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Thread: Help with L67 starting and rough idle

  1. #1
    Potential Tuner
    Join Date
    Jan 2021

    Help with L67 starting and rough idle

    Hello. I have 2003 Pontiac Bonneville ssei. Used hptuners vcm suite to try and diagnose hard starting and rough idle issues. I am new to this so i don't understand a lot. The scan shows that LTFT B1 is red on start up and idle. I replaced crank position sensor, MAF, o2, iat, fuel filter, I checked spark plugs and wires and just cut off the catalytic converter to straight pipe. Still the same issue. I noticed that when i increase the rpm, the LTFT B1 shows a normal reading. Does anyone know what is wrong or what I should try next?

  2. #2
    Tuning Addict 5FDP's Avatar
    Join Date
    May 2012
    Rogers, MN
    Did you check fuel pressure yet?

    And did you check for vacuum leaks??

    Intake manifold gasket leaks are very common on the L67. As well as the vacuum lines around the supercharger and boost bypass valve.

    Just note that cutting off the cat probably wasn't the best idea if you didn't have any codes for that problem. There is a cat test that HP Tuners is unlikely to remove and it may drop your fuel economy some.
    2016 Silverado CCSB 5.3/6L80e, not as slow but still heavy.