Subaru Repair: 1990 subaru legacy wagon auto trans, subaru legacy wagon, mass air flow sensor

Question
replaced mass air flow sensor, codes cleared, no codes present, car has intermittent running problem.. runs fine till warm then power loss wont upshift 20mph max speed then after sitting runs fine again.

Answer
What codes were set? Why was the Mass air suspect?  One thing that is very hard to diagnose on that year Legacy is sticky valves.  What happens is that after years of sludge build up on the valve stems and valve guides you can have a situation where the valve will "hang up" momentarily allowing a compression lost in that cylinder or cylinders.  Than the valve will close and all will be fine for a while.  If this is the case, the symptoms will be rough running (like one or more cylinders are not working)Usually starts after accelerating,  sometimes the problem will get so bad the car will loose all power and come to a stop.  Here is what you can try,   A fuel induction type service along with some sort of engine oil treatment followed by three tanks of the best premium fuel you can by along with a fuel system treatment.  What you are trying to do is clean the valve stem and guide with out removing the cylinder head and doing a valve job as that will be pretty pricey.

Hope that helps