Subaru Engine Hesitation

1999 Subaru Outback, 110k.

Noticed some engine hesitation last week. Pressed the gas pedal and it acted like it was not getting enough gas. No check engine light, or any other light came on. So, I took it to the shop.

Replaced the spark plug wires. Didn’t fix the problem. Replaced the spark plugs. Didn’t fix the problem.

Tried a new mass air flow sensor. Didn’t fix the problem. Tried a new coil (not exactly sure what that is), didn’t fix the problem. They didn’t charge me for the sensor or coil.

I’m not exactly clear on this next step, but they did a chemical test for O2 in the coolant (I believe), and it came back positive. However, the shop ruled out a bad head gasket. Huh?

I’m aware of Subaru’s issues with head gaskets. Any ideas?

Most of the time this problem is due to a clogged fuel filter. The fuel pump may be weak also.