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.
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?
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