CASE GM 8.1 LTFT - HOW TO KILL A CAT
Vehicle: 2005 Workhorse GM 8.1l
ISSUE:
Progressively high LTFT fuel trim cells at load (data below) Vehicle has 50k miles, orig owner.
NO MIL, Codes, Pending Codes
Initial drive-ability issue: surging from rolling stop until trans downshift; rolling through stop sign... Found misfires at idle mainly cylinder 6, no DTC. Found throttle body plate dirty with minor carbon buildup
.
ACTIONS
- Replaced fuel filter, fuel pressure check under load OK
- Replaced original sparkplugs/wires OEM AC Delco 41-983
gap .06
- Removed K&N, replaced with OEM AC Delco A1236C
- Cleaned Throttle body plate (CRC TB
Spray Cleaner)
- Compression test all cylinders 170-175.
- Borescope insp cylinders, nice and
clean piston heads.
- X66P injector cleaning asper GM TSB (10% solution with OTC 7448A)
- Tech 2 Injector Test: all injectors drop from 55 to 37#
- Injector leak-down test, 56# held 90 minutes.
- Replaced both 02 sensors with AC DELCO
213-1161 (possible silicon contamination)
- Cleaned MAF with (CRC MAF Spray Cleaner)
- Cleared fuel trim data w/ Tech 2
Misfires resolved, low rpm surging resolved.
DISCOVERED LTFT ISSUE:
Test drive San Diego to Jacumba on I-8 elevation change app 4,100. Vehicle runs normal.
STFT are high, then ECM quickly learns to adjust with high LTFT
• Cell 1 / IDLE
STFT oscillating around 0 +3/-3
LTFT 0 / 0
o2 sensors immediately respond to induced vacuum leak.
• CELL 5
STFT (not sure)
LTFT +7/+7
• CELL 14
STFT +2/+3
LTFT +17 / +16
• CELL 19
STFT 0/0
LTFT +20/+19
• CELL 21
STFT 0/0
LTFT +21/+17
• CELL 22
STFT 0/0
LTFT +17/+17
Video before Power Enhancement / Open Loop
NEXT STEP: Running theory is the K&N damaged the MAF creating an insulating film on the heat-wires causing the MAF to under-report and creating the lean condition at high load.
Replaced MAF with Delphi 25318411.
RESOLUTION:
Proof K&N filter caused this issue by damaging MAF Sensor causing sensor to underreport and create a lean condition. I lost my bank 1 catalytic converter because of a K&N Filter.
K&N Filter cost me over $2,000 in vehicle damage and that is parts only!
TAKEAWAYS:
- Do not count on the MIL
- Part of routine maintenance, check LTFT by Fuel Trim Cell at various LOAD factors
- GM 8.1 at idle MAF should be app 8 g/s
- GM 8.1 WOT MAF should be app 230 g/s
- STAY AWAY FROM K&N AIR FILTERS
THANK YOU!
SPECIAL THANKS!
Without the video below from Mike Becker at Wells Tech, I would still be chasing my tail going after fuel injectors under the false impression MAF sensor cleaning works; it does not.
Wells TechConnect Ep. 19--Does cleaning a mass air flow (MAF) sensor really work?
THANKS!
