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Thread: 2.0 LTG cutting throttle aggressively at half throttle

  1. #1

    2.0 LTG cutting throttle aggressively at half throttle

    Me and my buddy have been working on his 2019 Malibu and the car was running well only problem was that I couldn't get the manifold pressure up. Boost was reading around 20 psi but manifold pressure was only up to 8 psi. Car was running strong. Now we installed a catless downpipe and I deleted the codes for the cat and turns catalyst test off. The car runs great in wot but going from cruising to half throttle will aggressively cut fuel and close the throttle really quick. I could not find what table is causing this. I have tuned many LNF cars and I understand how the tables work in relation to each other. This one has me stumped on what is causing the issue.

    I lowered the wastegates duty cycle, maxed out over boost and underboost tables, lowered timing to avoid knock and also increased knock airmass tables a bit from -2 to 0 and no change. WOT runs great but half throttle does not. It does not knock when this happens but actual boost goes over desired boost by 2 or 3 psi. Any suggestions on what might be happening? I can send a log to anyone that wants to look over it. Thanks

  2. #2
    Post log and tune.

    LTG is GM software, LNF is Bosch software. Outside of the fact they both share the idea of ?torque control? literally not a single line of code is the same.

    If your engine is relatively stock, with stock turbo, there?s no reason to change the wastegate duty cycle.

  3. #3
    I used a bit of what i learned from the LNF on this and i believe that is my mistake. i did bring up the wastegate DC like i had done on my Solstice. here is a log of the issue. it only happened once on this log and i will also post the tune file that was used. i will bring the wastegate values back to stock and retest as i feel that is what is causing the throttle to close quickly.

    throttle cut.hpl
    stockj3 catless.hpt

  4. #4
    i'm pretty sure there's a variable that you can record that's wastegate dutycycle, i'd record that as well in future logs.
    Also if your car is 2016 or newer (E80 controller) that means you have a factory wideband O2 sensor, not sure which variable to record, or if you already record it, but find that signal and record it.

    If the throttle closing problem you are describing at t=11:23:06, then yes, that's because you messed with the wastegate duty cycle too much. if you have a stock turbo, on a relatively stock engine, only minor changes might be needed to that table, but maybe no changes. you put in 100% dutycycle in the 2.6 pressure ratio columns, which means it will try it's hardest to close the wastegate (maximum effort as deadpull would say), when the ECM sees all the cylinder airmass (you're reading 1.53 g/cyl) WAY above what you wanted, the ECM tries to gain control (and protect) the engine by closing the throttle.

    Here's my understanding of how this system works: accel pedal is converted to desired torque. The torque model converts desired torque into desired cylinder airmass. the boost controller converts desired cylinder airmass into desired pressure ratio. the wastegate base dutycycle table + a boost PID controller converts desired pressure ratio into wastegate dutycycle.

    Since you messed with the wastegate dutycycle, all the controls lose their mind because the engine isn't doing what they thought it would, and all the closed loop controllers scramble to gain control of the system however they can, in your case, closing the throttle.

    From the sequence of events i described above, you can see that if you want to get more torque, you need to ask for more torque, and allow a higher pressure ratio, not to mess with the conversion table that turns pressure ratio into wastegate dutycycle.

    There's something odd in your data, the MAP signal seems messed up, I didn't spend any time trying to figure out why it's wrong, but something is odd. when the throttle is open TIAP and MAP should match, only about 1 to 2 kPa different.

    EDIT: you're also knocking here and there, 2 to 3 deg knock retard, not sure if that's real knock, not sure it's acceptable, but it does it at high engine load (high boost, high cylinder airmass) so i'm not sure i'd be comfortable with that. this is a really long, complicated discussion, many opinions, few facts. Don't know if it's real knock or the sensors are too sensitive, don't have an answer for you, maybe someone else will comment on the knock.
    Last edited by TurboCobalt; 6 Days Ago at 04:00 PM.

  5. #5
    Thanks I was sure it was because my actual boost was exceeding desired boost and you have confirmed that with your explanation. So in order to ask for more torque, that would be the peak torque and the other tables in the torque management section correct?

    The MAP signal is also one of my concerns, it has been like that since I first worked on the car. I thought it was maybe not allowing airflow into the manifold and why it was reading that way. There is a major difference in kpa.

    As for the knock, it looks like real knock but it doesn't seem to go away when I pull timing either. The knock sensor settings have not been changed, if they are anything like the last knock sensors they are pretty sensitive. And I thought I was logging the o2 sensor I'll take a look again thanks.