I'm looking for ideas...I have an intermittent problem that my mechanic is not finding the cause of.
SS, 80k miles, auto

Current Symptoms: Frequently after startup the idle is just perceptibly louder than usual. Adding throttle and the loudness is more obvious - like I magically swapped in a bigger exhaust or one with a hole in it. You can also feel the loudness through vibration through the body of the car. Everything seems to respond to throttle as normal, transmission, turbo, as RPMs rise I can tell there is a loss of power when it's making this much noise.
These symptoms, if they appear after cold startup, seem to most often stick around while driving, and through warm re-starts. Occasionally it will start like this, and then after 20 minutes calm down. I've never had it running smoothly and then go to bad while driving.
Last week I had three good days in a row, thought it must have been some dirty something and cleaned out, but then it went back to two more days of rough.

History and other info:
No codes, never any codes.
This all started when I was having what I describe as surging or stutter under constant throttle climbing a hill ~2000rpm. tac would jump up and down ~200rpm with each one feeling a kick in the seat. Mechanic reported lockup converter was locking and unlocking repeatedly.
Full transmission service.
I found some mention of shudder or judder like this in old threads and dealers had always started with throttle body service and plugs.
So he cleaned the throttle body, and put some aggressive fuel system cleaner in the tank.
This created a very strong vibration even at idle - like an amplified version of my current problem.
Mechanic had it back several more times during that period, I'm not quite sure what else he was cleaning, but that really strong vibration either went away or minimized to become the current one - I'm not sure.
Even then I started it once (only once!) from cold and I could hear some clicks and ticks and it smoothed out while sitting in the driveway. I'm afraid I've had this car for 10 years and I still can't claim to recognize good clicks ticks and slurps at startup from bad ones - this motor makes more weird noises and always has...
I'm not sure where in there new plugs went in. So not confident if they helped or not.

New coils only went in last week, and certainly had no improvement over the day before.

Ideas of what it could be or next steps? Questions I should be asking my mechanic?

 

Carboned up intake valves, broken flex pipe, come to mind the flex pipe should cause a code. The fuel system cleaner , was that for DI engines, you mentioned spark plugs AC Delco’s? Or some other brand?
a broken front guide in the timing chain can cause havoc , but should show a code, has he inspected the timing chain?

 

Thanks Oldblue,

1. Mechanic was talking/ruminating about direct injection motors and their foibles, and I trust he put in the correct cleaner, but will confirm
2. Plugs: receipt doesn't say brand, will confirm
3. front guide bolt: I had read on here about the problems with the original bolt's design, so ordered it when I ordered the coils. and just today I gave them the redesigned one to put in - will ask him what he finds while in the area (though replacing that is all from the outside isn't it?) Will ask him about timing chain etc.
4. I couldn't get eyes on it, but by feel (shape of the top of it) I believe I have the newer timing tensioner design (2009 model year)

 

All HHR engines with VVT have the "new" tensioner. But they over extend too.

Loose bolts on the turbo?

 

Timing chain tensioner would be the new style , pull the valve cover and inspect the chain


This is a loose timing chain in my 2.4 same set up as your 2.0 this set a P0014 code.

 

1. DI fuel system cleaner yes.
2. ACDelco plugs installed (but the new plugs didn't introduce this problem, they were part of our process of repairs)
3. Front guide bolt replaced, old one not broken (but boy it's tiny, no wonder it breaks).
4. Mechanic says he was able to see the timing chain through various access holes during this process and it looked fine. I don't think he ever had the timing chain cover off.

Still no codes.
Drivability is okay, it's just very loud under reasonable load (2.5k RPM and up), and I feel when it's that loud it's not pulling as strongly as when it isn't.

And very strange to me that most common mode of it switching from loud to not loud is a cold startup (but not always).

 

Fuel system cleaner does not clean throttle body, intake runners, or valves.

Spark plugs need to be ACDelco 41-108.

Even though there are no codes, still check the flex pipe.

 

So still planning on getting on mechanic’s lift to check exhaust/flex pipe. Though they’re confident they would have gone over that already.

had several bad noisy vibratey days then Tuesday it smoothed out 5 minutes after starting and stayed smooth for the rest of my 20 min drive, and then again after warm start 3 hours later.
when running smooth definately have more power. Though boost and shift points all seem to be the same in both conditions.

then today after cold start, started fairly smooth. (For a cold hhr anyway).
pulling out into the main road up a hill, as boost rose and rpm passed 3k I could hear it “opening up” transitioning from smooth and quiet (just the normal rev noise level) to loud and vibrate.
it really seemed like the pressure of the acceleration this time opened something up.

what is in the path from turbo into cylinders?
where could such intermittent leaks blow off some air and mess with mixture?

could explain why in this mode it can tend to “hunt” a bit, could be ecu trying constantly to adjust mixture to changing air

or it could still be exhaust as motor would blow harder during acceleration too

 

I don't think SS has a flex pipe.
Everything that you describe should cause a code. Maybe not a CEL. You should scan while it is acting up. There is a lot of piping involved with the turbo/intercooler and many joints that require special clamps.
BTW, a HHR should start right up and idle smoothly even in cold weather. The don't need warming up, should get to 180F in less than 5 minutes.

 

To confirm, yes the LNF SS has a flex pipe, this photo is of a ZZPerformance down pipe