I have 2011 hhr if I start it after its set over night or sets for 3 or 4 hours it starts and runs fine for about 5 seconds then the engine starts missing like it revs up and down for about 10 or 15 seconds then clears up and able to drive it but it acts like something is holding it back loses power I've changed upstream o2 sensor put new throttle body on put new speed sensor and maf sensor still getting cil and code system too lean bank one any ideas of what my problem is

 

Welcome to the site, please post the codes you’re getting, just the codes, we can take it from there.

 

P0172 and p0455

 

The P0455 is most likely the EVAP vent valve (behind the gas tank).

You probably have a bad fuel pump. It should hold pressure. You may have a rusted out vapor return line

 

https://www.obd-codes.com/p0172

points to the MAF sensor, check to ensure the connector is seated firmly in place.

https://www.obd-codes.com/p0455

points to the EVAP system has a leak, somewhere, most likely the purge canister valve , but do check the purge vent valve under the hood . You should not be able to blow air through it from grey side to black side. Even a little bit can cause starting troubles.

 

The o455 code came up bc the gas cap but when I bought the car the check engine light was on said system too lean I changed o2 sensor (upstream) now it says to rich any ideas

 

Yes, reread post #5 .

 

Yes, read the previous post.
The codes actually are "fuel TRIM too rich"; that means the computer is adding fuel because the fuel ratio is too lean.

 

Which the MAF sensor is responsible for reading and adjusting.

 

Are you using an oiled air filter? (K&N, etc)

Inspect the flex pipe for cracks.