control
Check your Instantaneous fuel usage in PARK. Mine was 4.5L/HR
Check and set the AFR to 14.7. Raise this by 0.5AFR and check your instantaneouds fuel usage. Also listen to idle quality. Keep raising this by 0.5AFR until the car begins to chop and lope. This means you have reached a lean condition and should stop there. Time to back it off. Back the AFR off by lowering it 0.1AFR at a time until you have found the sweet spot for idle quality. Check your instantaneous fuel usage - it should have significantly decreased.
My sweet spot was 16.3 from 14.7. My instantaneous fuel consumption dropped from 4.5L/HR to 3.6L/HR.
Turn off the car. Go to your VE MAP and multiply your left 3 columns (all of them - 400RPM to 1200RPM inclusive) by your sweet spot equivalence ratio. For example, 14.7/16.3=0.9. Blend this into your next 3 columns.
Save and flash.
Startup the car and repeat.
It took me 2 goes - all of 10 mins to completely get rid of all my idle and off-idle issues. No more reverse surging, no more fuel stink on cold start, smooth return to idle characteristics.
Remember, your O2s are useless with large overlap cams so ignore what your LTFT/STFTs are reading. A large cam will ALWAYS run lean at idle and off-idle - according to the O2s.
Infact, I turned off my LTFTs all together - permanently - 1 year ago.
My thinking is the larger the cam the more you need to rely on open loop and stuff the O2s. Also note that your VE (MAFless) tune should be dialled in to begin with. I could run open loop all together but since I dont have a wideband I dont want to risk it. With my size of cam, it begins breathing efficiently approaching 2000RPM anyway so I chose to only go to 1200RPM for open loop.
Note that this is effectively a hybrid open/closed loop approach. You can have both - after all.
My fuel economy after a 100km round trip dropped from 24L/100KMS to 16L/100KMS. The car is much smoother off and return to idle.
All feedback welcome.
Special thanks to Martin Donnan. Thanks mate.