I think that it is a possibility if he has followed all steps to bleeding the system and still is having the reoccurring issue. Do you think that having a bad head gasket is out of the question? I would like to understand your thought process on this.
I know first hand chasing a "air pocket" or what you would believe to be one is aggravating. I have been down that road and replaced everything water pump,t-stat,radiator and all hoses and had it bled properly only to find out I actually had a blown head gasket.
All I am saying is that if you have new hoses and new cap and stat you will be able to bleed the system out. If your still having the same issue I would think that the air is coming from somewhere especially after replacing the cap and having a residue and slim on the cap already.
88stang88, you seem to understand what im saying perfectly. Let me just summarize because there are a few conditions to consider:
1) if one has followed all the steps [in proper sequence], and if his parts are new AND tested, unless they are Motorcraft; meaning that the stat has been boiled, and the cap has been pressure tested, and he still has a reoccurring overheating issue, then it is certainly possible that there is a head gasket problem.
HOWEVER, I stated earlier that the first thing to check for is a head gasket issue (e.g., water in oil, or oil in water). I wanted him to check this first because I had no idea how hot the engine was actually getting, regardless of the gauges, or how long he had been driving like that. Plus, its easy to check for those two issues. As a side note: head gaskets can be blown w/o manifesting those two conditions, but that is a more rare occurrence and I didnt want to complicate matters by suggesting that. So yes, a blown gasket is certainly possible, but I would follow the proceedure before jumping into that expense.
2) I'm sorry for your wild goose chase. That certainly stinks after all that work.
3) Yes, if the system has been bled correctly, and that's not regularly accomplished, and one still gets air from somewhere, then a head gasket is the prime suspect. BUT make sure its air, and not an inefficient cooling system. If there is any rust or scale one must flush the system properly and consider replacing the hoses as they will have built up sediment in them too. FURTHERMORE, I would pressure test the radiator to make sure that it can hold pressure and is not drawing outside air.
Residue or slim on the cap is certainly a problem, but one must determine what kind of residue/slime it is in order to properly diagnose the concern.
4) Lastly, someone else mentioned this point in the posts, and I wholeheartedly agree; don't overlook a worn out tensioner, and do not rely on the indicator arrows to help you diagnose. A bad tensioner will make a rumbling noise, or a kind of loose broken noise, and it will allow the car to overheat in 5th gear, but not necessarily 1st-4th gear. If one's tensioner is around or over 100k miles/12-15years old, just replace it and save the headaches.
Was that clear? Does that make sense? I hope the tensioner bit didnt confuse things. Please let me know if I failed to be clear.