Subaru Repair: Shuttering at 50 mph+, Subaru Expert

Question
I have an 04 Subaru Outback Wagon.  4 cyl.  144,000 miles.

I travel quite a bit in low and high attitudes in NC.  Over the past few weeks I've noticed the car will shutter or "surge" at speeds greater than 50 mph.  It may do it at lower speeds but I can't tell for sure or it's not as bad.  It seems to get worse at 60 mph and then 70 mph.  I can feel the rapid surging back and forth most noticably while maintain a cruising speed.  (With or without cruise control on)  It's almost as if there is a misfire or something but not quite that bad.

No check engine light on, all fluid levels okay and somewhat fresh.  I do have to use premium gas if I don't want to hear the engine knocking.  (It knocks at low octane levels)

I've tried to read up on throttle position sensors and then the Vehicle Speed Sensors.  Hard to tell if that would be my first course of action.  Unk how much these are.  Someone said it might be the front shaft sensor is trying to override the rear shaft sensor or vice versa, causing the TCU to not know what to do and the result is surging.  Any ideas on where to start?

Answer
Michael

I say this all the time, but there are some things that just cant really be addressed over the internet, and your symptoms are one of them.  There are so many possibilities.  You could have a misfires, but would typically also have a flashing check engine light which you did not mention.  You could also have a fuel delivery problem, or a transmission issue, but I wouldn't suspect a speed sensor issue.

I can give you a long list of things to look into, but to diagnose the problem you need practical knowledge of how all of the systems work and what sensor ranges are normal at given times.

You can physically inspect the plug wires, coil and spark plugs.

Not sure what to tell you about the knock, you typically don't have to use premium fuel on one of these cars .


Justin