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Thread: -22 LT Fuel Trim extended driving/high RPM only - cause/solution?

  1. #1
    Join Date
    Jan 2017
    Alabama, GA
    My Cars
    1998 BMW Z3 1.9L

    -22 LT Fuel Trim extended driving/high RPM only - cause/solution?

    1998 Z3 1.9 Automatic, 240K miles.

    Hello, I’m looking for cause and solution to hesitation at higher rpm after an hour or more of driving.

    I seem to be having a problem with a rich fuel mixture suddenly occurring after extended driving (an hour or so) at higher rpm (2,800-3,000+). The live data ST fuel trim will start decreasing to about -17, -20 then the LT fuel trim eventually does the same thing. In other words, I understand that the computer is detecting more fuel than air in the mixture and so it cuts back on fuel till the car starts to hesitates/bucks…then smooths out if I drop rpm or start heading down a hill.

    I took a pic of live data while it was happening…here is a summary...after about 4 hours of driving.

    ST FTRM1 = -14.1
    LT FTRM1 = -22.7
    Speed 63
    Fuel sys – closed
    O2s11 (v) .86
    Lt Ftrm1 1 = -12.5
    O2s1s (v) = 0.765

    Occasional P0101 code a month ago: Mass Airflow Sensor. Could this be the culprit? Should I try and drive at high rpm with MAF unplugged to confirm? The data that got saved with the PO101 code happened about a month or so ago and the code seems to have been triggered while the car is at rest.:

    Eng rpm = 880
    Cal load = 4.7
    Coolant = 892
    St Ftrm1 = 0.0
    LT FTRM1 = -3.1
    Veh Speed = 0
    Fuel Syst 1 = open

    Each of the O2 sensors will cycle up to about .8+ than cycle back down at all RPM while driving.

    Head gasket replaced at @ 200K, along with radiator, waterpump, thermostat, upper intake hose.

    Here’s how it happens. I make a 3 hour commute (6hr round trip) about 1 or 2x a week. On the first leg, the car may experience some occasional hesitation after an hour or more of steady 70-75mph (2800-3,000+ rpm). Cruise control activated, or unactivated. Then no problems for the next few hours…maybe the odd hesitation now and then. I’ll then drive off and on (city driving) for a few hours, no hesitation or problems. On the return trip, after an hour or so of highway speed/rpm (70-75mph, about 3,000rpm the car will begin hesitating more frequently. This occurs most often on long stretches of slow inclines, it goes away on a decline. It will be bothersome enough to slow down to 60mph (@2,500 rpm), the hesitation will gradually disappear as I stay at lower speeds/rpm. The above data was captured after a good bit of hesitation and then slowing things down.

    What could cause LT fuel trim to decrease significantly only at high rpm over extended driving periods?

    Also note, (and I know this may be a different, unrelated issue) car ran low on coolant and hot (225 – about ¾ up the dial) on extremely hot day in Atlanta after extended driving (6 hrs) about 2 months ago. I refilled reservoir, got home okay no overheat. Happened again, found autozone supplied thermostat and/or water pump leaking. Replaced both, seems the gasket on waterpump was not so good. No more overheat. Drove for a month with above hesitation conditions. Two days ago, upper radiator hose burst at idle (fortunately was looking at live data and saw temp at 225 before it even moved the gauge.) May not have burped all air out of system, causing overheat. Pulled over, limped home later. Replaced hose. No overheat (currently).


  2. #2
    Join Date
    Jan 2017
    Alabama, GA
    My Cars
    1998 BMW Z3 1.9L

  3. #3
    Join Date
    Aug 2012
    My Cars
    2000 m roadster 2010 x3
    I’d check all the intake boots for leaks/cracks. You could do a smoke test to locate.

  4. #4
    Join Date
    Sep 2018
    Houston, TX, USA
    My Cars
    1999 BMW Z3 Coupe 2.8
    Sounds like a vacuum leak with my basic understanding.

    I have link to a smoker if you need it

    Sent from my iPhone using Tapatalk

  5. #5
    Join Date
    Dec 2007
    Great Dismal Swamp
    My Cars
    E36/7 E36/8x2 E39/2 E46
    Fuel pressure way too high.

    Bad MAF, over-reporting airflow

    Someone stuffed high flow fuel injectors in because, well, racecar.

    In 40 years I've never seen a vacuum leak cause the fuel mixture to go 50%+ RICH.

    - - - Updated - - -

    I discount the possible bad O2 sensor since both are against the rich stop.


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