Runs well but it surges?

A broken cam? Where exactly was it broken and can you take a picture? That was really not expected, I was curious what the outcome of this would be. I can't believe there were no misfire codes with part of a camshaft not moving. I'd really like to see a picture of that. Thanks for letting us know what the fault was.

Oh wow, hard to believe all this caused was a little surging. I do wonder if this would have showed up on a cylinder leak down test, or possibly a running or cranking compression test. I can understand how the cam was semi locked in place, but even at the breaking point, I dont see anywhere that looks like it was lacking oil at the cap location, or even any worn lobes. So the right end there, thats where the cam actuator is on this engine correct? And the cam sensor is at the other end where the notches are.
And you didnt find any stuck valves or anything like that on the broken side of this?

Its difficult to say is the metal was porous at the breaking point, since obviously there was some movement against the two broken ends together, I see some metal flakes inside the holes. It looks like it might have just been a manufacturers defect, but I would really inspect for anything that could have caused this. Maybe still do cylinder leak down testing to make sure no valves are effected, you dont want to get this all back together and find out it now sets misfires codes and something else Id be concerned with is all that metal flaking, Im sure some of that migrated into the oil journals and down into the oil pan.

Just off hand, and checking some other information, I found some mechanical causes of this such as a valve spring, rocker arm seized up or connecting rod coming loose, but I think you would have heard some thing like that, I mean a rod thats loose would be making some serious noise. And the breaking point does not look like there was a lack of oil there, it doesnt look burned or gouged where that cap was. And camshaft bearings will make some serious screeching noises when they start to go bad. I had one that I was almost positive it was the serpentine belt screaming, but it turned out to be a cam bearing.
I did a search too just to see if there were any other causes I wasnt thinking about.
Thank you for sharing these pictures though, very interesting case study, its a first for me, considering there were not codes, I assume it wasnt making any apparent noises either.

I also wonder if this engine had been apart before, and something wasnt torqued to spec, thats something I else I saw. If dont find any other issues, I guess we go with manufacturer defect.
Are you planning on just replacing the cam? If so, I would measure the clearance on that cap where it broke. Id use Plastigauge to do it, if I spelled that correctly. I dial bore gauge would work, but the plastigauge would give a more accurate reading. You might even want to cut open the oil filter and see how much metal particles are in between the layers of the filter. Hopefully the engine is damaged. But keep me in the loop as you go, and thanks again for posting the fault. I really appreciate it. Not the outcome we thought at all.

Okay, so it has the same setup as a Suzuki Forenza, with the valve springs under the shim caps. I had one of those recently with a broken timing tensioner that smashed all 16 valves, so I remember the design. I wasn'tt sure if you were sending the head out to get machined or not, that was a good call. I'm sure its okay then. Just make sure to do another oil change after the first start up so if there's anything particles left over, they can be cleared out. Did you find any coolant in the oil due to the head gasket? Although contaminated oil can cause a cam failure, I'd think it would look scored under that cap as well, but it looks surprising smooth. Do you have the valve clearance specs to check the shim caps, or did they take care of all that for you?
I have another guy with this same exact year and he's been experiencing a surging in hot weather only, and he never did figure out what was going on with his car, although the engine has been apart a few times because a shop didn't set the timing correctly when doing the belt and cam actuators, but I told him about this case and he was pretty interested. I dent think he any cam issues, but since that breaking point was under the cap, I'm wondering if he or the shop possibly missed seeing it.

Oh boy, the opposite end where the cam sensors are it has notches as well for the flat camshaft hold tool. There is one for the front as well (diagram 2), but its really tough to say what happened here. Something just doesn't seem right though.
So, what did the machine shop do exactly? Just resurface the head? I wonder if they used an impact gun when removing the cam bolt, I mean this is a lot of internal damage and I just don't see how a cam broken in half didn't set any cam position sensor codes or anything at all. With no marks or signs of overheating under that cap where it broke, this area in diagram 3 where the notch is, looks like there was serious vibration on the cam actuator, you can see it looks like the lower notch wasn't affected like this. But we don't have the entire history of this vehicle, such low mileage for such major problems. To be honest, when these cars start having problems, it doesn't seem to end, I don't know if it's the quality of parts GM used on them or what. Going forward if you are going to keep the car, I'd use a really good synthetic oil and there was even a TSB on using ACDelco oil filters only. Unless you fix it and sell it. It's got low enough mileage, I did a search on internal engine damage and really only came up with the oil filter TSB, but I'm sure more in-depth searches will show more issues with other vehicles. I just don't want to see you have more problems in the future.
If you give me the vin # I can run it through service info and see if any recalls or service records, come up. But that crack there seems like the actuator on that cam must have been vibrating on that notch to make marks like that. Maybe the bolt wasn't torqued to spec, makes me wonder what else might be damaged as well. Was there a magnet in the oil pan on this? Or any metal particles you noticed in the oil. See if the VVT solenoid screens have anything in them as well.

I didn't find any open recalls and there is a mile long list of TSBs, I ran through them really quick and didn't see anything related. But where that cam broke, I can't see how it wouldn't have set misfire codes and VVT codes. These cars set VVT or cam timing codes at the slightest hint of being off. Does the cam have a marks on either end like it may have been dropped? I know I'm kind of throwing the machine shop under the bus here, and I may be wrong, it could have just been a cam that was manufactured where it was weak at that point, but even with the way the broken section has that small point that looks like it could lock in place, I really don't think that would hold the whole in place, in time. Theres just too much force it has to overcome. If you think about how tough it is to turn an engine over by hand that has the spark plugs still in. I do think I see some small rust points, but if you think about it, this camshaft snapped in half, and then just stayed perfectly in place!?? And there are no other issues with the end that broke off, I'm just skeptical. I did a search for broken cam videos, and they are all pretty much the same, a no start condition with a really bad cranking cadence.

https://www.youtube.com/shorts/phcGOwswGWM

I don't like the look of that, maybe at least look up the machine shop and see if there are any reviews about it. With the price of parts these days, they may not want to say anything. But yeah, I agree with you. Can't really prove anything without the before pictures. You could bring it to their attention and see how they react. It's your call, I sorry it turned out this way. It was a cool case study, but not so much when you have to pay for it. I do appreciate you sharing the end results though. Not every day you see a cam snapped in half, I'm definitely saving the pictures. If it's possible, you may want to get rid of the vehicle after it's repaired, I know these have a lot of problems, but so does my vehicle.

Okay, well I do honestly wish you the best, I think this one was a learning experience for us both. I would like to know if the surging issue is resolved when you get things back together. Along with the fuel trims numbers if you're okay with that. Have a good evening.

Al

Does this vehicle actually have an IAC valve along with an electronic TB? I cant imagine it does, but for whatever reason they show one in service info according to that VIN #.
But heres the TB relearn procedure, that will need to be done since everything has been disconnected (diagram 1). The rest is just service info on an IAC, but I dont see one in the wiring diagrams. It looks to be for certain engines, probably built in other locations.

After the TB relearn, if its still surging, along with the Fuel trims, Id be watching what the oxygen sensors are doing, since they will be the the main feedback sensors in Closed Loop operations, they are not used in open loop. But you should see the ECM go into closed loop pretty quick after start up, "modern" vehicles should warm up the oxygen sensors fast enough to go into closed loop after a couple minutes. Its almost unbelievable how much work you've put into this car and still have issues. But watch what the 02s and fuel trims are doing during the surge.

Im sure we went over this, I dont remember everything, but also check if the surging still occurs with the purge valve blocked off, on a short test drive. Blocking it off from intake manifold vacuum and any air or vapors it might be pulling from the Evap system(tank vapor) would help rule that out. Sometimes if I suspect an Evap system leak, I will check the fuel tank pressure sensor when I turn the key off, just to see if pressure goes to atmospheric pressure quickly. A very small leak you most likely wont see with just that data pid; it should set a code for that as well.

You mentioned it going lean on heavy accel, see if its still doing that. That would explain why it still surging, the fuel system and sensors are pretty much all you have left at this point. If the injectors are clogged up a little, or the fuel pump is having an issue, it would definitely surge and you would see the fuel trims go positive along with the injector pulse width would increase as well. I really want to see what the final issue is with this vehicle.
I am also wondering about the I/M readiness monitors at this point, if any of them are reading either not Ok, or NA. If the ECM is losing its memory when the key is off, it would lose the I/M Readiness self tests it has already run, with any learned memory. You might have to dig through the scan tool options to find the Readiness monitors, ie Catalyst, Oxygen sensors, Fuel, Evap, etc. When the monitors run and it passes the self tests, they should read Ok, or Pass (Passed).