I have a 2008 Chevrolet HHR with 172,000 miles on it with an automatic transmission, runs, starts fine, no noises, clunks, misfires, or codes. It shifts at regular intervals with no delays, clunks, or any other issues. But, between 40-50mph the rpms fluctulate around 100-300rpms until it shifts into 4th gear around 50mph. Once it shifts into 4th gear there are no issues, and no issues at lower gears. There is no delay in shifting, it shifts at 50mph almost on the dot.
It might be related, or might not, but at higher speeds it struggles going uphill. This is really only noticable when I have cruise control on at highway speed. When I have cruise control off I don't notice it but when I have it on at (albiet very fast) highway speed it has to rev around 4000-5000rpm to maintain that speed.
I last got an oil change 1000 miles ago, transmission flush at the same time. But this problem started prior to the transmisison flush, and has not gotten better or worse.
At first glance it seems to be maybe a dirty MAF or vac leak but after the statement "This is really only noticable when I have cruise control on at highway speed". i suspect a computer control issue. If there are no codes or CEL then my suspicions says it has something to do with the overdrive. If you have it on all the time or maybe it just has started to malfunctioning. Eventually can lead to engine surging. Can also check engine speed sensor.
May be the torque converter locking and unlocking. It feels like a mild gearshift.
Once you hit 45+ mph, the torque converter is locked out, and the rpm's will drop a few hundred.
And when going up a hill it will sense a need to unlock, adding a few hundred rpm's.
Perfectly normal to operate that way. Even more so as the car ages.
As others have said, that little bump in RPM and drop in RPM is likely the torque converter lockup clutch engaging, which makes the crankshaft and transmission input rotate as one piece. That saves gas by eliminating energy losses through tranny fluid. A torque converter is basically an enclosed set of fans, one spins with the engine and pushes hydraulic fluid over a stationary fan that routes the fluid over a 3rd fan attached to the transmission, and it transfers energy from the engine to transmission while allowing the speeds to be different.
Torque converter lockup will disengage if you push the pedal to a certain point, putting it in Overdrive with the engine and tranny rotating at different speeds. It does that to let you accelerate a bit better. Pushing it even further will downshift the transmission into the next lower gear. All perfectly normal.
How fast are you going to make the engine need to be at 4 or 5k RPM to hold highway speed going uphill? This sounds like either you're driving like a maniac and the HHR is having to downshift into 3rd to maintain a very high speed, or there is an electrical/ transmission problem, especially with cruise control on. I had an HHR years ago and I don't remember mine ever driving like this.
Lol, well to be fair it had to do that when I was going 85mph. At 85mph it runs about 3000rpm, but if I am going uphill on the highway it revs 4000-4500. Again, this is only with the cruise control on. It does not over rev if I am just manually applying acceleration.
I was thinking torque converter as well. But I wasn't exactly sure. I have an exhaust leak that is currently getting fixed right now, but I doubted that was the issue as the leak is coming from just behind the muffler. Not close enough to the engine to cause a bunch of issues.
Lol, that sounds about right for cruising RPMs at that speed. Does it slow down a little and you're not realizing it when you push on the gas vs the cruise control in this situation? The car may be driving itself more aggressively and is downshifting to maintain that speed, whereas if you're the one driving, you may be pushing on the gas to account for the hill, but you're gradually slowing down to 80 while going uphill because you're not on the gas enough. 80 vs 85 isn't that noticeable in the car. Basically you're pushing it down enough to knock the T.C. lockup off, but it's still slowing down a hair, and the computer is making sure it doesn't go below 85 by going completely into 3rd from OD.