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Thread: Fuel Trims Readings odd

  1. #1
    Fuel Injected!
    Join Date
    Apr 2015
    Location
    Albuquerque, NM
    Posts
    52

    Fuel Trims Readings odd

    Some back round, Over paid for a 1999 Chevy Tahoe 2dr, I have not drove it, because of brake problems (working on it when weather permits). I bought an OBDLinkMX+ to Log data. Looking at the data idling and up to temp the fuel trims are odd, long term are -15 both banks and sort term are close to 0, it also pops when I let off the gas after revving the engine. I also had the battery disconnected, not sure if that makes a difference on OBD2.
    From what I been reading it could be a leaking fuel pressure regulator. I don't know what the fuel pressure is yet. New plugs, wires, cap, roter, and O2 sensors.

    Is it normal for the long term trim to be so far off and the short term trim to be good?

    Thank You Dom
    Last edited by Dom; 12-24-2019 at 08:18 PM.
    1993 Chevy Blazer TBI Vortec long block 5.7
    16168625 BDJZ 46mm TB 18 PSI fuel
    Camshaft, Hydraulic Roller Tappet, Advertised Duration 288/308, Lift .431/.451
    Chevrolet Performance Vortec TBI Intake Manifold
    shorty's, y-pipe and 3" full boar muffler

  2. #2
    Fuel Injected!
    Join Date
    Dec 2013
    Posts
    87
    not normal, but check you fuel pressure first.

  3. #3
    Fuel Injected!
    Join Date
    Nov 2019
    Location
    Bossier City, Louisiana
    Age
    65
    Posts
    339
    Not normal but also not excessively bad either. Does it have anything other than a stock, original air cleaner system such as a cold air intake? I had a plastic pipe elbow and pipe to a K & N air filter that made the air through the mass air flow (MAF) sensor too chaotic. This drove my LTFT numbers in excess of +25 much of the time. It also did not help that I had over-oiled the K&N air filter such that excess oil was confusing the tiny MAF sensor wires. Some spray can stuff from a company called "CRC" with their Mass Airflow Sensor Cleaner took care of that.

    But the biggest change occurred with me removing the plastic elbow and pipe and mounting the K&N cone filter straight onto the valve body. My STFT and LTFT numbers are now random values mostly plus or minus five.

    Rick

  4. #4
    Fuel Injected!
    Join Date
    Apr 2017
    Posts
    233
    Quote Originally Posted by Dom View Post
    Some back round, Over paid for a 1999 Chevy Tahoe 2dr, I have not drove it, because of brake problems (working on it when weather permits). I bought an OBDLinkMX+ to Log data. Looking at the data idling and up to temp the fuel trims are odd, long term are -15 both banks and sort term are close to 0, it also pops when I let off the gas after revving the engine. I also had the battery disconnected, not sure if that makes a difference on OBD2.
    From what I been reading it could be a leaking fuel pressure regulator. I don't know what the fuel pressure is yet. New plugs, wires, cap, roter, and O2 sensors.

    Is it normal for the long term trim to be so far off and the short term trim to be good?

    Thank You Dom
    That's how trims work. LTFTs adjust to keep the STFTs close to 0.

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