What performance factors can be affected with an exhaust manifold leak?

Some potential solutions:

EGR (Exhaust Gas Recirculation): You mentioned that you cleaned the EGR, but it's still showing as a fault. The EGR system recirculates exhaust gases back into the intake manifold to reduce emissions. If the EGR valve is malfunctioning or clogged, it can affect engine performance.

Here are some steps to consider:
Check for Vacuum Leaks: Inspect the vacuum lines connected to the EGR valve for leaks or cracks. A vacuum leak can disrupt EGR operation.

Test the EGR Valve: Use a vacuum pump to check if the EGR valve opens and closes properly. If it doesn't, consider replacing it.
Clean the Intake Manifold: Sometimes carbon buildup in the intake manifold can affect EGR flow. Clean the intake manifold thoroughly.

Thermostat: You've already replaced the thermostat, which is good. A faulty thermostat can cause overheating or poor engine performance. Make sure the new thermostat is functioning correctly.

System Voltage Low: This could be related to the charging system (alternator, battery, or wiring).

Here are some steps to troubleshoot:

Check Battery Voltage: Measure the battery voltage when the engine is off and when it's running. It should be around 12.6V when off and around 13.8-14.4V when running.
Inspect Alternator: Ensure the alternator is charging properly. If it's not, consider replacing it.

Check Grounds and Wiring: Loose or corroded connections can cause voltage issues. Inspect battery terminals, ground connections, and wiring.

Idle Issues: If the engine won't idle properly, consider the following:

Idle Air Control Valve (IACV): The IACV controls idle speed. Clean or replace it if necessary.

Throttle Body Cleaning: A dirty throttle body can affect idle quality. Clean it using throttle body cleaner.

Vacuum Leaks: Check for vacuum leaks around the intake manifold, hoses, and gaskets.

Timing Chain: The B12D1 engine uses a timing chain. However, if it's stretched or worn, it can cause issues like poor performance or stalling. Consider having the timing chain inspected and replaced if needed.

Remember that diagnosing engine issues can be complex, and it's often best to consult a professional mechanic or use diagnostic tools.

code p 0174 ( which is probably what you meant to say) refers to system too lean bank 2====causes --intake /exhaust leak---fuel pressure/pump---injector/s---AIR system---hose connection/s
possibel items
intake ---leaking vacuum hoses ,booster,vent controls , transmission actuator
exhaust cracked flexible section in exhaust pipe connected to exhaust manifold
fuel pressure -- faulty pressure regulator ( would affect both banks
pump-- unlikely as again it would affect both banks
injectors --very possible --not working properly , blocked
HO2S sensor as well as it controls the injector operation to that bank in the exhaust manifold or before the cat converter

check for a leaking manifold gasket or cracked manifold. If it is a front wheel drive car there may be a flexible bit near the engine to allow the engine to move in the mounts . This bit of flexible exhaust is known to crack under the brading.

Coolant leaks,When the intake manifold gasket cracks or blows out, coolant will leak out of the intake manifold. You may see puddles underneath your car or notice smoke or steam coming off of the engine and exhaust from the coolant dripping onto it and then burning.Overheating, If the intake manifold gasket fails and allows coolant to leak out into the engine, your car will begin to overheat. If your car frequently runs low on coolant or begins to overheat, check the intake manifold gasket.Running poorly, When the intake manifold gasket falters, it affects the way the motor handles its emission gases. The change in pressure can affect the vacuum in the intake as well as how smoothly your car runs. Mainly, it will idle roughly and may hesitate, cough or sputter.

Sounds like more water than can get out a crack. Check if you have a frost plug on the block has come loose and leaking. There will be about 3 on each side of the engine block, located just below where the exhaust manifold mounts to the block. It's a pretty easy repair, provided there is enough room to work where it is located. Sometimes a smaller such plug will be found on the rear of the engine. The coolant must be leaking externally and running and dripping off your exhaust .

Loss of Performance

Since the manifold is an important component of the engine's exhaust system and helps ensure that the engine receives the proper amount of backpressure (the pressure developed by the exhaust system as gasses are evacuated), a cracked manifold can negatively impair engine performance. A crack is, in essence, a vacuum leak and may prevent all of the engine functions from operating correctly..
plus go to this link and have a read up will help you..

Symptoms of Cracked Exhaust Manifold

A cracked exhaust manifold should not effect your motor unless you have an exhaust driven turbo or something depending on exhaust back pressure to be there. I know where I live you can not get a vehicle with exhaust leaks infront of the passengers compartment inspected though so you may want to have the leak fixed for passenger safety.

could be just head gasket and bad exhaust manifold gasket (coolant leaks past head gasket into exhaust port then leaks out of exhaust manifold gasket)
or it possible that head is cracked too
Only way to know for sure is to remove head and inspect it

You may have a crack in crossover pipe on driver side near exhaust manifold where it bolts to crossover pipe flexing from engine movement opens crack on acceleration, check for cracks on both sides driver and passenger.

if the engine is still running lean, I would replace the Oxy sensor (s),
Then check for a vaccum leak, I found a very large leak on my intake manifold... there is a cap on a nipple on the engine side lower intake manifold (very difficult to get to) and I could not see it until I removed the intake and looked at it.
On my engine, this cap made of rubber was rotten and leaking vaccum.

This helped me 100%