runs well but it surges?

Hello, do you have a scan tool to look at some live engine data? Check the Long Term and Short-Term Fuel Trims at idle, if they are high positive numbers at idle, you may be dealing with a vacuum leak, raising the idle up to 2500rpm and the fuel trim numbers come down towards 0% is a good indicator of a vacuum leak. But looking through the live data should show something standing out if the issue is that bad. The fuel trims will tell you how much the ECM is compensating the air/fuel mixture due to it being off for whatever reason, start there, I'll see if service info has any type of values chart showing what good live data should read. Does it idle above 700 RPMs when hot as well?

https://www.2carpros.com/articles/automotive-engine-vacuum-leak-symptoms

Good advice, I will try these suggestions and get back to you.

These little engines react quite a bit to vacuum leaks, or any air leak that is past the Mass Air Flow sensor, any leaks beyond the MAF are considered "false air" and will affect the fuel trims, more at idle, so anything that looks odd we can look into further and see if that's what you're dealing with. Do you have a smoke machine?

Just yesterday I replaced a Purge valve hose going to the intake air boot that had caused the fuel trims to go up to +25% at idle. The engine was running extremely rich, it was so bad I couldn't even run the vehicle in the shop right with the vehicle at the open garage door. Once the repair was made, the Long Term went to +10% but the Short Term went to -10% so total fuel trim was close to 0%, the engine computer just needed to relearn its fuel strategy at that point.
Even a stuck open Purge valve is technically a vacuum leak due to the fresh air entering the Vent valve and through the Evap system.

I always assumed any air leak beyond the MAF would set a DTC, since I wasn't getting any codes, I didn't think there were any mechanical problems. No, I wish I did have a smoke machine but I'm not a shop, I just work on my own stuff and family and friends.

This is a diagram of the Evap vapor management system. Another thing you can look into if you don't see anything obvious on the live engine data is the Mode 6 data, under the Mode 6 data any self-tests that have failed can also give you some direction, you'll need to dig through the scan tool a bit to find it sometimes, but it should be there for a 2010 OBD2 system.

Thank you much for your quick and useful responses.

Okay, let us know what you find.

ST trim 1 to -2
LT trim -3 Steady
748 rpm
MAF 2.62 g/s
Trims 0 at 2500 rpm
There is no vacuum leak, I sprayed carb cleaner all over the engine with no change in RPMs. I can't find mode 6 on my scan tool, it's a cheapy Xtool D8. Evap system tested good.

Can you graph sensor signals with that scan tool? APP should set codes, yes, how was the Evap tested? I've seen the purge on these put on backwards before if it's been taken off before, very strange the hoses can hook up opposite, but it's possible and manifold vacuum pulls the valve open when on reversed.
Your Fuel trims look normal, even cylinder misfires can cause fuel trims to go either direction. You could try graphing the APP, looking for dropouts and correct voltage levels. But I would expect a code for an APP as well.
Also graph the Oxygen sensors, they should respond very fast and go rich under wide open throttle and then lean when decel due to injector cut-off. Slow response could be an oxygen sensor heater fault. If they go lean under wide open throttle, then you could have a fuel pressure/volume problem. So, you can see these cars have a lot of different problems, some have one after another. Are there any other details you can give?

As mentioned previously according to the scan tool the evap system is good. It runs pretty good in all other conditions. Only on acceleration do I notice any issues. I never thought of fuel pressure, is there a port on these little cars to test it?

Never fully trust a scan tool test, it's only one part of the diagnosis. When it comes to the Purge valve on these, they can stick open if the charcoal canister fails, you shouldn't be able to blow any air through it one direction toward the intake manifold if it's not energized. But keep track of the direction the lines are on it before disconnecting. I have seen them stick open due to a small piece of a failed charcoal canister, check that the purge isn't stuck open by any amount. but there looks to be a fuel pressure port on the rail, here's the service info on it. Make sure it holds fuel pressure at key On as well, if there's a bad check valve in the fuel pump assembly it will drop off pressure quick, this is a return less system so if pressure drops off too fast I block off the fuel rail to see if it's the pump or injectors leaking down.

https://www.2carpros.com/articles/how-to-test-a-fuel-injector

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

I haven't done the pressure test, but I graphed the fuel trims, and it appears to go lean on accel. But I'm not sure I trust my scan tool, it seemed to respond slowly and kind of hard to read. Also, my APP is good.

Okay, try graphing just one oxygen sensor, if you're only looking at one data PID at a time, the scan tool will refresh the live data much faster. Are you getting a 0-1volt signal from the front Oxygen sensor in live data? Some info is showing this an Air/Fuel ratio sensor for the front oxygen sensor, which is what I would think on a 2010, but then theres some showing it a narrow band regular oxygen sensor up front. There are a bunch of different designs when it comes to this vehicle. Under the basic generic OBD2 live data it will most likely be a 0-1v signal, under the manufacturer side of a scan tool (so advanced data) it will show either a 0-1v signal or an A/F equivalence Ratio signal and works off current flow. But if you're seeing a high negative fuel trim number under wide open throttle, then checking the response of the oxygen sensor would be my next check, because a slow responding sensor can cause a lack of fuel, if the oxygen sensor heater is not working the sensor wont respond fast enough either. Sometimes I'll make a vacuum leak for a few seconds while monitoring the 02 and make sure it will go lean, and rich, since these little engines only have that sensor for feedback as to the air fuel ratio.

I did create a vacuum leak, and it did go lean and did go rich on fuel trim (+32). Basically, according to this scantool the is no issues but it will still surge while under heavy acceleration getting up to speed. I did a valve cover gasket and checked all other sources of vacuum and there is no leak That I can find without a smoke machine.

Okay, since you're dealing with something that is not within parameters of setting any codes, I think you should run through a VE test and see if the engine is breathing correctly, and that the MAF is sending a good signal, it may be under reporting air flow but not a point yet to where its setting a code. Especially if the MAF has been replaced with an aftermarket sensor in the past. This is a link to a VE calculator, the button at the top of the page will show the exact instructions for running the test, basically you're going to go to wide open throttle and record the highest RPM achieved, along with the MAF grams per second (g/s), intake air temp reading, engine size, and the Baro reading you can look up at the time of the test either on your local weather or some scan tools will give the barometric reading. Because you're going to be at wide open throttle, the baro and MAP should be reading the same. Wide open throttle will allow for atmospheric pressure to be equal to the intake manifold pressure, When you record scan tool data, you will want to scroll back through the live data until you get to the highest RPM reading and use the rest of the live data readings from that point as well. This test should also reveal a bad MAF reading if that's the case. The estimated Volumetric Efficiency (EST_VE) % will come up low if either the MAF is reporting incorrectly, or if there is a restricted exhaust due to bad Catalytic converter which can be backed up by a back pressure test at the front 02 location.

https://atgtraining.com/atg-volumetric-efficiency-calculator/

I did the VE test, it may or may not have been WOT. It was very near redline and screaming. Here are results. I'm not a certified tech but I consider myself a fair mechanic, but this little car is testing everything I understand about cars, but I am an old school points ignition type mechanic mostly but I'm learning more all the time.

Okay, this peak RPM is good, I know doing this test the vehicle does seem like its screaming, that's why we record the live data and scroll back to that highest RPM to get our data, you can see it would be very difficult to get accurate data at that one point. I would be curious to see if the Calculated Load was close to the Theoretical Load.

Is this data from the manufacturer's side of the scan tool or global OBD2 scan data? If you're on the Chevy side I would try this test again under global OBD2 scan data, just to be sure you're not getting any substituted values in there and grab the Calculated Load and Long Term/Short Term Fuel trims as well. I'll say this is a difficult vehicle, without any obvious issues showing under up. No codes, 02 seems to be responding correctly. But the comment on the fuel trims under accel has me wondering about the MAF not reporting correctly or low fuel volume. I assuming the 02 is going lean as well under accel, possibly staying low voltage the entire time when it should be going rich.

I found a good video that will help with the VE test a bit more. but since this is just under some code setting criteria and making a call on expensive sensors or a cat without being sure can be costly. In the video he goes into the fuel trims more and what to watch for if the MAF is under reporting air flow, right around 6min in, he explains about the ECM and its fuel calculations. If the MAF is under reporting air flow going into higher RPMs you should see some correction in the fuel trim numbers due to the 02 feeding back a lean exhaust. That wouldn't really show up for a long term (learned value) so much because the vehicle is rarely at these high numbers.
I added the OEM wiring diagram for the MAF if you need to check its powers and ground.
I'm really curious to see what the fault is on this vehicle, it's a close call, but the fuel trims under acceleration is a clue here.
Being at almost 84% VE, I don't think it would be a cat, but if you want to double check that, you can do a back pressure test at the front 02 location. You shouldn't be over 1.5psi at idle, I use an old 02 threaded section with an adapter for my vacuum gauge, you don't need to keep it on there long, just long enough to get a reading. I have seen people melt their vacuum gauges with just leaving it on the exhaust. Most restricted exhausts will show a much higher number right away and its almost something you can hear on these smaller engines; with that sort of hiss they make when the exhaust is just pushing out any place it can. Let us know what you find.

https://www.youtube.com/watch?v=CfwTc7Im0dE&list=PLfPHKlLGXDPIVTQBWC0Lwq9P7SBm8vuH-&index=10

Well I give up, this little car has beaten me. My little scan tool won't let me go into global mode, or graph more that one PID at a time, or record more than one so I can't get accurate readings. also, I don't have the means to take physical cat pressure even though I don't believe there is an issue there. I guess I'll just have to live with this little surge upon acceleration, there seems to be no other problems with the car.

I wouldn't give up on it, you have a lot of information already, I understand how difficult things can be when working with limited scan tools and other testing gear, I've had to work through all the same situations as well. We can't all afford $5,000 scan tools. none of mine are that expensive. But keep it simple, you were able to see the fuel trims go positive under acceleration, correct? So that means the exhaust has to be lean, For now graph your rear 02, and give it some heavy throttle while monitoring the short term trims, if it goes positive more than +10-15% and the 02 goes to low voltage you know the ECM is compensating for a lean exhaust. So that kind of leaves you with fuel pressure or the MAF is under reporting air flow.

I don't think the cat is the issue either, if you mess with the VE numbers just a little, such as changing the MAF reading just to 50g/s increases the
est_ VE to 97.86%. With that said, you could run to AutoZone grab some MAF cleaner spray (follow the instructions on the can) and see if just cleaning the sensor makes a difference. It's good to have a can of MAF cleaner anyway. Check the MAF voltage and ground, use a test light across the two pins of the MAF connector, orange/black is B+ from a 10-Amp fuse, and the black wire is the Ground.

On these more modern vehicles, I see a slight amount of carbon build up around the edge of the throttle plates and throttle bore being cleaned with some carburetor clean on a rag make a hug difference, just from getting the air flow turbulence corrected. Don't spray the throttle plates directly, you don't want the cleaner getting into the TB motor. I've started to look closer at these little things and I am always surprised at how much better the engine runs after I'm done. But never give up, then the car wins and that's just not allowable. =)
Here's a few more guides that can help. I really want to see this thing fixed, it helps others with this same year/model in the future that might run into the same problem.

https://www.2carpros.com/articles/repair-lean-mixture-codes-p0171-or-p0174-on-some-manufacturers

https://www.2carpros.com/articles/mass-air-flow-service

https://www.2carpros.com/articles/engine-surges

After all that I pulled the head to do a gasket and found a broken intake camshaft but yet it was running. Imagine that. I had to do the head gasket because it was overheating bad.

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.

Yes exactly, but the camshaft was still rotating. It broke in such a way as to be keyed together. Also, it did no damage to the journal. I was blown away. You can see the rub marks where it was contacting.

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.

This is an overhead cam so there is no rockers or push rods, it's direct cam lobe to valve caps/cups and since it just came back from the machine shop, I assume they did a full inspection before machining. so, I wasn't informed of any valve issues, but I will thoroughly check things out.

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.

Strange enough I didn't get coolant in the oil, this whole thing has been very odd from the start. It started out being oil in the coolant which led me to change the oil cooler but at the time I didn't know I had another issue which was the overheating. Before that I replaced the valve cover gasket but didn't see the broken cam. The machine shop didn't see it either until he loosened the cam to do the machine work.

I was preparing to begin reassembly when I found this on the (Exhaust) camshaft.
(See pic). What is going on with these camshafts? What could have caused this kind of damage but still run okay? Also, it was in perfect timing when I disassembled it. Notice almost no wear on the lobes or journals.

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.

KL1TD5DE8AB089950
Yes, I just had it resurfaced but I would have thought that finding the broken camshaft would have raised curiosity and caused them to at least check the rest of it out. I checked the screens and all is clear. I did use an impact to remove the actuators, but it didn't take too much effort to remove. And they didn't seem to be loose as to be banging around. None of this makes any sense. It's all new to me as I'm not used to working on these little motors, I've had big V8's most of my life.

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

Yes I did find something on the actuator end, but also, I can't swear that it wasn't already there when I took in. I agree with all that you are saying but also, I can't prove or do anything about it if it was dropped at the shop.

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.

I got the car for a really good price but now have way more money in it than I had planned. I now have 2 new camshafts in a resurfaced head, head gasket set including head bolts, new timing belt kit includes idler and tensioner pulleys, new thermostat and housing, front struts, complete front brakes, front tires. I think I'll keep it and try to get some use out of it knowing it's a gamble. Thank you for all your hard work and research and advice and even opinions.

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

Well after 2 new camshafts, head resurface, timing belt and pulleys, o2 Sensor b1s1, cam sensors, new gaskets and seals and thermostat and housing, and a throttle body cleaning, it runs like top, finally idles smooth but guess what? Still surges on acceleration slightly, I haven't gotten fuel trim readings yet, but I'm convinced it has something to do with the TB.

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),