Coolant loss
 

I have a 2005 that I replaced the engine in, engine was from a salvage yard with roughly 100,000 miles. Before installing I replaced the water pump and timing belt (the original engine had a timing belt failure that bent all of the intake valves). The coolant level keeps dropping (roughly a quart very month or two) but there are no obvious leaks at any of the hose connections or at the heater core so I suspect internal engine leakage but there is no evidence of coolant in the oil, no smoke on start up, engine runs very smooth, and no obvious problems with power/performance. The only other clues I have is a code for cylinder 1 misfire and a code for a bank 1 downstream oxygen sensor fault.

Any insight would be appreciated.

I don’t have an answer for you, the coolant has to be going somewhere. You might try a GM cooling system sealer, available at the dealer. They look like a pair of horse pills. They designed it to solve cooling system problems in their aluminum V8 engines in the 80s.

Check the orings on the coolant pipe in the back. Had a leak coming from there for a while that I couldn't track down because the coolant was burning off the block before hitting the ground

Thank you for the insight. I will do some additional investigation and advise what I find.

Check for residue on the back of the block and subframe

If it's boiling off at a fast enough rate you can often smell your way to it by chasing that nasty metallic glycol rotting vegetable smell mixed with damp leather boots and a tinge of rancid pumpkin pie (no whipped cream)

I inspected the car again and still am not finding any evidence of where the coolant is going. All the radiator and heater hose connections appear dry and clean (no residue). I drained the engine oil and will let it settle in a container to see if there is any evidence of coolant in the oil. I generally do not like to use coolant leak stop but decided to try Bars Leak head gasket sealer as a last ditch effort to seal the leak before tearing into the engine (or something else). My senses may be deceiving me but I swear I can smell coolant sometimes like it is leaking onto the exhaust manifold (or something hot) but so far no visual evidence. It is hard to trace the smell to a source. if the leak sealant doesn't correct it the next step may be to remove the rear exhaust manifold heat shield to see if there is any evidence on the manifold hiding under the shield. It could also be the front manifold but that one is harder to access.

Bars Stop Leak is a definite no no. That crap will stop up your heater core and then you're going to have a royal pain in the neck job replacing that because you have no heat. I would NEVER use it in anything. Moroso makes a superior block sealer, but you have to follow the directions exactly for it to work. Right now, I can't recall the exact name. I'll try to find a link for you, though. Here's what I'm talking about. Please take note that it will NOT work with antifreeze. The cooling system has to be clean in order for it to be successful. I lot of diesel shops use this stuff. If it works on the high compression diesel engines it should work in a gas engine as well. I know an ASE certified master mechanic in Sacramento that swears by this stuff. There's also a lot of videos on Youtube about it. Here's the link. https://www.moroso.com/ceramic-engine-seal35500/

Have you thoroughly inspected the expansion tank itself? Some folks have chased disappearing coolant only to find that the expansion tank has been chafing against the body or something else and created a pinhole leak in a spot not visible under nunder circumstances. In this scenario, the coolant escaping does so as vapor and leaves little if any trace. You'll smell it but not trace it unless you are waiting there to smell it.

SWAG

I know it's impossible to check everything at once.

If you let it idle for half an hour, starting with a full expansion tank, does it overheat?

As Rubehayseed suggested I now appear to have a heater core problem. The heater worked great yesterday after the installation but today my daughter reported that there is no heat. I checked on the Barsleak website and they clearly state the product will not block the heater core but I am guessing that is what happened. Would there be anything else that may cause the no heat condition? I would like to explore any options before I replace the heater core but it is very cold here (high 15 F today) so heat is a priority. It appears I can get a heater core with all the related parts on E-Bay for about $60 but probably will not order until after Christmas to avoid the order getting lost over th holiday weekend.