I have these 2 pids. My cheap scanner did not show the P2187 and the gas cap looks fine and is tight. I'm assuming the P2187, "System to lean at idle Bank 1," might indicate a leak near the intake manifold. My cheap scanner shows graphs of things that might cause the P0451. The quiet parts are idle and the noisy parts are driving. Don't know if that will help?

Any hints on where to start looking would be appreciated. If it matters, it showed up shortly after replacing the radiator fan.

 

I’d start by inspecting the EVAP system starting at the rear under the car make sure the hose connections are clean and tight, look for cracks or damage to the EVAP canister the solenoid then look up under the hood at the EVAP valve it should be closed so you can’t blow air through it

 

P0451 is fuel tank pressure sensor performance not EVAP leak. Usually that comes from a wire or a fuse, sometimes the sensor.
Asanti found that his MAF sensor was dirty/disconnected https://www.chevyhhr.net/forums/ss-s...ss-auto-50932/
Also make sure the lower MAP wires didn't get moved around and shorting. Look for vacuum leaks between the MAF and the #1 O2 sensor, including the PCV hoses.
Could be low fuel pressure.
Here is a pdf from OnDemand:
p2187.pdf
link in my sig to free shop manuals.

 

The EVAP system utilizes a vented reservoir (usually called a canister) to store excessive fuel vapors until the engine is being operated under the appropriate conditions to burn them thoroughly. It is designed to capture fuel vapors before they escape into the atmosphere. The charcoal canister, the EVAP pressure sensor, the purge valve/solenoid, the vent control valve/solenoid, and a complex system of metal tubes and rubber hoses that stretches from the fuel tank to the engine compartment are all crucial to operation of the EVAP system. Fuel vapor pressure, created when fuel is stored, acts as the propellant and causes the vapors to vent through a network of metal tubes and rubber hoses; eventually reaching the canister. Fuel storage tank vapors are discharged through the pressure relief valve (in the top of the fuel tank) and forced through a complex web of metal and rubber lines to the charcoal canister, where they accumulate. The canister contains a charcoal element which absorbs fuel vapors and holds them for release at the appropriate time. The purge control valve/solenoid is responsible for regulating intake vacuum to the EVAP canister so that fuel vapors may be drawn into the engine when conditions are ideal for them to be burned instead of being released into the atmosphere. The PCM electronically controls the purge control valve/solenoid which is the heart of the EVAP system.


It’s a complex system

 

It turned out to be the fuel tank pressure sensor and this is what the same scan looks like when it's working properly.

 

Thanks for the follow up and confirming the fix!! Keep it clean and drive it often!