GXP or NA?
I’d be hesitant to try this on the side of the road. If you had an air compressor and a vacuum bleeder tool like an AirLift, it’s easy to bleed the system. But, you may try the DDM method.
Basically, you disconnect the coolant line that goes into the head. Front of the engine, top drivers side of the head, connection points up and down. It’s a fairly small hose.
With that disconnected, remove the 10mm bolt below the reservoir that holds it to the bracket and disconnect the coolant level sensor that plugs into the reservoir. Fill the reservoir, the lift it up so that the coolant level is higher than the top of the engine. Coolant will (quickly) start leaking or gushing out of the head where you disconnected the hose.
Put the hose back on, lower the reservoir, reattach everything and run it for a bit, keeping an eye or temp and coolant level. When reattaching everything, try to keep the reservoir as high as possible until the hose is reattached. If the reservoir isn't the highest point in the coolant circuit, it'll suck air in. (Hooray, physics.) This is just an overly-wordy way of saying that water runs downhill.
This can be aided by squeezing the radiator hose with your hand a lot, or so it has been said. Obviously, don’t get your hand caught up in the belts or fan.
It helps if you have the heater on full blast while doing these things.
I had limited success with this method, but it’s probably the easiest thing to try without access to a shop.
Note: if you have a GXP, there is a TSB regarding coolant filling. There are a pair of check valves that need to be inserted in the coolant hoses. Available from GM, but I had to order mine as no one had them in stock.
Here’s the video on the DDM method:
https://youtu.be/YF5Ka4oeCxA
Hopefully goes without saying, but never open the reservoir when the engine is hot. It is under pressure and will spew ~5-10 feet in the air and give you severe burns.
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