...2) We checked for CO levels and it showed a positive result.
Showed a "positive result", meaning it has CO2 in it? Or a "positive result" meaning "good - no CO2"?. Either way, I don't understand the significance of that. But I just meant is it properly full of coolant, or is there air where coolant should be? Is it full (of coolant)? (Perhaps you meant CO, not CO2, which would be an indicator of a leaking head gasket allowing combustion gases into the coolant jacket. That's not what I was meaning by "trapped air". By "trapped air", I just meant the coolant jacket of the engine not totally full of coolant.)
3) Can't really say that much as the engine has not been disassembled.
Yeah - I wouldn't expect you to know the answer. I was informing you of that as something to be on the list of possible causes if no other cause can be found.
4) Again, I don't know as we haven't checked inside the engine.
Again, just a possibility to be aware of since you don't know the history - *or* if you did somehow know that the heads had ever been removed, then the possibility of swapped head gaskets would be high
on the list of possibilities.
5) Timing belt wasn't changed.
Unrelated to the current symptoms, if you mean it never has been changed (since engine was new), then it's important that it be, along with the water pump. Related to the current symptoms, if it is old/original, it may be stretched and could have slipped a tooth or two and could be causing symptoms.
6) No, unfortunately I did not, the "mechanic" who did the job just showed me the engine and said it would work perfectly.
OK. No problem. Just would have helped to know.
On another note, one of the local mechanics told me that exhaust might be blocked somehow, not letting air out and that we should cut out the catalytic converters. Do you think that might help?
Only if they are clogged. Ignoring the overheating, have you been able to drive it at highway speeds enough to tell if it runs OK at higher engine rpm under heavy acceleration? If so, then the exhaust is not clogged. If the exhaust were partially clogged, the engine would have trouble accelerating, especially at higher rpm's.
...Again thanks for the reply...
You are very welcome. I hope you figure out the problem.