171, 174 on Windstar 95

[SebringYves]

I still have the code 0171 and 0174 on my 95 Windstar (185.000 miles).
As mentioned before I have a new fuel pump, new fuel filter, new air filter and cleand fuel injectors. Now I cleand the MAF sensor and found a little hole in the elbow from the intake manifold which I stoped up. The I run the van for about 50 miles without CEL coming up. So I tought the problem is resolved. Yesterday I started the van and after 5 miles the CEL is coming up again. What can I do??? Should I check the HO2 sensors. Can they be cleaned? How can I find an air leak?? Once a member wrote to spray some carb cleaner into the engine area and the carb cleaner will be sucked in. But will this clog the leak?
Should I disarm the manifolds and pu new gaskets? Could it also be a leak in the exhaust? Should I check this?
Any ideas???