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Thread: Throttle "kick" during accelerations over 50 mph

  1. #1
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    Default Throttle "kick" during accelerations over 50 mph

    This is a 2004 XC70 2.5T with 120K miles, well-maintained. B5254 T2 engine for CA cars.

    About a week ago, out on the freeway, I noticed a recurring "lurch forward" during times when over 60 mph and accelerating. It is so short-lived (<0.25 sec), I would call it a "kick". I get maybe 3 kicks in 10 seconds..they are randomly spaced. As soon as I back off the accelerator, they stop. The kick feels like a lurch forward, then immediately a compensating reverse kick. The average acceleration is positive, but not smooth...peppered with jerks. There isn't much sound, except a dull thud on each kick.

    If you accelerate very slowly, there are no kicks. When I tried cruise control, I couldn't produce a kick, but then the acceleration rate is limited.

    I hooked up my iCarSoft 906 ODB-II scanner, and started looking at ECM traces. The one that looks wrong is Absolute Throttle Position (see attached pix). At the same instant I feel the forward kick, there is a short-lived spike in the throttle position. I looked at MAF Airflow, and confirmed that there is an accompanying spike in air intake.
    Click image for larger version. 

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    I wanted to see if there was any correlation to RPMs, so I went into manual shift mode on the Geartronic. The same kick happens in 4th gear at roughly the same velocity, so the RPM doesn't seem to matter.

    I next looked at something called Calculated Load (the amount of power being demanded of the engine). I found out that, in 5th gear, the kicking starts when you exceed 25% load going anything over 50 mph. In 4th gear, it doesn't kick in until hitting 35% load. This is a completely repeatable defect.

    I know very little about the Bosch engine control system. I'm hoping someone out there knows how the throttle is controlled, i.e. what sensor inputs are being crunched. Is engine load an input? How do you go about diagnosing a spiking problem in the throttle control system?
    Last edited by pbierre; 06-30-2019 at 04:52 PM.

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    Could this be hysteresis in the throttle-body? My understanding is that the throttle plate is driven by a servo-controlled-motor. Let's say during high acceleration, the computer is telling the throttle to open up wider, but there's a mechanical snag. The Throttle Position Sensor immediately picks up the lack of response and ramps up the motor current. The throttle flap breaks free and overshoots -- the Position Sensor immediately senses the overshoot and brings the flap back into position. Result: a very-short-lived, transient power "kick".

    The problem I'm having with this theory is, why wouldn't I see it happening at speeds below 50 mph?

  3. #3
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    The TPS itself could be at fault, not familiar with the Volvo TPS but other cars with a TPS problem lurching is not uncommon.
    Current fleet: Countless P.O.S's, Rust buckets, Junk cars,( 50W Oily cesspool Sludge) Stolen and other assorted rubbish Volvo cars, 1928 Jed Clampett Tourer, ( 450K, original rust and decay, 40W Straight Bacon Grease),

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    Here are a series of two articles explaining Bosch's Motronic Engine Management System concept:

    http://www.autospeed.com/cms/article...art-1&A=108379

    http://www.autospeed.com/cms/A_108380/article.html

    The ME7 engine control system was introduced in the 2001 XC70s. It's as complicated as I imagined. The "Calculated Load" is a total torque demand quantity calculated in software, combining internal loads (engine friction, AC compressor, PS pump, transmission shift-points, etc) with the accelerator pedal, brake pedal and cruise control (indicating driver's desire for acceleration v. decelleration v. steady-eddy). Environmental factors such as ambient temp., barometric pressure are considered.

    Just to indicate how subtle and far-reaching these torque load calculations are, the ECM algorithm is told by the tranny when a gear-shift is imminent, so the torque is lowered during the gear change and resumed at a different level matched to the new gear ratio. That's how it's possible to accelerate smoothly thru 4 gear changes without feeling them occur. No NASCAR driver can duplicate this smoothness, because he can't coordinate 10 effector variables in real-time.

    The throttle opening angle is one major effector, but then the ECM algorithm is also able to vary input and output valve phase angles, vary the spark timing phase angle, vary the fuel injection start-phase angle and duration angle, and vary the turbo boost pressure. Ahhhhg!

    The good news is that this maze of inputs-calculations-outputs is operating under the vigilant supervision of hundreds of Diagnostic Trouble Codes (DTCs).
    There is redundancy built into the most crucial input sensors, for accelerator position, and throttle flap position. In theory, then, you should see a CEL well before the redundant sensor goes bye-bye.

    The article states that the ECU algorithm does spike filtering in its calculation of instantaneous torque demand, making it unlikely that the ECU is feeding transient spikes into the throttle's set-point, ones big enough to cause the engine to kick.

    If you have VIDA or a Volvo-specific OBD-II scanner, you'll be able to graph this "calculated load" or "torque demand" curve. It can have small step-jumps in it, for example, you should be able to see smallish total load change when the AC pump comes ON, or when the tranny shifts gears. Bosch-Volvo's system concept is that you never feel these discontinuities happening below the system level. Smooth driving-physics is the materialization of the concept.

    After reading these articles, I'm feeling like upgrading my diagnostic tool to a laptop Dice/VIDA. So I can hoover up the same parameters Motronic engine control employs.

    Meanwhile, I'll remove and clean the throttle body as a "naive guess" exercise. The Bosch writeup failed to mention Motronic failure modes that DON'T trigger a DTC -- that would've been helpful.
    Last edited by pbierre; 07-01-2019 at 05:32 PM.

  5. #5
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    Quote Originally Posted by AKAMick View Post
    The TPS itself could be at fault, not familiar with the Volvo TPS but other cars with a TPS problem lurching is not uncommon.
    The TPS (according to Bosch's article cited) has two redundant potentiometers, so that a CEL is thrown at the first instance of the two not agreeing. But, the 2 potentiometers share mechanical parts, and that could portend a single-point failure with no DTC thrown (e.g. hysteresis in the wiper arm rotation).

    Your comment was just the type of practical experience I was hoping to hear about. Thanks.

  6. #6
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    Problem resolution:
    I guessed that the "stutter" above 50 mph was unreported misfires. I replaced plugs and coils, air filter, and cleaned throttle body. The stutter is gone.

    Lesson 1: not all misfires result in throwing a DTC (I was experiencing a handful in 10 seconds).
    Lesson 2: follow the PM schedule for new spark plugs & coils every 100,000 miles (mine failed at 120,000)
    Lesson 3: the Bosch engine management system is a complex labyrinth of inputs and outputs -- but "computed engine load" drives spark timing,
    valve timing, fuel injection timing. So, don't be surprised at a failure that appears load-dependent (mine was).

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