ST and LT fuel trim values 89 Jeep 4.0 (mt2500)

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  • my2monkeys
    Junior Member
    • Dec 2012
    • 2

    ST and LT fuel trim values 89 Jeep 4.0 (mt2500)

    Hi, I've been checking data on my new mt2500.

    Closed Loop after less than 1 minute on cold start.

    O2 volts switch back and forth quickly.

    CTS reads 180* when IAT reads 120-130*

    But LT fuel trim is at 89 and ST fuel trim never gets above 100

    Are these values a concern?

    The Renix Fuel Injection Manual says they should read around 128 optimal but atleast 115-120.

    Any insight is appreciated.

    Colin 1989 Jeep XJ 4.0 Renix ECU

    MT2500 primary and T/S cart. up to 1998
  • crackerclicker
    Senior Member
    • May 2008
    • 400

    #2
    ECM is subtracting fuel. Check your MAP sensor reading, and for vacuum leaks.

    Comment

    • Bob's Garage
      Senior Member
      • Nov 2007
      • 3257

      #3
      Agreed, your vehicle is running rich. As Cracker indicated, the MAP is king over fuel control on vehicles with no MAF sensor. The whole vacuum system is suspect on those old Jeeps. Every rubber piece from the MAP sensor supply hose to the PCV needs to be inspected first.

      We recently did a Carburated 1987 Wrangler 6 cyl. It had leaky intake, exhaust and an EGR gaskets. The exhaust leaks had skewed O2 SENSOR readings to the point that it was always reading zero at idle, adding to the problem. The point is, these old vehicles can really have more than one leak.

      Do not replace MAP sensor first. Tee in a vacuum gauge and see if voltage readings are correct in comparison to vacuum reading. Find a chart. Vantage Pro or any SO tool with CTM should give you the values

      Good Luck, Bob

      Comment

      • my2monkeys
        Junior Member
        • Dec 2012
        • 2

        #4
        Thanks for your replies, sorry it took so long to update.

        My problem was the Neon 703 injectors I had installed a week before my

        purchase of the mt 2500.

        Not knowing what a proper running motor's data was supposed to be, I noticed

        the ST and LT trims degrade over a few days of driving.

        So I put the old injectors back in and a new Bosch o2 and it went directly into Closed Loop.

        I was due for a smog check so I was glad I had the Brick to tell me I had f-up with those 703's.

        The rig was running ok in open loop so I didn't even know it would have failed CA Smog.

        So after that it passed smog with good numbers at 425,000 mi. on original motor

        Comment

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