2005 GM 8.1 LTFT Case

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

1. Replaced fuel filter, fuel pressure check under load OK 
2. Replaced original sparkplugs/wires OEM AC Delco 41-983 gap .06
   
3. Removed K&N, replaced with OEM  AC Delco A1236C
4. Cleaned Throttle body plate (CRC TB Spray Cleaner)
   
   
   
5. Compression test all cylinders 170-175.
6. Borescope insp cylinders, nice and clean piston heads.
   
   
   
   
   
   
7. X66P injector cleaning asper GM TSB (10% solution with OTC 7448A)
   
   
   
8. Tech 2 Injector Test: all injectors drop from 55 to 37#
9. Injector leak-down test, 56# held 90 minutes.
10. Replaced both 02 sensors with AC DELCO 213-1161 (possible silicon contamination)
    
    
    
11. Cleaned MAF with (CRC MAF Spray Cleaner)
12. 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!   

The proof is in this video.

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!

Scotty Kilmer

Eric O at South Main Auto

Ivan at Pine Hollow Auto Diagnostics

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?

CONFIRMED CALIBRATION

THANKS!