If you've spent any time on these forums or searched for anything related to "surging/hesitation", you'll find a plethora of information, but no real fixes. I've been dealing with it for years with my TL, and only this past weekend did I actually find the solution.

I've toyed with everything from spark plugs, to thermostats, to EGR valve, to MAP sensor, to APP sensor, to dirty throttle body, to replacing the battery. Of all the things I've tried, the only thing that helped was a new battery - but that was only temporary.

I adjusted my valves a few weeks ago for the first time (at 173,000 miles). I wasn't as precise as I should have been, and actually set a few of them too loose. There was a slight tick before the adjustment, but it was worse after I adjusted them. Interestingly enough, the surging got worse.

About this time, I also ordered an Autel AP200 OBD reader, so I could view some live data while I was driving. What I noticed (and long suspected) was that a significant amount of ignition timing was being pulled while driving (I was watching the "timing advance" and "knock retard" properties). Last weekend I adjusted my valves to be in spec, and noticed a marked difference in how it was driving. Also zero valve tick. Timing was still being pulled while driving, but it seemed less.

Now that I've driven about 100 miles, it's a completely different car. It started feeling better and better as the ECU "relearned" the correct timing advance.

What was happening was the knock sensor was hearing the valve tick, and kept trying to pull timing to prevent detonation. Since it had been going on so long, the ECU had learned to almost permanently retard the timing, also trying to constantly adjust it as you're driving. This is why the surging/hesitation is so unpredictable...it's constantly toying with the timing retard (or "knock retard") in an attempt to prevent detonation (which wasn't actually happening - it was just valve tick).

As some have noted, WOT (wide open throttle) minimizes the surging/hesitation. The reasoning behind this is under WOT the ECU goes into "open loop mode," disregarding some sensor data and defaulting to pre-determined fuel maps, etc. So rather than constantly toying with "knock retard," it sets to a default position and holds there, making acceleration more constant.

By the way, this is also why some have claimed that replacing the battery or installing "better grounds" fixes the issue - because disconnecting the battery resets the ECU and may temporarily negate the issue...until the ECU continues to retard the timing. I experienced this just last month when I had to replace my battery. It was good for about a day or two, then it just got worse and worse.

All I can say is I forgot how fun and relaxing this car was to drive...and surprisingly torquey!

If you have ANY valve tick, adjust the valves and get them within spec.